Now that we have finished our journey through the past, some may be asking what the Future holds. Ultimately, it is practically impossible to make any completely accurate predictions, considering there is an awesome number of factors at play. Certainly, we, humans, are the biggest creators of uncertainty. We have an incredibly plastic set of behaviors and, even being all under the same species (Homo sapiens), we sometimes operate almost as if distinct types of organisms. Due to this fact and the many behavioral sets the components of humanity can assume, everything regarding our future becomes much harder to pinpoint. Despite this, it is possible to paint a few broad strokes, especially when these are dependent on external factors at play. But, before we explore some of these “broad strokes”, it is important to highlight the sheer adaptability of our species, which resides in great part at the core of all this uncertainty, the just-cited behavioral plasticity. As a result of such plasticity, we as a species were able to come out of Africa and colonize practically all continents (with the notable exception of Antarctica) without undergoing too significant phenotypical variations. All of this was undertaken thanks to our ability to fit into new environments, not by changing through natural selection, but rather through changing our actions and, in what would become especially more common as time progressed, changing the environment itself.
As a result, humankind is exceptionally versatile and, dare one say, hardy, not due to any special physical properties, but simply due to how we react to and shape our surroundings. Due to this, humanity’s extinction is likely to be exceedingly difficult. Do not confuse this, dear reader, with a lack of hardships: those may absolutely come and perhaps be more devastating than anything we as a species have endured before. Our entire civilization might collapse, most of our peers might perish, but our species, Homo sapiens, will likely eke out, not unharmed, scarred, but alive nevertheless. What could be such a dramatic hardship?
Well, perhaps the one closest by and, tragically, a possible cataclysm purely created by ourselves, is that of nuclear war. Quite a recent threat, there were a few, very well-known and documented instances in which a nuclear exchange almost occurred during the Cold War, a time when both the United States of America and the Union of Soviet Socialist Republics (USSR) were engaged in a global standoff for supremacy, one in which a direct conflict between the two superpowers was mostly avoided, in a twist of fate, exactly by the nuclear weapons that put humanity at the precipice. Since the beginning of that worldwide confrontation, such arms of mass destruction have gradually made their way into the hands of several other countries, and to this day, others are still trying to develop their own. While one might argue that the tense state humankind found itself in during the Cold War is behind us, recent developments in the international scene suggest that a new, similar age is dawning, and the fact that nuclear threats have been thrown out with increasing banality by the Russian Federation, the nation that inherited the stockpiles of the deceased USSR, still puts the whole of our species in a rather uncomfortable situation. The situation is potentially made more serious by the recent rise in artificial intelligence, which, increasingly integrated into our daily lives, is also present in the military sector, where it could, coupled with inadequate human supervision, possibly aggravate a cascade of bad decisions by relaying incorrect or incomplete facts.
In the case of a nuclear war, predictions are also difficult due to a lack of experimental data and due to the many variables in such a conflict, such as the number of weapons that would be detonated, which areas would be targeted (civilian and military targets or just military ones), and so forth. However, despite these uncertainties, it is plausible that such a conflict would stay restricted to the Northern Hemisphere, as the Southern Hemisphere does not contain any countries wielding nuclear weapons, and, consequently, targeting such nations would possibly be considered counterproductive. Consequently, the North would likely bear the brunt of the cataclysm, especially because, as previously mentioned here, the air circulation is roughly isolated for each set of latitudes and thus the South would be roughly spared, at least initially. First of all, not only would struck areas be affected by radioactive fallout (the radioactive particles dispersed by the blast), but the explosions would probably cause massive fires, triggering a great release of toxic gases such as carbon monoxide, and, should industrial areas be affected, many other toxic volatiles, such as sulphur and nitrogen oxides, which would make rain highly acidic. Most devastatingly, though, the enormous amount of soot and ash would lead to the formation of black clouds, which, by blocking sunlight and radiating heat back into space, would cause temperatures to drop significantly, akin to (but not equivalent!) the winter that occurred 66 million years ago and that resulted in the extinction of the non-avian dinosaurs as well as many other creatures, as more detailed in this chapter.
The fact is that, potentially, such dark clouds could be sufficiently heated up by sunlight to the point of going into the upper atmosphere. Should that happen, then they could spread globally, covering the Southern Hemisphere in darkness and bringing with it radioactive particles formerly confined to the boreal parts of the Earth (those that would more profoundly affect the South would be the ones with longer half-lives, such as strontium and cesium isotopes, with the first being chemically similar to calcium and thus accumulating in bones as well as in other calcium-containing tissues, and the second, chemically similar to potassium, accumulating in soft tissues, especially muscles). In the whole world, the areas that would be most affected by cooling would be continental ones, as those near the coasts, due to the thermal buffering effects of the ocean, would probably manage to have more stable temperatures. Even more devastation could ensue as a result of the arrival of nitrogen oxides in the ozone layer, leading to the formation of oxygen from ozone and thus a depletion of the protective gas layer. However, the clouded skies themselves could probably filter part of the increased UV radiation, and so the situation could be somewhat mitigated.
While such a scenario is undoubtedly extremely grim, there are some hopeful points, especially because the Southern Hemisphere houses countries with significant agricultural capacities. Brazil, for instance, has an extremely large land area and plenty of natural resources, apart from an exuberant agricultural sector that produces not only enough for the country itself, but a very significant amount for international markets. This has been made possible by the great technological advancements undertaken by such a sector in the last few decades of Brazilian history, developments that allowed agricultural food production to increase by 4 times and the cultivated area only by 68%! All of this means that, even in such a tragic global context, Brazil could still potentially have the capacity to feed not only itself, but maybe even more disadvantaged countries. Despite these positive points, though, a major portion of the country’s agricultural production is localized in more interior portions, ones that would be more greatly affected by the drop in temperatures, and, additionally, Brazil is a politically and socially tumultuous nation, a feature that could prove disastrous amidst a cataclysmic landscape. In South America, Uruguay and Argentina are also two other States that have powerful and technological agricultural sectors, one that also supplies their internal demands as well as foreign ones. While Argentina, due to its more southern position, would probably face harsher temperatures, Uruguay could be in a better situation, especially since, unlike both its larger neighbors, it has better socioeconomic indicators, potentially making it more resilient in such a desolate landscape.
Plausibly even more promising than those South American nations would be the Australasian ones of Australia and New Zealand. Not only is Australia home to a technological and self-sufficient agricultural sector, but it is also a highly developed nation, putting it quite a bit ahead of its previously mentioned peers. However, its quite large size, much like Brazil, puts many of its most interior areas in a worse situation when it comes to falling temperatures. At the same time, though, its isolation from the rest of the world could prove particularly valuable, especially due to the huge amount of refugees that would certainly arrive in the South and which could put more strain on already highly fragilized countries, potentially hastening societal collapse. About New Zealand, while not completely self-sufficient like Australia, it is in regard to many food sectors, and, as its larger neighbor, it is also isolated from the globe. Additionally, its small size means that it could have its temperatures buffered by the surrounding oceans, potentially making its climate more hospitable and less extreme. While the detailed scenario above would likely push humanity back several decades and maybe even centuries, we as a species could probably make it out and, as mentioned before, not unharmed, not without scars, but alive nevertheless.
Right on the contrary of a nuclear exchange, an asteroid or comet impact is another event of potentially cataclysmic proportions that could only be stopped or mitigated by our decisive action. Fortunately, though, the most massive of such astronomical bodies, those ranging in the tens of kilometers in size (such as that responsible for the just cited Cretaceous-Paleogene extinction), are quite rare and expected only to make contact with our planet once every about 100 million years, which would still give us, in theory, some 34 million years before another encounter. Unfortunately, though, there are many more of these objects still sizable enough (kilometers in size) to produce plenty of damage, potentially damage enough to cause a civilizational collapse in a much more powerful way than the nuclear scenario above. For these astronomical bodies, it is estimated that an impact occurs every about 700 thousand years, a much shorter timeframe. Even smaller asteroids or comets, with hundreds of meters in size, could generate massive casualties if they hit heavily populated areas or nearby locations, with an estimated frequency of impact of one in about 20 thousand years, an even shorter timeframe. Well, thanks to our ever greater technological advances, it is possible to deal with some of these extraterrestrial threats, and there are plenty of proposed ways to do so (with a variety of different mechanisms), especially if such a hazardous object is discovered years in advance, which would allow its orbit to be slightly tweaked and a collision avoided.
Thankfully, there might even be a way to deal with smaller comets or asteroids that were not discovered years in advance, perhaps just months, weeks, days, or even hours. This would be achievable with modern technological capabilities and would be done via the positioning of several tungsten cylinders, which would ideally slice through the incoming object. Interestingly, they would not need to be at high speeds, since the great velocity of the asteroid or comet would already offer plenty of kinetic energy for their action. As these hardy cylinders went through the astronomical body, it would be fragmented into several smaller pieces that, should it already be in Earth’s vicinity in the context of a surprise impact, could be effectively burned up by our planet’s atmosphere, preventing greater damage. In the case of more distant objects, the penetrators could fragment it with much less hazard, neutralizing the comet or asteroid before it gets too close, apart from being able, in such circumstances (of about 60 days in advance), to pulverize something as big as 1 km in size. There are some ways the use of these objects could be maximized, such as by putting them on standby on the Moon, where launching them, due to the natural satellite’s lower gravity, would be made much easier. Even so, it would be possible to get them into space from Earth with an adequate spacecraft. Overall, these penetrators are extremely promising, being cost-effective and quite achievable.
Now it is time to focus on a significant threat already ongoing, that being of anthropogenic global warming. Kickstarted by the Industrial Revolution, it has been caused, as several other warming events of the past, by a great increase in the release of greenhouse gases, especially as a result of the burning of fossil fuels, such as coal and petroleum. This climatic event is part of a greater process of extinction that has accompanied humanity since it dispersed out of Africa. While it started with the megafauna of the areas where our ancestors colonized, it has continued since then, now occurring not only as a result of global warming, but also due to massive changes in the environment promoted by human action, such as deforestation, the building of dams, the introduction of invasive organisms (a particular problem for once isolated, island ecosystems), and, in a more primordial aspect, the continued hunting of several creatures.
How the release of greenhouse gases affects the biosphere is multifactorial. While there is the obvious warming, which can negatively affect several organisms not only by increasing temperatures but also by changing weather patterns (making some areas drier and others wetter), as well as by causing the deglaciation of ice caps (which helps raise sea levels and leads to habitat loss for polar species), there are also non-thermal effects of the greenhouse gases. Carbon dioxide, for instance, dissolves into ocean water and turns it more acidic, compromising the formation of carbonate skeletons as well as eroding existing ones, affecting organisms such as corals, shellfish, and foraminiferans, among many others. Speaking of corals, they are also affected directly by the higher temperatures in the form of bleaching, in which they expel their dinoflagellate symbionts, as better explained in our Devonian voyage. Despite this, it appears that crustaceans, despite also containing carbonate shells, have increasingly thrived in more acidic oceans. Other marine creatures are also thriving in hotter oceans, these being cephalopods, which often prey on crustaceans, and jellyfish, considerably expanding their populations.
Some of the most currently affected animals, if not the most, are amphibians. The reason for this is two-fold: global warming and an emerging, deadly fungal disease known as chytridiomycosis, which may also be associated with the world’s temperature rise. Regarding the first reason, it has mainly affected tropical amphibians, which typically have smaller distributions than temperate ones, especially those located in restricted habitats, such as mountainous environments. Climate change has affected them in various ways, such as by augmenting dryness in certain times of year, which can be especially disastrous to these vertebrates, since, as their name already makes clear, they are amphibious in nature. Regarding the second reason, it has also put at risk especially tropical amphibians, probably because they live in areas where multiple species coexist, meaning the infection of a single species commonly goes on to affect the other coexisting species. Well, what is the causer of chytridiomycosis anyway? It is the species Batrachochytrium dendrobatidis, a member of the fungal phylum Chytridiomycota. This phylum of fungi is quite basal (its hyphae, unlike those of ascomycetes and basidiomycetes, for instance, are non-septate and fused together), probably arising during the start of the Phanerozoic, and most of its representatives are aquatic, though others live in terrestrial environments. However, potentially the most peculiar feature of these organisms is that they are the only fungi to exhibit flagella, an ancestral trait shared with animals that was lost in the more derived fungal lineages.
This locomotory structure is especially important for the life cycle and pathogenesis of B. dendrobatidis, for it is using its flagellum that the motile stage of the fungus, lacking a cell wall, finds a suitable host. As it does so, it interacts with components of the amphibian’s skin, such as mucin, a glycoprotein that, due to its great hydrophilic properties, is responsible for what we know as mucus. These interactions lead it to suffer a great transformation: it loses its flagellum and starts building a chitin cell wall, all of this occurring within an invaded epithelial cell. This new sessile form, feeding on keratin, grows more and more, asexually multiplying until it eventually releases several motile forms, which then go on to restart the cycle, either reinfecting the host or a new individual altogether. The main factor behind the pathogenesis of this parasite is the fact that amphibian skin displays various functions apart from just constituting an external covering, such as breathing and osmotic balance. Infection results in thickening of the skin and apparently compromises its ability to maintain the just-cited osmotic balance, often causing the hosts to die.
However, not all amphibians respond in the same way to this pathogen, and some, like the cane toad (Rhinella marina), are not much affected by it (as will be better explained). As a matter of fact, this anuran, originally native to the Americas (with a wide distribution ranging from Texas all the way to the Amazon), has become an invasive species in the Caribbean, Hawaii, and even Australia, going against the trend in amphibian loss. One of the reasons this tetrapod has undergone such success is due to its extremely high fecundity. Not only does it breed year-round, but more than 30 thousand eggs can be laid at a time! This is a key survival adaptation against B. dendrobatidis, which, though not affecting adults much, can be highly harmful to tadpoles. However, these come in too great numbers for the adult population to be affected, and besides, they are often found in warmer microhabitats that prevent fungal growth.
Regarding the adults, these large, bumpy-looking toads are mainly insectivorous, often outcompeting native amphibians and also leading to the death of many native predators. This is due to the toxin these animals secrete from their extremely enlarged parotid glands (these form two verrucous lumps around their heads), which, known as bufotoxin, exhibits a mechanism of action similar to the drug digoxin (extracted from the eudicot plant Digitalis): it inhibits the sodium-potassium pump, causing a buildup of sodium and later of calcium in cardiac cells, which slows down their conduction and may lead to cardiac arrest.
Shifting away from amphibians, but taking advantage of the mention of the cane toad, it is time to mention a few other organisms that have prospered not despite humanity, but thanks to it. These lifeforms will probably stay with us for as long as we are around and are actually advantaged by disturbed as well as heavily modified environments resulting from human activity. Perhaps the most iconic of these is the infamous brown rat (Rattus norvegicus), a fairly sizable rat (though far from as large as previously seen Papagomys armandvillei) originally found in the forests of Northern China. However, by the eighteenth century, it had made its way to Eastern Europe and, from there, it colonized the rest of the world, often as unintended passengers in ships. Though far removed from its original environment, this rodent had no problem adapting to the new habitats, becoming inseparable from its human commensals. As a result, these mammals can now be found everywhere except in Antarctica, being ubiquitous in areas subject to human habitation. As occurs with the cane toad, part of the reason for their success is their frequent reproduction, which occurs up to seven times per year, based on a competition of males for access to females, which, nevertheless, mate with multiple members of the opposite sex.
Females are the ones to partake in parental care, with the offspring being nursed in often complex burrows with specific chambers where the adults also inhabit. Such burrows can be complemented by a variety of materials, be it vegetable matter or human discarded waste. Additionally, they are commonly located near water sources, and, indeed, these rats are very capable swimmers. Apart from dealing well with water, R. norvegicus is also an excellent navigator as a result of the combination of several specific traits, such as a great sense of smell, great tactile sensations (both from their whiskers and paws, allowing them to transverse even completely dark spaces) as well as good memory, which leads to remember not only their complex burrows, but also the complex sewers and other convoluted human structures they routinely travel through. In regard to food, they are extremely opportunistic and are not picky eaters, being true omnivores, though apparently exhibiting a preference for meat and even engaging in predatory behaviors, such as catching fish with their paws and going after birds.
Another creature commonly associated with brown rats, both in the environment as well as in disgust, is the German cockroach (Blatella germanica). This hemimetabolous insect is native to Southeast Asia, where it occupies a wide variety of habitats, ranging from rainforests to caves, all the way to drier areas. As a result of human activity, they also disseminated far and wide, now also being found, like the brown rat, on every continent except Antarctica. This is no coincidence, for these creatures are intolerant of the cold and so, even more than the brown rat, need human presence for their survival, especially in temperate areas. Like occurs with the brown rat and the cane toad, they are avid reproducers, breeding throughout the year and with high promiscuity, each generation intermingling with the next, to the point that a single cockroach may leave, at the end of its life (which does not pass over two hundred days), up to 10 thousand descendants! Like the brown rats as well, they are ample opportunists, consuming human foodstuffs as well as waste, apart from also feeding on stranger materials such as soap and even glue.
Yet another animal very often found in association with humans is the common pigeon (Columba livia). Native to Europe, North Africa, and the Middle East, this primarily seed-eating bird of the order Columbiformes has historically built nests in the crevices of rocky cliffs near the sea, eventually being domesticated by humans and spread across the world. However, many domesticated individuals either escaped or were intentionally reintroduced to the wild, leading them to adapt to the new environments they found themselves in and grow significantly in number. Curiously, though many of the new areas lacked the characteristic rocky cliffs that these theropods used as nesting grounds, human buildings served as fine analogues, and now pigeon nests are widely distributed around cities, with skyscrapers inadvertently acting as cliffs once did. Unlike the previously mentioned species, C. livia, like most other birds, is monogamous, and once a pair is formed, it stays so for life, raising their chicks cooperatively and defending them from any intruders with pecks coupled to other aggressive behaviors. Here we thus see a creature that, though far removed from its original habitat, managed not only to survive but also thrive by subverting spaces far removed from the function of nesting sites. It is a testament to the versatility of life and also to how our species not only annihilates but also creates new opportunities for life.
Not only animals have benefited from humankind, but many plants have as well, and just like domesticated animals, many of them have undergone profound modifications. The arguably three most important plants for humanity are all grasses, thus members of the Poaceae family, explained in more detail here. Our interactions with them stretch back many centuries, and they have shaped us as we have shaped them. The first of these is Asian rice (Oryza sativa), the most widely cultivated form of domesticated rice and, as the name suggests, a native of Asia. Its cultivation by Homo sapiens started from some 8 to 15 thousand years ago, and this crop grows in both tropical and temperate climates, setting roots in a variety of soils as long as these are capable of flooding. This is because rice is a semiaquatic grass, growing in areas where few other plants can.
One of the ways it manages to do this is through the formation of gas-filled spaces along its body known as aerenchyma, which allow air and thus oxygen to be transported from the leaves (in contact with air) all the way to the roots (often submerged). Apart from this, rice exhibits a higher tolerance to alcohol (formed by submerged plant tissues as a result of alcoholic fermentation, itself a result of the lack of oxygen) and has its growth stimulated by submersion, helping it break the water line faster. These peculiarities did not go unnoticed by humans and, indeed, most rice is cultivated under a considerable film of water, which prevents the growth of weeds and other pests that could hamper production. Regarding some of the changes rice underwent during its domestication process is the fact that it is now mainly auto-pollinated, leading to inbred populations that conserve traits seen as more useful by our species. Wild species of the genus Oryza, however, are mainly pollinated by the wind, as other grasses, exhibiting higher genetic diversity. Though most cultivation of this plant still takes place in Asia, it has spread worldwide and is thus another example of a lifeform brought to new heights thanks to anthropogenic intervention.
The second of these fundamental grasses is, of course, common wheat (Triticum aestivum), the most widely grown wheat by far. This plant originated in the Middle East and, unlike O. sativa, which typically requires warmer conditions, is well-adapted to temperate climates. From that place of origin, the plant disseminated, developing several varieties adapted for drier and colder or less cold climates. Drier wheats, for instance, have reduced size, requiring less irrigation. Wheats adapted to the coldest climates possible fruit in just three months, allowing them to complete their life cycles before winter. Wheats adapted to cool but still not as cold climates, have longer life cycles and actually require a period of lower temperatures in order to properly develop. Like occurred with rice, wheat is primarily self-pollinating, and its domestication came at a significant price, for, unlike wild varieties, the inflorescences of common wheat, which eventually develop into their grains, are not brittle and can be collected whole, which greatly facilitates harvest. In wild wheats, such structures fall apart easily, and the grains disperse, helping secure the new generation and possibly the colonization of new areas. T. aestivum, though, is mostly dependent on us for its propagation in contrast. Despite this, significant parts of the world’s surface are now dedicated to the growth of this plant, and it has thus achieved unprecedented population numbers and geographical distribution.
Finally, the third of these Poaceae representatives is corn (Zea mays). From the get-go, it appears different from the two grasses analyzed previously due to its larger height (up to 7 meters) and less bushy aspect as a result of the tall main stem. It also has a fairly distinctive reproductive structure: the male flowers sit at the top of the plant, and the female ones appear as almost hair-like growths (apt for catching pollen) emerging from a protective covering of leaves. These leaves protect the corn cob, made up of various ovules that, upon being fertilized, turn into the seeds that we eventually consume. Native to the New World and more specifically to Southwestern Mexico, this phototroph was initially domesticated some 9 thousand years ago, undergoing a great transition, as its teosinte ancestors (wild species of Zea) were bushy in aspect and had small cobs containing only a handful of seeds that easily detach. Despite these changes, corn, to this day, still practices mainly cross-pollination, unlike its wheat and rice relatives. Tragically, teosintes in their places of origin (around Central America) are now endangered, just going to show the dual action our species can have. Indeed, we have turned into the greatest ecosystem engineers of all time. Not only do we change the distribution of organisms in major ways, but we also change their appearance through intentional and systematic processes of artificial selection, giving rise to exaggerated phenotypes that would have never arisen without our interference.
While our power not only to destroy, but also to actively create has been made clear, we also have the power to preserve, something that has become ever more present as time goes by. While currently biodiversity is still decreasing and a significant number of species are steadily losing numbers as a result of anthropogenic activity, our actions have also generated some positive results, especially for some of the most threatened species, which, due to heavily directed conservation efforts, have had their extinction risk reduced, though they have not yet fully recovered. These initiatives have been directed mostly towards birds, amphibians, mammals, and warm-ocean corals, with other animals receiving less attention. Despite this and the still much greater number of species with decreasing counts, there are quite a few real success stories that go to show that extinction is far from unavoidable: coexistence is not only a possibility, it is a reality in many cases. Two of these examples come from the United States of America. The first one is regarding the American alligator (Alligator mississipiensis), a darkly colored crocodilian with a broad snout that can reach sizes varying from 2.7 meters for females to some 3.7 meters for males. These opportunistic carnivores, which inhabit various bodies of water along the Southern United States, including brackish and saltwater environments, were considered endangered during the first half of the twentieth century as a result of excessive hunting, motivated mainly by their scaly hides and meat. However, by the second half of such a century, conservation measures were put in place, and now these semiaquatic archosaurs have a total population of five million, potentially even increasing their northern range due to the increasing global temperatures.
Another American animal that formerly faced the threat of extinction is the bald eagle (Haliaeetus leucocephalus), which is amazing to think about given its great importance in American symbolism. Nevertheless, this bird of prey, which has a wingspan up to 2.3 meters, nests mostly on tall structures, such as conifer trees or human-made towers, usually near bodies of water, be they lakes, rivers, or full oceans. In the past, these fellow archosaurs probably reached numbers of about half a million in North America as a whole, but hunting, as well as the use of the now-banned pesticide DDT (which caused their egg shells to become excessively brittle) led their numbers to fall to only 412 nesting pairs in the United States by the end of the first half of the twentieth century. However, several actions, such as captive breeding programs, reintroductions, and protection of eagle habitats, have caused their numbers to once again rise close to their past levels, and now they sit at more than 300 thousand individuals, with more than 71 thousand nesting pairs. This is an extremely poignant example of our power not only to conserve, but also to promote effective recoveries.
Another example is that of the humpback whales (Megaptera novaengliae) from the Southwestern Atlantic. These large mysticetes are very well-known cetaceans, existing in several roughly isolated populations in the Atlantic and Pacific oceans. They are migratory and spend most months in high latitudes, either around the Arctic or Antarctica, where the productive waters allow them to engage in their filter-feeding lifestyle. During the winter, though, when the environment becomes much harsher, they migrate to tropical areas, which, despite lacking as much food, offer adequate nursing grounds for their young. Since the 1800s, whalers killed these mammals, increasing very much in activity at the beginning of the twentieth century and, over the next years, up to 60 thousand individuals were exterminated, to the point that less than one thousand of the giant synapsids remained. In response to this dire situation, international action was taken, and the action of whaling in the region became illegal by the 1980s. Since then, their numbers have not yet reached their former scale, but now number at 25 thousand, a considerable improvement nevertheless.
Despite this, anthropogenic global warming is another threat to be considered, especially due to its ability to disrupt the extremely productive marine polar ecosystems. Antarctic krill (Euphasia superba), diminutive, shrimp-like crustaceans that constitute an important component of the diet of these massive mammals, are particularly adapted to survive in their icy environment. For instance, their larvae are dependent on the ecosystems that form on ice for food, especially because they are unable to accumulate the energy derived from the consumption of blooming phytoplankton during the warmer times of year. Not only this, but ice often provides shelters where they can appropriately develop. While adults are schooling filter-feeders that can survive even without ice grazing, these invertebrates generally only tolerate a fairly narrow range of cold temperatures, even to the point of longer exposures to 3.5 degrees Celsius being detrimental to their health. Simultaneously, though, it has been suggested that warmer temperatures could increase the availability of food, an event that would compensate for the higher metabolic rates these animals would acquire, since they are ectotherms (this effect is mentioned again later). Additionally, though commonly found as extremely large schools in the water column, these arthropods can move to deeper and colder waters, and this could perhaps become a more generalized behavioral adaptation to the ever-hotter climate, a behavior that would also restrict their consumption by more surface-dwelling animals, thus potentially amplifying effects along the food chain.
As part of the larger biosphere, we too are, of course, affected by the climate changes of our own making, effects that have been widely publicized, such as more extreme weather events in certain areas, as well as increased drying or wetting in others. How these modifications impact us varies. Some impacts are quite direct, such as the health implications of hotter temperatures, which can prove lethal in many cases. Other impacts are less so, being often tied to climatic effects on crops and other food sources. Certainly, though, some of the greatest impacts will be those regarding sea level rise, fueled not only by the melting of ice caps but also by the thermal expansion of the oceans directly due to the increased temperatures. Even under a scenario of deep and rapid cuts to greenhouse emissions, which is probably too optimistic, there would still be grave dangers by 2100.
In Europe, for instance, a considerable part of the Netherlands would be overtaken by seawater, including cities like Amsterdam and Rotterdam. Indeed, as the name already suggests, a considerable portion of this country sits below sea level, and the Dutch have a long history of fighting against the sea, with several engineering feats that have actually managed to reclaim land and that began around one thousand years ago (another massive show of human ingenuity and versatility). This history continues to this day, and this nation is already preparing to face the new challenges through the Delta programme, a national effort to further safeguard the country against the new oceanic threats that lie ahead. It is to include the reinforcement of dikes as well as the construction of new seawalls, apart from other barriers, like sand dunes. Even living protections are being taken into consideration, for coastal vegetation placed in front of flood defenses can help safeguard against the sea, especially by reducing erosion.
Another part of Europe that would face existential challenges would be the Italian city of Venice and the surrounding areas, which, like the Netherlands, already sits at a disadvantaged position, being in the middle of a lagoon, near swamps and marshes as well as to the Adriatic Sea. Like the Netherlands too, the city has not only remained above the water, but has also flourished during its long history, in great part thanks to many engineering feats. These continue to the present day in the form of the MOSE project, which started in the 1980s and consists on 78 gates that stay submerged and only appear during moments of high tides, blocking the three connections the lagoon has to the Adriatic without occupying any land space. Though these gates have been roughly effective in protecting the lagoon, some flooding has yet occurred, and as sea levels rise even more and weather events become more extreme, they will be put to an even greater test, especially because many land areas around the lagoon may flood, which would expose it to seawater from several more sides, making its containment quite a bit more difficult.
Asia, though, especially its eastern and southeastern portions, is the area of the world that will be most affected by rising sea levels, a situation that is made much more serious due to the great population centers that exist there. China, for instance, has several river deltas with a high risk of flooding, including virtually the whole Shanghai municipality, which, harboring the largest Chinese city, is home to about 25 million people, situated on the Yangtze River delta. On the northern wing of the same delta, the city of Yancheng, home to almost 7 million people, is also at risk of being inundated by the advancing sea levels, along with large stretches of neighboring lands that comprise the Yancheng Coastal Wetlands reserve. The problems are far from ending there. The more northern city of Tianjin, home to almost 14 million people, and situated fairly close to the Chinese capital of Beijing, is also going to be subject to almost total flooding, even in this more optimistic scenario being analyzed.
As such, the situation for the People’s Republic of China is indeed extremely dire, for it is in real danger of losing major population as well as financial hubs. As such, protective measures are already being undertaken, such as the construction of floodwalls and other defensive structures. But other initiatives, such as the Sponge City one, are concerned not necessarily with sea level rise, but with floods that occur as a result of heavy rains and other such events, which, as mentioned previously, might become more common with global warming as a whole under specific contexts. Basically, such an initiative seeks to change the urban landscape, which, normally impermeable, is more conducive to rapid and intense floods. Consequently, it aims to integrate permeable pavement with more natural areas, like artificial ponds, wetlands, and rain gardens, which, historically ignored, truly act like sponges, soaking up excess water. Such excess water is stored in specific areas underground and is later released when conditions stabilize. While this approach is not immediately adapted to marine advance, it shares elements of the defensive approaches employed by the Dutch, which also take into consideration the importance of natural environments to restrict the advance of the sea.
All around the world, there are plenty of other examples, such as the capital of Thailand, Bangkok, which also faces existential threats, to the Bengal Basin, which, concentrating the massive populations of both Bangladesh as well as Western Bengal, is truly an area that, if not addressed properly, carries the potential to trigger one of the largest humanitarian catastrophes in history. These grim scenarios are not restricted to Asia and Europe, though. Many island nations in the Pacific, especially due to being islands fairly small in size, also face similar existential issues, as, even despite the fact that their countries might not be completely submerged, their populations are heavily concentrated along the coast. The United States itself is to face several problems regarding marine transgressions, especially on its southeastern coast, which is full of swamps and low-lying areas, making places such as Louisiana and the southern tip of Florida particularly at high risk. Overall, it is clear that, in the following decades, countries all around the world will have to potentially reinvent themselves as a result of the occurring and coming changes and it is likely we will see a new age of truly massive and unprecedented infrastructure projects to keep the sea at bay, the success of which will once again show humanity’s great resilience, but it is probable many will fail or not come soon enough, possibly triggering great catastrophes in which maybe millions will lose their homes and become displaced.
As a result of these expected pressures, there has been considerable global momentum for the decrease in the use of fossil fuels and ever greater increases regarding renewable energy sources, which not only help prevent further damages in the present, but can also prove pivotal for future populations here on Earth, for, despite this modern-day climate change, glacial periods will eventually resume (though the next one will be delayed by the current bout of warming) and those who will be here to see it should have the fossil fuels to potentially prevent the advance of great ice caps and the significant disruptions that would emerge not only from this, but also from the receding sea levels. Currently, we are in a time of great transformation for humanity and for the Earthly environments, and a key opportunity to reconcile both like never before is appearing and apparently being exploited, despite some setbacks here and there. Even cities, which are traditional holdouts for the three key animal commensals mentioned before, have increasingly been adorned by green spaces filled by native species, which not only help with factors such as water drainage (as previously mentioned) and heat regulation (artificial building materials, such as asphalt and glass, can concentrate a lot of heat in urban environments), but also serve as key environmental holdouts, further strengthening biodiversity and the ability for man-made and natural habitats to harmoniously as well as synergistically coexist.
While the role of fossil fuels has been widely mentioned as a key driver of the current aggravation of the greenhouse effect, about one-fourth of such gases, like methane, come from the agricultural sector, especially the production of livestock, as ruminants, due to the anaerobic metabolic processes that occur within their gut, are key releasers of the mentioned gas. Modern agriculture, despite all the great advancements it has been subject to, is still a great driver of deforestation and other forms of environmental degradation in many areas around the globe. Fortunately, there are new farming techniques being developed, which not only take up much less space but also inhibit the release of even more of such thermally active gases. For plants, vertical farms have been appointed as a promising opportunity, and they could even be developed within cities. For livestock, cell cultures grown in laboratory have also been cited as a new source of meat, something that could prove itself even more interesting resulting from the possibility of restraining diseases associated with these animals, such as avian flu, caused by influenza viruses (as mentioned here), and mad cow disease, a lethal pathology caused by prions (mentioned here and here). Despite these clear benefits, these novel methods are still not very widespread, being, additionally, too concentrated on very developed nations, while some of the greatest dangers to natural environments come from less developed countries (such as the ongoing process of Amazon deforestation, in great part associated with the Brazilian agricultural sector).
Even in the first, disastrous scenario imagined here, it is probable that, ultimately, our reach of the final frontier would only be delayed, that final frontier being, of course, space. While during the Cold War and the space race inside it, the exploration of the cosmos was a mainly governmental endeavor, the private sector has become increasingly active in regard to space-faring. Indeed, we are currently seeing the first space tourists and, over time, these, even if brief, voyages beyond Earth will become ever more significant. However, where the potential truly lies is not in simple space tourism, but in activities that might guarantee for ourselves a future that is not intrinsically dependent on that of our home planet. Perhaps one of the main initiators for this new era of human existence will be economic opportunity, such as the mining of asteroids and other astronomical bodies, like our own Moon, which contains an isotope of helium (as a result of its constant bombardment by the solar wind) that is part of the fuel for nuclear fusion reactions, processes that might become a significant source of clean energy in the future.
While not a reality currently, the means for doing so will certainly become available over the coming decades, and the incentive is already there. Some astronomical bodies such as asteroids are incredibly rich in metals and other materials, to the point that many of them (like the large asteroid Psyche 16, which is to be visited by a NASA spacecraft and possibly has a value worth in metals like iron and nickel bigger than the entire world economy) might very well cause a collapse in the price of these materials here on Earth, due to their sheer abundance. However, these materials would not be lucrative if they could not be returned, a massive logistical endeavor that would probably render the return of many to Earth not advantageous. Despite this, platinum-group metals, which are incredibly expensive and have a wide variety of applications, have been considered as some of the compounds that would justify the whole process in the first place, especially because it is expected that the concentration of these precious metals would be significantly higher in such astronomical bodies than in Earthly ores. Either way, one big benefit of asteroid as well as lunar mining that might boost it further is the complete lack of environmental degradation it entails, a feature that could lead it to become a significant form of resource extraction even under higher operating costs, which is especially poignant considering the current and plausibly future “zeitgeist” concerning these issues.
However, what might drive space mining to become a particularly solid business is the construction of ever more space habitats. In our current time, such structures have already been built, but the development of extensive mining operations in space could allow for the necessary material abundance and ease of transport (made easier due to the much lower gravity of asteroids and natural satellites such as the Moon) for truly enormous space habitats to be built, ones mimicking Earthly environments and with artificial gravity, seeking to emulate the authentic Terran existence. Once again, private enterprise might be key in developing these vast space settlements, and already in the modern world, the billionaire Jeff Bezos has explicitly vested his interest in this proposal. These truly space colonies would provide many great advantages. Not only would they resemble what we find here on Earth, but they could sustain much greater numbers of people and industry, considering the unimaginably vast resources of the Solar System and the heavy implementation of previously mentioned technologies like vertical farming and meat cultures. Over time, the space population could grow to dwarf Earth, but that is, of course, only its potential. As has been shown on our planet, population growth tends to decrease in more developed nations, and, seeing as these would be highly developed structures (as can be expected from the sheer tremendous amount of effort it would take to build them), they might be quite empty if modern patterns remain.
The greatest advantages of all, though, would not be this potential for unchecked and virtually unlimited growth, not at all. The greatest advantages would be the sheer level of control we would have over these habitats. With completely artificial climates and environments, no natural disasters would occur, and zoonotic diseases, which constitute the most destructive to have ever afflicted humankind, could be greatly hampered through the realization of extremely tightly controlled introductions of fauna, which, either way, would not come into contact with great wild expanses potentially serving as reservoirs for deadly pathogens. Additionally, as mentioned earlier, making humankind much more diffuse rather than concentrated on a single planet would be extremely beneficial for our continued survival. While now a sufficiently large asteroid strike or extremely intense volcanism may cause a mass extinction event that might carry us along with it, these would no longer be a problem in space. Even if a space habitat suffers a catastrophic occurrence, many more would still exist. It is truly taking a cosmical approach to the old saying of “do not put all your eggs in one basket”.
But how would such things even exist in the first place? Fortunately, there are already many proposals regarding how these could be shaped and protected from significant cosmic threats, such as impacts, radiation exposure, the solar wind, and temperature extremes. Certainly, the most iconic of such proposals is the O’Neill cylinder, proposed by the physicist of the same surname. His proposal consisted of an enormous rotating cylinder (its rotation leads to the creation of artificial gravity), up to several kilometers long and also a few kilometers in diameter. It would be composed of aluminum plating and extremely large glass panels, allowing a view of the outside and the incidence of natural sunlight. Since such an initial idea in the 1970s, addenda have been made to it in order to improve safety standards and safeguard the structure against the cited threats. Such changes include the removal of the enormous glass windows and a double reinforcement of the structure with aluminum panels, shielding it from micrometeorites and radiation. Despite this, there would still be natural sunlight, which would arrive through a series of reflective structures.
Diving into more detail, between the inner aluminum hull, which would be in contact with the artificial environment housing humans and other organisms, and the outer aluminum hall, a cavity would exist, one in which air would circulate, helping regulate temperature, and also from where condensed water would be expelled, being thus essential both for the artificial atmosphere and for the artificial water cycle. Regarding the outer hull, it would be adorned internally by layer of foam glass, which would help maintain the internal temperature of the cylinder due to its low thermal conductivity and also help cushion impacts. Externally, a structure covered by lunar soil and adorned with solar panels would not only offer further radiation as well as solar flare protection, but would also serve as a fundamental source of energy generation. According to this idea, the smaller cylinders would have a length of 2.3 kilometers and a diameter of 900 meters, but, even farther in the future, these could acquire sizes of up to 8 kilometers and diameters of 3.2 kilometers, capable of housing up to 250 thousand individuals!
While these structures would be free-floating, others could exist in the hollowed-out interiors of mined asteroids, where the remaining material would offer great amounts of protection against both radiation and impacts from other astronomical bodies. These could be especially local, being built from materials extracted from the host asteroid itself and assuming a torus shape, a geometric arrangement that could be described as roughly resembling a donut. Like for the cylinders, natural sunlight would be beamed through via the use of several mirrors while artificial gravity would be achieved through the habitat’s rotation. The asteroid surface itself could contain several solar panels, fueling the structure inside.
Apart from these various advantages, space habitats would also be ethically superior. With them, we would not be planetary invaders, instead observing everything from afar and minimally intervening. There is a great reason for this: as will be cited later, a few astronomical bodies in our own Solar System might be home to alien lifeforms, including Mars, the planet that has been considered a constant target for colonization. Not dwelling on it would not only protect us from any potentially harmful organisms, but it would also protect potential alien organisms from potentially harmful ones we could introduce. While it is possible that alien life could hardly interact in any meaningful ways with us and fellow Earthlings as a result of vastly different evolutionary trajectories, it is also possible they could, especially considering a shared general chemistry, which is likely to be the case, seeing as many of life’s building blocks (as cited here) are already present in space. As a result, there might be a particularly dangerous middle line: one in which we are close enough to interact, but the existing interactions are extremely destructive due to evolutionary incompatibilities.
Here on our planet, we already have terrifying examples of this fact. Generally, parasites, over the course of their evolution, tend to suffer a process of attenuation, one in which they are able to proliferate and continue their life cycles while maintaining the overall health of the host, especially because, if the host dies too fast, they may become unable to continue living themselves. However, in certain circumstances, especially when organisms are not commonly parasites or not commonly parasites of a specific host, the interactions may be particularly destructive. To give “face” to one of such examples, we have the amoeba-like protozoan Naegleria fowleri. This microorganism is a predator that lives in warm freshwater habitats all over the world. Its surface is adorned by sucker-like structures that actually serve as dynamic mouths, enabling it to nibble on other microbes.
While such a lifeform would never normally have even the chance to be a parasite, it can be in a very particular scenario: when a human deeply flushes its nose with water, like during a dive into a warm body of water. After being introduced into the nasal cavity, this protozoan steadily moves up in the direction of the olfactory nerve, potentially lured by neurotransmitters, which cross-react with some of its receptors that never evolved to sense neurotransmitters in the first place. From the olfactory nerve, it eventually reaches the brain, where it proceeds to feed on human cells and provoke an extremely intense inflammatory response, one that, in almost all cases, proves fatal. This whole process is a complete disaster, a complete accident, bringing two species that otherwise would never meaningfully interact, two species that underwent completely different evolutionary processes and that, tragically, interact in the most disastrous way possible. Examples such as this one raise the possibility that a potentially similar scenario could play out with alien organisms, both to our chagrin and theirs as well.
Apart from this quite reasonable source of worry, the colonization of planets and moons with a more definite intention also proves quite problematic due to the following: gravity. While in space habitats, the gravity can be essentially customized, the same cannot be said for astronomical bodies, whose masses we can barely change, certainly not on the scale to change the gravity that occurs at their surfaces. Lower gravity than Earth, as is the case on the Moon and Mars, can have several pathological effects, such as weakening of the musculoskeletal system and the yet not very well understood spaceflight-associated neuro-ocular syndrome, which causes swelling in the optic nerve and the retina and is likely associated with several factors related both to individual predispositions as well as the microgravity habitat. While currently the stays in lower gravity are only for a few months, these changes could turn a lot more definitive and serious in people who are subject to such environments for their whole life. Some of these changes could be offset by the installation of centrifuges that mimic Earth's gravity and by the installation of metal coverings in space suits specifically to increase their weight.
Even if all these issues eventually get resolved, the fact is that the Martian environment, for instance, considered the prime planet for settlement, would not be by any stretch habitable, being extremely hostile and dangerous for humans and many other Earthly organisms. Consequently, colonies would also be closed habitats, with the difference being located on solid ground instead of free-floating in space. However, there is the possibility, probably confined to the far future, that the red planet would be subject to terraforming, a massive planetary-wide process with the aim of turning it into a truly habitable world, one in which people would be able to freely traverse its surface, ideally completely transformed, full with vegetation, permanent bodies of water, and prowling creatures.
While such a colossal undertaking would maybe stretch the boundaries even of an extremely advanced, spacefaring civilization, it could theoretically be done. Certainly, the first thing that would need to be changed on the red planet would be the addition of an atmosphere. With an atmosphere, it could warm up, and liquid water would be able to stay liquid on the Martian surface, as the current Martian atmosphere is too thin for it to be that way. A possible alternative for rebuilding the Martian atmosphere would be quite a wild one: the collision of asteroids rich in light-weight elements. These could be captured from the outer fringes of the Solar System and then transported all the way back. Such transport would be assisted by the Sun’s gravity and would need to be carefully done, especially because an impact with too high velocity could heat the astronomical body’s atmosphere too much, causing even more of it to be lost to space. For such an endeavor, it is possible the planet would need to be evacuated beforehand, especially because of the cataclysmic proportions asteroid collisions can assume, leading to earthquakes and volcanic events.
With humanity’s ever greater advances, it might come a time when we leave the Solar System and start venturing out into planetary systems beyond. By then, it is hard to tell what things will be like, how cultures will have developed, if populations will be much higher, taking advantage of the vastness of space, or if they will be smaller, though spread out. It is even hard to say what will be of humanity here on our own home planet. With time, it is probable that space will take the lead as the main pole of human ingenuity and progress, while Earth might be considered way more bucolic by comparison, a place where things still happen, but where the focus may have shifted: instead of continued expansion, the main point could become about coexistence with nature and its preservation, especially because, after all, the space for wilder things and great technological projects would frankly just be space. Of course, this is just a thought, and, in the end, predicting humanity is likely much harder than predicting anything else due to our sheer ability to change and not only change, but think about our changes and how and why they are occurring. As Earth continues its incessant shift in climates and continents, maybe existence here will become harder, and only fewer and fewer people will choose to do so. In the end, our cradle might become marginalized, more of a symbol than anything else, a place looked upon with admiration and veneration, but one which progress and time will have left behind.
In space, actually, the incredible behavioral repertoire of humans might actually achieve its greatest potential and fullest expansion, especially in space habitats. With customizability only limited by our biological constraints, which themselves could be modified through genetic engineering (also ever more developed), humanity could become so much more than what it is today, perhaps in extremely disturbing and alien ways, but in diverse ways most of all. It is hard to imagine what people unbound by planetary and biological chains could achieve, but it could be much more than what is currently imaginable. Maybe some would seek ever greater integration with AI and machines, becoming authentic cyborgs. Maybe others would try and completely abandon gravitational constraints, choosing to live in and adapting their bodies to microgravity environments, perhaps becoming lanky beings that constantly float around in space habitats. And some would likely stay just as we are today, perhaps with some changes here and there, but roughly indistinguishable.
While this might occur with spacefarers, planet dwellers, such as the Martians, would be significantly more limited in regard to such “creative expression”. Such populations might develop a sort of frontier society mentality, akin to those that developed in the United States and in the Japanese island of Hokkaido. Such a mentality, based on individual pioneerism and the colonization of new and harsh environments, could probably apply to such planetary dwellers, going out of their way to build a life in situations far harder than those that would be faced by those who choose to settle in space. However, once again, this is a generalization, and what does not work for us humans is generalization. As such, this is just an overall idea and far from a hard-established fact.
Either way, as humanity disseminates more and more through the cosmos, we will become as fragmented as we once were and as distant as we were before the appearance of light-speed communication. This will be, of course, a result of the limits imposed by the speed of light. The closest planetary system to ours (which will be explained in far more detail later) is about 4 light-years away, meaning it would take eight years in total for a message to be sent and a response to be received. This is only regarding communication: transport will be much harder. Given enough time, it is likely that humanity will form a fragmented cosmic empire, one composed perhaps by several different species of human, reproductively cut off from each other and maybe only communicating sparsely, with messages taking years, decades, and even centuries. It is strange to think that now we are more connected than ever and that our brightest future means a breakdown of this and a return to older conditions, when messages were not instant and communication was a very much longer and arduous process.
As we spread far and wide, it is especially important to consider where we might find life. In our own Solar System, just as cited before, there might be other astronomical bodies harboring life besides Earth. One of these is the iconic red planet of Mars, as also just cited before. As explained in the first entry, Mars was much more Earth-like in the past, with a reducing atmosphere similar to that of primitive Earth and a far warmer climate, enough to harbor large bodies of liquid water. Indeed, indications of prehistoric Martian life have been found. Certainly, the most poignant of these has been the discovery of a mineral formation in a crater. While there are abiotic explanations for such a geological finding, the biotic explanations are, excitingly, more straightforward and in greater accordance with the environment in which the structure would have been formed (a sedimentary context with available water under an Earth-like, habitable climate), apart from the fact that it shares similarities with equivalent formations dated from Earth before the Phanerozoic, hinting at the fact they would have been formed by lifeforms with equivalent metabolisms to some regard. This, especially after the first entry, may not come as such a great a surprise, since life on our world appeared shortly after its formation, raising the chances that it could also arise fairly easily in other celestial bodies with similar conditions, as Mars truly did have.
While the existence of prehistoric Martian life is quite plausible, whether any organisms managed to survive the drastic changes the red planet underwent is more uncertain, though certainly possible. Mars as a whole is a very inhospitable planet, with a very thin atmosphere that makes it cold and subject to great temperature fluctuations. Despite the thin atmosphere, which, without an ozone layer, allows plenty of UV radiation to reach the surface, its surface is often subject to dust storms, some of which may be powerful enough to cover the whole planet. Apart from this, it is also very dry, home to desolate but still varied landscapes, some of which are clear evidence of its past, such as ancient river channels and lakebeds. Even so, liquid water ephemerally occurs at the surface, being very salty and unable to stay in the liquid phase for long as a result of the low air pressure, leading it to quickly evaporate. More substantial amounts of water are found at Mars' poles, which have ice caps that, during colder times of year, are superficially covered by carbon dioxide in its solid phase, popularly known as dry ice, and which turns directly into gas once the weather becomes a bit warmer once again. Though these might not resemble habitats particularly conducive to a biosphere, there are areas where life could have hung on despite the tremendous changes around it.
These areas would be below the surface, some deeper, and others way more superficial. The most superficial ones are likely in mid-latitudes, not as far as the poles, but not close to the Martian equator either. In these locations, there may be snow and subsequent melting. But, while exposed water would evaporate as previously mentioned, a film of snow and dust above it could offer some stabilization and also filter excessive UV radiation while allowing some sunlight to pass through (additionally, the dust, becoming more concentrated as the ice sublimates, could, due to its lower reflectivity, increase the absorption of sunlight, helping in melting). In these microhabitats, Martian photosynthesizers could have persisted, perhaps being the base of miniature food webs, all contained within these compact environments, similar forms of which have been observed here on Earth in glaciers and ice caps, for instance, and which sometimes contain great microbial variety, with cyanobacteria usually being predominant. During colder times of year, these pockets would freeze, and their microorganisms, probably with anti-freeze mechanisms in order to have survived thus far, would enter into dormant states until liquid water returned (what also happens here on Earth in these ephemeral microhabitats).
Additionally, findings of seasonal methane emissions on Mars also raise the possibility of more diverse biotas. While methane can be produced by purely geological processes, the involvement of biological ones, so abundant here on Earth, cannot be ruled out. In this context, the most expressive methane-producers of our planet are the methanogens (previously mentioned here), anaerobic archaea that obligately release methane as part of their metabolism, being widespread in the Earthly environments and even present in the microbiota of animals (including in ours). Some methanogens can extract energy from simple compounds, using hydrogen as an electron donor and carbon dioxide as an acceptor for example, with these two substances being abundant on the red planet (carbon dioxide is the predominant gas in the Martian atmosphere and hydrogen can be produced by a wide variety of different processes, including the breakdown of water by radioactive compounds below the surface). Due to these properties, it is believed that methanogen-like microbes may be the source of these gases seeping from the Martian underground.
These organisms, if they truly exist, might be found in association with many other microbes in deeper subsurface environments, where photoautotrophs do not predominate. These habitats would probably be hypersaline hydrous bodies close to areas with high concentrations of radioactive materials, with the high solute concentrations impeding water from freezing even at temperatures below zero, and the many radioactive substances increasing the breakdown of water, thus offering hydrogen to the tentative methanogen-like beings. Apart from this, the deeper subsurface locations would be more insulated against the cold Martian surface and closer to its hotter mantle, potentially opening niches even for thermophilic lifeforms. However, these environments would not be devoid of threats: while the radioactive materials could be essential in securing a hydrogen source, their breakdown of water also leads to potentially very disruptive reactive oxygen species, which would be possibly countered by mechanisms to repair genetic material or adequately scavenge them, and, besides this, perchlorate salts, which are highly oxidant, have been found in Mars and could also be highly disruptive if present in high enough quantities. On a brighter note, though, the red planet is rich in sulfur, and many compounds based on this element are used by chemolithoautotrophs not only to generate energy but also to fix carbon, highlighting the possibilities Mars has of harboring even more expressive remnant microbial communities.
Moving farther away in the Solar System, we arrive at the largest planet: the gas giant Jupiter. With a beautiful and intricate marbled surface containing a mosaic of colors, this, like other gas giants, is extremely peculiar when compared to rocky worlds such as our own and Mars. Lacking a solid surface, it has kilometers and kilometers of cloud after cloud, with its composition being primarily of hydrogen and helium, the same gases that also make up most of the Sun. Apart from these gases, it counts with many others, such as sulfurous and phosphorous ones, which are in part responsible for the striking colors of this ginormous and striped world. Such clouds follow specific distributions: the uppermost ones are composed of ammonia crystals, the middle ones are composed of ammonium hydrosulfide crystals, and the lowermost are composed of ice crystals as well as of water vapor. Apart from this vertical layering, they are also segregated horizontally due to Jupiter's fast rotation, which is only 9.9 hours and creates its characteristic stripes. Despite these atmospheric dispositions, some meteorological events are truly transgressive: massive storms, such as the planet's Great Red Spot, visible from space. This red spot is an enormous anticyclone (twice the size of Earth), formed due to cold air spiraling outwards, and has been ongoing for hundreds of years, being also very large in vertical terms, extending several kilometers beyond the top ammonia clouds. Due to the lack of a solid surface, winds do not lose speed to attrition with land, thus achieving incredible speeds and proportions exceeding Earth's biggest storms by an incomprehensibly wide margin (as can already be concluded by the example of the Great Red Spot), which makes Jupiter's atmosphere dynamic but quite violent.
At much deeper portions of Jupiter, pressures are so great that hydrogen turns into liquid, forming a massive ocean that surrounds its core, which, though composed of heavier materials like metal, does not have a sharp, well-defined boundary, instead intermingling with the hydrogen ocean around it. Not only is the hydrogen liquid, but it also shares electrons with fellow hydrogen atoms as a result of the tremendous forces, which approximate them enough to allow for this sharing. As a result, the hydrogen ocean is electrically conductive, having, like metals, an electron sea between atoms. This, coupled to the gas giant's fast rotations, leads it to harbor an also massive magnetic field, extending all the way to Saturn's orbit! Not only does the great strength of this field lead to significant heating in the upper atmosphere radiating from the planet's poles (caused by the particles that concentrate in such regions, also the ones responsible for auroras, as better explained in this chapter), it also traps a massive amount of charged particles around it and accelerates them to great speeds. As a result, Jupiter's surroundings are extremely dangerous, flooded by a flurry of potentialized stellar wind particles, which bombard its moons and any orbiting spacecraft with intense radiation.
With a total of 95 moons, Jupiter has plenty of natural satellites, and four of these are particularly notable. They are the four Galilean moons: Io, Europa, Ganymede, and Callisto. Io, the one orbiting closest to the gas giant out of the four, is actually the most volcanically active astronomical body in the entire Solar System, with a constantly renewing surface dotted by various volcanoes, some spewing lava kilometers high and others forming vast lava lakes, which fill in craters or simply spread across smoother areas. This hellish landscape is a result of extreme tidal forces not only from its host planet but also from its fellow moons. These are so extreme that the moon itself bulges in and out by 100 meters, much more extreme than Earth's advancing and receding watery tides. The lava itself has an unclear composition, but it probably contains a mix of sulfur and silicate compounds, the former of which explains the varied coloring of the moon, marked by pastel hues of green, orange, and brown, for instance.
Moving on to Europa, we justify our visit to Jupiter, as this moon has a plausible chance of harboring life. Instead of Io, Europa's surface is an icy one, crisscrossed by several lines and splotches of reddish material that, though also somewhat unknown in its composition, probably represents a mixture of sulfur compounds, salts, and the ice underneath, further modified by the great exposure to radiation. What is most exciting about this natural satellite though is not its surface, but what likely lies underneath, protected from the harsh external environment promoted by Jupiter's very strong magnetic field: a salty ocean, 60 to 150 kilometers deep (Earth's oceans, by comparison, are just 3,6 kilometers deep in average, with the deepest portion close to 11 kilometers in depth), despite the fact this body is slightly smaller than our planet's moon. It remains in liquid form as a result of the previously cited tidal heating (aggravated by the fact that Europa has a more elliptical orbit, causing it to stretch and contract more significantly while it moves around Jupiter), in contact with a rocky crust that overlies a mantle and a deeper iron core, a structure roughly equivalent to that of Earth, apart from the obvious difference of a completely flooded surface under thick layers of liquid water and ice.
This contact between the probable subsurface ocean and the solid interior is fundamental, as it allows for geological processes such as the ones that originated hydrothermal vents here on Earth, areas where life could have plausibly begun, as discussed in much greater detail here. Indeed, there is evidence for this sort of geothermal activity on the icy moon. For instance, there has been the detection of thin water plumes ejected from the frozen surface up to several kilometers high, and, additionally, the surface of Europa is young in geological terms, indicating it is being recycled and, thus, that the object is active to some degree. This recycling of the icy crust is likely associated with the tidal processes and probably occurs due to the aforementioned stretches and contractions the moon is subject to. These probably crack the crust and, apart from making the many fissures observable from space, may drive fractured portions below, exposing pieces of the subsurface ocean that then proceed to freeze over, replenishing the fracture with new ice. This fracturing process is not only mechanical and likely also occurs due to increased heating of the subsurface ocean itself, which then proceeds to melt the frozen layer and decrease its thickness, further helping in eventual fractures.
These occasions might introduce several oxidants that build up on the icy surface into the ocean, and such introduction, associated with periods of increased eccentricity, would be coupled with simultaneous greater hydrothermal events on the ocean floor, since tidal heating affects the moon as a whole, likely increasing the frequency and intensity of these other phenomena as well. This flux of oxidants could perhaps be a basis for occasional blooms of Europan life dependent on them for essential chemical reactions and during these instances, these speculative organisms could perhaps proliferate to significant degrees only to enter less active or even dormant states until they repeat. For most of the time, though, the great majority of possible organisms in this natural satellite would probably lead their existences around hydrothermal vents, potentially in ecosystems resembling the abyssal ones of Earth. If more complex forms of life could develop within such environments remains an open question (so does if there even is life on Europa in the first place), seeing as, here in our planet, vents often harbor animals, sometimes in great numbers, but these colonized them from other habitats, immersed in the context of a much larger world with a much greater variety of habitats and energy sources.
Europa is likely not the only moon of Jupiter to have a subsurface ocean. Ganymede is the third of the Galilean moons in distance from its host planet, being the largest moon in the Solar System and actually larger than the planet Mercury itself! Like Europa, it has a structure roughly equivalent to that of Earth's, with an iron core, mantle, and layers of saltwater and ice above. However, unlike Europa, Ganymede, due to its much larger size, has higher gravity, and this has deep implications for its astrobiological potential, for it, apart from higher gravity, also has a thicker covering of water (both in its liquid and solid phases), as much as 800 km in depth. These two factors contribute to the formation of exotic forms of ice around the rocky crust of the moon, which arise not due to low temperatures, but due to the great pressures found at such depths. There are different possible arrangements for these exotic ices.
While the disposition of the upper layers is more generally accepted (an icy crust, in contact with space, covering a salty ocean below), the same cannot be said for the lower layers, which may be either layer upon layer of exotic ice (thus causing the salty ocean to a be thin sliver of liquid water compressed on both sides by icy crusts) or an alternation between layers of ice and liquid water very rich in salts, which, under pressure, might be even denser than some forms of exotic ice. Either way, the presence of these solid phases of water below the main subsurface ocean compromises chances for the development of abyssal life as may have occurred in Europa, since they impede the intimate contact necessary, for instance, for the origin of hydrothermal vents or for the exchange of nutrients between Ganymede's interior and its bodies of liquid water. The last Galilean moon, Callisto, though smaller than Ganymede, may follow a similar pattern of a subsurface ocean isolated from the rocky crust by a layer of exotic ice. However, in Callisto's case, even the existence of an ocean is far more contentious, and it does not suffer from such a great amount of tidal heating as the other moons, which are engaged in an orbital resonance (meaning they have synchronized orbits) that further exaggerates the tidal processes they are subject to.
As we move even further from the Sun, we arrive at another gas giant: Saturn. With its very noticeable rings, composed of debris as small as dust particles to as large as mountains, it has many similarities to Jupiter, also having a short rotation period of only 10.7 hours and a characteristic striped pattern, though way more subdued in color, with a more uniform coating of yellowish and grayish tones. The atmosphere is also quite similar and, like Jupiter, its deeper portions have an ocean made of electrically conductive liquid hydrogen, which, unlike Jupiter's, though, is much smaller in scale, thus leading to an also smaller magnetic field. Another similarity, however, is that this gas giant too has a very large number of moons, an even higher number than its inner cousin. Once again, it is due to these moons that we have been brought to this massive, gas planet. Perhaps the most promising of these in regard to the possibilities for life is the small natural satellite of Enceladus, an extremely reflective icy world with yet another subsurface ocean (as a matter of fact, the most reflective astronomical body in the Solar System).
Similar to what occurs with the first three Galilean moons, Enceladus is also engaged in an orbital resonance, which promotes eccentricity of its orbit and, like occurs with Europa, causes it to stretch and contract as it gets closer or farther to Saturn. This tidal heating is likely what maintains Enceladus' ocean, also likely driving hydrothermal vents that have plausibly proven to be essential for the origin of life. Excitingly, this tiny astronomical body offers even more signs of astrobiological potential than Europa, which has been assessed by significant plumes originating from beyond its frozen crust. These plumes do not contain only water, but several other compounds, including organic ones at significant densities. Not only this, but traces of silica in the ejected material also point to contact between the ocean and the rocky crust below, strengthening even more the possibilities of hydrothermal vents. Additionally, signs point not to any hydrothermal vents, but perhaps even those of the cooler variety, formed by serpentinization processes, and that may have been, in particular, the original sites of Earthly life. Oxidants, as occur in Europa, are also present in Enceladus' ocean, and these might drive an even greater variety of chemical reactions. The source of these substances is not very well-known, though, and, while they might originate from the surface (this is likely not as a significant source as it is in Europa due to less radiation exposure), they might also be formed from the radioactive breakdown of water promoted by radioactive elements present at the moon's core. Overall, this roughly unassuming astronomical body has some of the strongest indications for an extraterrestrial biosphere outside the Earth and, if it exists, such biosphere would be potentially quite similar to the putative Europan one, abyssal in nature and possibly more limited due to the moon's smaller size, but, even so, a biosphere either way at the end of the day.
Apart from Enceladus, Saturn also has another interesting moon, not only from an astrobiological perspective. Such a moon is Titan, Saturn's largest and the second largest in the Solar System, just after Ganymede (still bigger than Mercury, however). Just from the outside, we can already see this natural satellite is highly distinctive, for its surface is obscured by a hazy, light brown layer: the manifestation of a dense atmosphere, denser than Earth's even! Like Earth, however, its atmosphere is mostly nitrogen, and this gas assumes an even higher percentage. However, apart from this inert substance, it also has a very substantial amount of methane in its composition and this helps explain the golden haze visible from space, which is produced by chemical reactions, powered by UV radiation and particles from the stellar wind accelerated by Saturn's magnetic field, involving nitrogen and methane, which lead to the formation of more complex hydrocarbons that may remain suspended in the atmosphere (forming the haze) or precipitate to form dunes on the surface of the moon (in the past, during periods of greater methane synthesis by methanogens, Earth likely underwent haze episodes similar to this, as explained here).
Occasionally, methane also suffers precipitation, condensing and forming clouds that then feed lakes, rivers, and even seas of more methane along with ethane, liquid bodies that sculpt the surface and play a part in the moon's not water, but hydrocarbon cycle. This is made possible by the extremely low temperatures of the astronomical body, allowing materials that we normally only think of as gases to play other roles. This extremely unusual arrangement for a moon undoubtedly raises the question: how did this atmosphere develop on Titan of all places? The most well-supported suggestion is that the moon's gas envelope has emerged from the heating of great amounts of organic/nitrogen-containing material trapped deep in its interior during the process of its formation, all the way back during the early Solar System (as explained here, organic compounds are easily found in the cosmos). As these deposits warm up, they release volatiles that eventually end up on Titan's surface, constituting a process of outgassing that eventually builds up the currently observed atmosphere, also helping replenish the methane, which, in geological terms, gets quickly broken down by radiation as well as stellar wind exposure, as just described.
This strange environment, almost like a parody of that of our own planet, sits on top of an icy crust and, below it, once again there may be a subsurface ocean, possibly containing salts as well as ammonia (the latter would also be derived from the vast organic/nitrogen deposits just mentioned). While the combined thickness of these water layers (both those in solid and liquid phases) is expected to be quite significant, at 470 kilometers, the presence of exotic ice around the moon's rocky interior is less certain than in Ganymede for instance and such an icy layer may be entirely absent if Titan's superficial frozen crust is thin and its ocean not too salty. This, of course, raises its potential for habitability, incredibly dependent on the contact between water and deeper portions of the natural satellite, as previously mentioned and explained. Once again, if life has developed on the deepest portions of Titan's ocean, it would bear similarity to the putative lifeforms of both Europa and Enceladus, all occupying reasonably similar, completely abyssal environments. However, in Titan's case, there would be some peculiar conditions. First, any possible organisms would be subject to much higher pressures as a result of the moon's thicker covering of water, as well as higher gravity, and second, the potential presence of ammonia in the also potential body of water would perhaps also lead to some physiological changes, especially as it may increase the alkalinity of the liquid medium.
Curiously, though, Titan's surface might also harbor another, much more exotic biosphere, one which uses not water as a solvent, but rather methane along with ethane. This already is a very great distinction, since water is a polar molecule and these hydrocarbons are apolar, meaning that the cell membranes of any lifeforms living in this environment would count with a radically different structure, perhaps with fatty acids in an inverted distribution: their apolar tails in contact with the also apolar medium and their polar heads in contact only with each other. Nucleic acids, the building blocks of genetic material, would also be subject to great changes, considering they are polar compounds that dissolve in water. One way these substances could be modified to dissolve into the hydrocarbon medium would be through the addition of hydrophobic components into their structures, additions that could render their outer portions apolar and their inner portions still polar enough to engage in base-pairing through the formation of hydrogen bonds. One may wonder why these compounds would be used by lifeforms evolved in such a different environment, so unlike Earth's, and it is possible they truly would not, but these are mentioned here to offer more familiar, but still plausible examples.
Either way, the temperatures on Titan's surface are way too low for classic, thermally-driven reactions to occur (temperature is a measure of the motion of particles and, for chemical reactions to occur, particles must collide, a process that is incremented by their movement) and, as such, quantum tunneling predominates. Well, what is quantum tunneling? It is a process that occurs as a result of the wave-particle duality inherent to all particles (here, the term is used to designate subatomic compounds, atoms, molecules, etc, all of which exhibit such a duality despite their great variation in size) and, in it, a particle crosses an energy barrier despite not having enough energy to do so. In the extremely frigid rivers, lakes, and seas of Titan, quantum-tunneling-driven reactions might be predominant and thermally-driven ones completely absent, further contributing to the constitution of a completely alien biosphere (even so, quantum tunneling is still an important process in Earthly chemistry, despite the predominance of thermally-driven chemical reactions).
What is especially interesting to consider is the possible interplay between these two biospheres. When potential hydrothermal vents located deep in the moon's probable subsurface ocean turn inactive, for instance, the abyssal organisms might become buoyant, through the accumulation of lipids or even gas bubbles, for example, and gradually make their way to the surface. As they go, they might form various intracellular compartments that keep their most fundamental processes running while they suffer radical transformations in their membranes, for example, preparing for the apolar solvents they might encounter if they completely make their way to the surface. Eventually, these dual lifeforms may completely transition into apolar-adapted organisms, with nucleic acids modified with hydrophobic compounds and their proteins completely restructured. With even more time, they may even find their way back to the depths, providing an important flux of organic material from the surface to the subsurface ocean. While such a life cycle may seem extremely convoluted and is completely speculative, convoluted life cycles do occur on Earth and sometimes involve great transitions, such as from the surface all the way to the clouds, as will be mentioned later in this tale.
Now, it is time to leave the Solar System and direct our attention to the nearest star system (about four light-years away, meaning it takes four years for light to reach it), one containing not one, but three stars: two Sun-like ones, orbiting around each other, and, farther out, a much smaller one, orbiting around its larger companions. This star system is known as Alpha Centauri and its most relevant star (Proxima Centauri) is actually the smaller one, for it holds Proxima b, a super-Earth (planets larger than ours, but still sharing structural and compositional elements) occupying the habitable zone (a region around a star where water can be found in the liquid phase at the surface of an astronomical body, being neither too hot nor too cold). However, since Proxima Centauri is so much smaller and also cooler than our Sun, Proxima b's orbital period is only eleven days! In fact, this star is called a red dwarf (the Sun, in contrast, is known as a yellow dwarf), the most common star type in the Milky Way (up to 70% of all stars) and too dim to be seen with the naked eye from our planet. All of this is a result of its lower mass, which means it burns through its hydrogen at a much slower pace than our Sun, shining far less brightly, but living for much longer: instead of billions of years, it possibly extends to trillions!
For the planets in the habitable zone around them, the very short orbital periods lead to an important gravitational effect: tidal locking, which occurs as a result of the great proximity between the two astronomical bodies. This phenomenon causes the planet's orbital and rotational periods to become synchronized and, in essence, leaves it with one side permanently facing the star and another side facing the darkness of space. From the get-go, Proxima b is already faced with conditions quite dissimilar from those of Earth, despite the aforementioned similarities. However, there is one more peculiarity regarding these "cool" stars, and one that is probably the biggest hurdle for the development of life around them. Such a peculiarity is the fact that these astronomical bodies are exceptionally violent, not rarely undergoing flare events, in which their emission of more potent forms of radiation (such as UV and X rays) increases exponentially (in the case of Proxima Centauri, it underwent a flare event in which its brightness increased by 14,000 times), and also coronal mass ejections, which, composed of plasma from the star's surface, intensify the stellar wind.
This is especially bad for prospects of life on inner planets (such as those in the habitable zone), which receive much greater radiation and stellar wind exposure than more distant ones, greatly increasing their chances of having their atmospheres along with water and other volatiles, stripped off by a combination of the intense stellar wind and of the breakdown of gases by the radiation. Over time, planets around these astronomical bodies may undergo several reshufflings of their atmospheres, as the tumultuous events from their host stars may dramatically alter their chemical composition. Analyses indicate that certain gases might be produced as a result of these stellar phenomena, mostly being hydrogen and nitrogen oxides, which may grow to significant concentrations even in previously reducing atmospheres. This may be an additional source of stress for any potential organisms dwelling on such planets, which would need to be incredibly hardy and adaptable in order to eke out a living among such erratic conditions.
Well, if conditions around red dwarfs are so erratic, is there any chance for life even developing there in the first place? For this question, we will take a closer look at Proxima b and see how it may, despite its hostile star, offer some possibilities for life to begin and, perhaps, even flourish. First of all, it already shows some promise by being slightly larger and more massive than Earth, features that allow it to better hold on to its atmosphere, especially since mass is more important than a magnetic field for preserving such a gas envelope. Second, Proxima b, as a result of the higher iron content in the stars of its triple system, probably has an iron core bigger than that of our own planet, and this core, likely also partially melted, thus offers substrate for the generation of a significant magnetic field. However, due to the tidal locking process, this magnetic field is also likely multipolar (instead of Earth's dipolar magnetic field, with poles on our planet's North and South Poles), thus leaving many parts of the atmosphere exposed and thus more vulnerable to damage by the stellar wind. Third of, Proxima b, as other Super Earths, may have even more intense plate tectonics than our own planet, due to a combination of high convective stresses, which are correlated to planetary size in a directly proportional manner, and lower crust healing, more expressive in cooler planets (as mentioned in the first entry, high temperatures augment crust healing, with this likely being the reason Venus lacks plate tectonics). Plates exert several important functions, such as recycling carbon (preventing runaway greenhouse effects) and, on the opposite side, promoting outgassing, which may be especially important on these planets by helping replenish their constantly harassed atmospheres. Additionally, tectonically driven geological events can also provide important environments for microbes, such as by leading to the formation of hydrothermal vents, already very discussed in this and previous entries and very plausible sources for the beginning of life in the first place.
In the case of tidally locked planets, atmospheres and even oceans exert another very important function apart from those intrinsically linked to habitability: heat distribution. This is extremely important due to potentially mitigating the extreme temperature differences that would otherwise arise between the eternal day and eternal night sides, with the first developing potentially scorching climates and the second potentially freezing ones. But with the existence of an atmosphere and oceans, the picture changes and heat is better distributed along the planet as a whole, helping it achieve more uniform temperatures and, in cases of hotter worlds, even deglaciation of the night side. As a result, tidally locked worlds may not be as extreme and unforgiving as one may initially think, though the differences, especially in ecological terms, between both sides would still be enormous.
In the case of Proxima b, how much water it has is completely unknown and, as such, it may be all the way from an oceanic planet (a scaled up version of the icy moons observed in our Solar System, though maybe with an exposed ocean instead of one concealed by ice) to something more akin to Earth, which is much drier, enough to the point of allowing for the existence of continents (fortunately, super-Earths might actually be capable of housing much more water than our planet without being completely flooded thanks to their higher gravity, which drags water further down, making ocean basins shallower and potentially driving water into the mantle). However, even if it originally was an oceanic world, it may no longer be so as a result of its harsh host star, which might have led it to lose a significant portion of its volatiles. On one hand, this could be good news, since, as said previously, thick coatings of water combined with higher gravities can lead to the formation of exotic ices that compromise exchanges between the water body and the more interior parts of the planet. On the other hand, this could mean that Proxima b, potentially once with oceans, is now extremely dry, akin to what happened to Mars.
Unfortunately, despite its higher mass and possible magnetic field in some form, this planet will also probably lose, along geological timescales, whatever currently remains of its atmosphere (if any was there to begin with), for the activity of its host star is simply too strong and erratic. With an atmosphere gone, water cannot stay in the liquid phase, either being lost alongside it or freezing over. Overall, the long-term scenario is particularly similar to the Martian one, but, as we saw with Mars, the possibilities for life are far from over and, actually, might be even increased as a result of the increased plate tectonics (though these would potentially halt with the absence of lubrication from water, as will be cited again later in this entry). Apart from a Martian-like biosphere, extensive subsurface oceans might develop from the remaining water and, once again, we draw parallels to the icy moons of our Solar System. Consequently, Proxima b might not support a thriving biosphere such as Earth's, but an interesting one nonetheless, one in which hardy organisms have been shaped by stellar cataclysms, eking out a living in secluded environments that, though hidden from the surface, might harbor abundant and diverse biotas. Besides, one cannot disregard the extremely long life of these red dwarfs: though life may have a particularly hard time developing and establishing around them, it has a lot, but indeed a lot of time to do so, and this may increase its chances rather significantly.
Leaving the Alpha Centauri system, we travel quite a bit further, reaching the TRAPPIST-1 system, located about forty light-years away from our Solar System. Here, we once again find a red dwarf, an ultra-cool one with a very reduced size, only a bit larger than Jupiter itself! This, of course, is far from the only remarkable finding in this system, for it contains seven rocky planets in orbit, all forming a compact arrangement that likely leaves all planets tidally locked (out of the seven, the fourth, fifth, and sixth planets are located in the habitable zone). The compactness of this system produces other tidal effects, for the planets of TRAPPIST-1 are engaged in several orbital resonances, like the first three Galilean moons. As such, every time the first planet completes eight orbits, the second planet completes five, every time the second planet completes five orbits, the third one completes three, every time the third one completes three orbits, the fourth one completes two, every time the fourth one completes three orbits, the fifth one completes two, and so forth, to the point that all planets are engaged, in one way or another, to a resonance pattern. This is extremely important, for it shows the great tidal effects these worlds are subject to, tidal effects that may, like in the icy moons of our Solar System, lead to subsurface oceans, particularly in the outermost planet, though these could also be widely present even in those of the habitable zone should they lack atmospheres capable of generating greenhouse effects.
Either way, another very important characteristic about this system is its age: it is older than ours, ranging from possibly 5.4 billion years old all the way to 9.8 billion years old, with the most accepted age being somewhere in the middle, at around 7.6 billion years old. Not only does this mean that the planets have found extremely stable arrangements in their resonances, but that any findings of possible atmospheres on these planets leave it clear that the gas envelopes have endured many billions of years of stellar flares and coronal mass ejections (importantly, it appears that TRAPPIST-1 is a bit less active when compared to other ultra-cool red dwarfs, but that does not mean flaring and other extreme events do not occur: they still do, but not to such extreme extents). Additionally, red dwarfs tend to wind down as they age, meaning that the TRAPPIST-1 system is probably far more hospitable now than it has ever been in the past.
But, before we proceed with atmospheric findings, it is first important to discuss the possible structures and compositions of these rocky worlds, for they, despite being all roughly Earth-sized (some larger and some smaller), possess key differences in density that imply poignant distinctions. These differences, generally preserved for all the TRAPPIST-1 planets, refer to their lower densities when compared not only to Earth, but to the other rocky planets of our Solar System (though the densities of the planets are all similar in general, the first, second, and fourth planets have the highest densities). These lower densities indicate two things: either that they have considerable amount of volatiles (such as water), which would potentially make some of them more akin to the icy moons we have thoroughly discussed, or that they have lighter materials in their interiors, such as completely oxidized iron (in contrast to the iron in the interior of our planet, which is in a reduced state), in which case they would lack cores and have more extensive mantles extending all the way to their centers. In reality, they may have a mix of both conditions rather than extremes in either direction, especially since, for their interiors to be completely oxidized, they would need to have been formed far away from their host star, in oxidizing conditions, and then migrated inwards, which is in accordance for the three outer planets, but the four inner ones might have formed closer to the star and as such might not be so enriched in these oxidized compounds.
Importantly, the three inner planets, even with significant volatiles in their composition, likely lost all of them as a result of the high temperatures they are subject to, discounting the effects of flares and coronal mass ejections that would have only intensified these losses. For the other planets, they might have retained some of these elements despite the intense effects of radiation exposure and the stellar wind, especially if they had great amounts from the get-go, and, perhaps, the loss of some of their water coverings might have helped at least some of them escape from the formation of exotic ices around their rocky crusts. As mentioned with Proxima b, the fact that they have around Earth's mass would have helped them hold on, at least for some time, to their atmospheres and, perhaps, by this time, they are getting some relief as a result of TRAPPIST-1's older age. Coupled to this, they might have, similar to Proxima b, multipolar magnetic fields, though, unfortunately, oxidized iron is a poor conductor, and so this might significantly impair their ability to generate such fields despite molten interiors, as there would be far less movement of electrically conductive materials. Overall, the final verdict is likely similar to that of the previously observed red dwarf planet: atmospheres lost over geological timescales and the remaining water freezing over, though with considerable chances of subsurface oceans as a result of the intense tidal interactions and, apart from this, the plate tectonics these worlds might exhibit as well (they have sizes conductive for the formation of these geological features and, besides, the planets in the habitable zone and beyond likely have cooler temperatures preventing crust healing).
Well, this is only a prediction. What are the real findings regarding the atmospheres of these exoplanets? For the first planet, TRAPPIST-1 b (a little larger and more massive than Earth), two main scenarios have been pointed out: the first one, considered more plausible, is that the planet has no significant gas envelope, being a bare world like Mercury, and the second one, considered less plausible but still within the realms of possibility, is that the planet has a thick, carbon-dioxide rich atmosphere that, combined with gases like hydrogen sulfide, generates a haze due to similar reasons to that of Titan's (the reasons being the breakdown of atmospheric gases by incident radiation). While the first scenario may seem exceptionally bland and a confirmer of the most pessimistic predictions regarding the long-term effects of the violent red dwarfs, it still is interesting due to the fact that TRAPPIST-1 b's surface seems very young, rich in ultramafic rocks, which are igneous rocks composed mainly of silicates, such as olivine (first mentioned here), being dark-colored and with very low reflectivity. This young surface, as well as the presence of igneous rocks, is a strong indicator of ongoing volcanic processes on the planet. Indeed, this is not entirely unexpected, especially since this world is subject to extreme tidal stresses like the moon Io: not only is it engaged in an orbital resonance, but it also sits the closest to the host star, suffering the most tidal stresses from it. Apart from this, its close proximity to the very powerful magnetic field of TRAPPIST-1 also makes it subject to induction heating, caused by the effects of alternating magnetic fields on conductive materials, which might be present in the planet to some degree despite its conceivably oxidized interiors.
For the second planet, TRAPPIST-1 c (also a little larger and more massive than Earth), one could say the results are even bleaker, though not unexpected, as it appears that this planet too is another bare one, likely with no atmosphere to speak of and a completely exposed surface. For the third planet, TRAPPIST-1 d (a little smaller than Earth, but significantly less massive), the results are inconclusive and include a variety of possible atmosphere configurations. One possibility is that the planet has no atmosphere whatsoever, and this is very plausible, especially considering its lower mass, which would make it holding on to any gas envelope much harder. Alternatively, it may have a very thin, Mars-like atmosphere, a Venus-like atmosphere with clouds high in the air, or a water-rich atmosphere also with high-altitude clouds, among other possible configurations. The latter scenario is interesting to consider, especially because of this planet's low density and high predicted amount of volatiles, which would be in accordance with a water vapor atmosphere.
For the fourth planet, TRAPPIST-1 e (a little smaller and less massive than Earth), the results are a bit more promising, and this can be somewhat expected, as it, in the habitable zone, receives less influence of the violent outbursts of its host star than the three inner planets. The results basically indicate a fifty-fifty chance of the planet either being airless, with no gas envelope to speak of, or with a nitrogen-dominated atmosphere, an arrangement closer to that of our planet and the moon Titan. Apart from these findings, the possibilities for a Mars or Venus-like atmosphere were basically excluded, reinforcing a potentially more habitable world. Additionally, there is some speculative evidence for the presence of methane on the planet, which may derive from a source similar to that of Titan's (large organic deposits in the astronomical body's interior) or, in a more Earth-like fashion, from volcanic events and serpentinization, both of which are certainly possible when both tidal heating and plate tectonics are taken into account (in an even more speculative fashion, maybe this highly putative methane could be a product of the metabolism of methanogen-like organisms).
Even though the presence of methane is possible, the occurrence of a haze like that of Titan's is unlikely, but it could also be a periodic event, as was the case for Earth during its early days. Besides, methane, if truly present, could be of importance on TRAPPIST-1 e by granting it a more significant greenhouse effect, which is especially necessary due to the fact that, although in the habitable zone, this planet receives less starlight than Earth, which means it has a tendency to be a cooler world. However, the intense stellar events would quickly degrade any methane present in the gas envelope and, as such, it may not serve such an important function as a greenhouse agent as one would initially be led to think. Actually, the contrary might occur, as a result of the formation of reflective photochemical hazes. Importantly for TRAPPIST-1 e, is the fact that it has one of the highest densities in the system and thus, though still possibly containing significant amounts of water and other volatiles, it has potentially lost enough to prevent the formation of global oceans deep enough to promote the formation of the previously discussed exotic ices. Not only this, but its drier composition might have led it to avoid an oxygen-rich atmosphere during its formation (oxygen is formed through the starlight-mediated breakdown of water), which could be potentially quite harmful to life as a result of the very reactive properties of such gas.
If any organisms are indeed present and if any of them are photosynthetic, they may exhibit a very dark, perhaps even black coloration, for TRAPPIST-1, as well as other of the smaller red dwarfs, are very cool and much less bright than the Sun, meaning that photosynthesis, to successfully occur, would probably require absorption of light from all the visible spectrum and beyond. As a result, the speculative photosynthetic lifeforms would not reflect any light, absorbing all of it and thus appearing black to our eyes (the same could apply to any similar beings in Proxima b, for instance). Some of these organisms may exhibit heavy forms of radiation protection, such as the pigment melanin and sophisticated, efficient mechanisms of genetic material repair, for though they gain their sustenance through a dim star, such a star is also erratic. Alternatively, if some are multicellular and more plant-like in aspect (possible through convergent evolution), they may simply count with dispensable, above-ground body parts that may be easily regrown through more permanent and protected parts below the surface (as happens with grasses here on Earth, as better explained in this tale).
Here, on Earth, there already are a wide variety of light-absorbing pigments, especially among anoxygenic photosynthetic bacteria (as already cited in the first entry). A few of these, known as the green sulphur bacteria (family Chlorobiaceae), are actually very well adapted to grow under low light environments, such as those that would be present on planets around red dwarfs. They thrive in the deepest portions of lakes, in the last layers of biofilms, and around sulfur springs, the latter being locations rich in sulfur, which can be very important, since these organisms, instead of using water as an electron donor (as occurs in oxygenic photosynthesis), actually use various other compounds, including sulfur itself, sulfide, hydrogen, reduced iron, among others. Curiously, some of them are not even green, being actually brown, and these, apart from containing a type of bacteriochlorophyll, are particularly enriched in carotenes, pigments that help give their distinct brown hues and, additionally, that are especially adapted for absorbing the remaining electromagnetic waves not filtered by water or fellow photosynthetic layers above. Consequently, on our own planet, there are already examples of darkly colored photosynthetic lifeforms exquisitely adapted to low light conditions, and, as such, these might serve as a useful analog for what phototrophic life might look like around especially dim red dwarfs such as TRAPPIST-1.
Apart from this, it is also interesting to consider that tidally locked worlds, such as TRAPPIST-1 e, offer very contrasting light intensities as a result of the eternal day and night sides. Consequently, the more darkly colored photosynthesizers might cluster around areas closer to the night side, termed the twilight zone, where there is a constant, perpetual starset. In contrast, photosynthetic organisms located on the day side, having to deal with perpetual light, might possess different light-absorbing strategies. For example, the more typical photosynthesizers of Earth, cyanobacteria and photosynthetic eukaryotes (the various algae and plants), mostly use chlorophyll a (with contributions from chlorophyll b as well), which reflects green light. However, most of the light in the visible spectrum emitted by the Sun is green, and so a question arises: why are these autotrophs "wasting" the more abundant green light? The answer might likely be that, though absorbing green light would provide the most amount of energy, it would lead to great fluctuations in light absorption, such as from a cloud passing overhead.
These great fluctuations could either severely hamper energy production or cause excessive activation of the photosynthetic apparatus, potentially leading to excessive production of free radicals, quite damaging to organisms, as seen many times before. However, by absorbing mostly the red and blue components of the visible spectrum, such photosynthetic beings are mostly buffered from the wildest fluctuations while still absorbing good amounts of the visible light sent out by the Sun. What is interesting is that the same principles apply even for the more atypical photosynthesizers, such as the green sulfur bacteria and other anoxygenic photototrophs. What this all means is that, maybe even around these very dim stars, those phototrophs most exposed to starlight would perhaps be redder in color, for this is the part of the visible spectrum most emitted by red dwarfs. By reflecting red light, these beings could avoid those wildest of fluctuations and thus avoid the aforementioned problems in their photosynthetic apparatuses. Another interesting point to mention is that planets with cooler tendencies might potentially be in great trouble if oxygenic photosynthesizers ever arise in their potential biospheres. On Earth, they were responsible for the Great Oxidation Event (read more about it here), which led to a great mass extinction not only due to oxygen production but also due to the planet freezing over. In worlds like TRAPPIST-1 e, these snowball periods could be much more frequent and perhaps much more persistent, potentially creating problems for more expressive biotas. Another side effect is that, unlike Earth, which has had more greenhouse periods than ice age ones, these might be the contrary, perhaps with rare or completely absent periods of warmer temperatures.
For the fifth, TRAPPIST-1 f (slightly larger and more massive than Earth), and sixth, TRAPPIST-1 g (even larger and more massive than Earth), planets, much higher amounts of water are expected, especially for TRAPPIST-1 g. As a result, they have a higher chance of still possessing very deep, global oceans (for TRAPPIST-1 g, its ocean might have a depth of 670 km) as well as atmospheres containing not only oxygen, but also water vapor (this would be the case especially for TRAPPIST-1 g, which might have a steamier atmosphere, while TRAPPIST-1 f might have one that is proportionally richer in oxygen). Consequently, both these planets would likely have potentially very thick layers of exotic, pressure-driven ice around their rocky interiors, a feature that is not very conducive to life as has been previously stated multiple times. Apart from this, atmospheres containing great amounts of oxygen would also not be ideal, as just cited. How the surface of these oceanic worlds would look is unknown, as, though in the habitable zone, they receive far less starlight than Earth and thus might count with frozen surfaces hiding subsurface oceans beneath, especially as a result of tidal heating effects. Alternatively, the possibly rich in water vapor atmosphere of TRAPPIST-1 g might grant it enough of a greenhouse effect to have a more liquid or at least slushy covering.
Regarding the seventh planet, TRAPPIST-1 h (with a roughly similar size and mass to TRAPPIST-1 d), this is the least known of the TRAPPIST-1 planets, and it is expected to contain a great amount of volatiles in its composition. As a result, it might have followed similar developmental patterns as TRAPPIST-1 f and g, though its smaller mass might have made it harder for it to hold on to an atmosphere, though. At the same time, however, its greater distance from the host star might have made it more shielded from the atmosphere-stripping effects. Overall, though, it is far from the habitable zone and, though it might have a subsurface ocean as a result of tidal heating, there likely is a layer of exotic ice around its interior, making it potentially almost like an upscaled Ganymede.
Now, it is time for us to go even more distant, to the Kepler-186 system, located about 500 light-years away from our own! Kepler-186, the host star, is yet another red dwarf, but just barely: with about half of the Sun's mass, it is on the border between a red and an orange dwarf, the latter being a star halfway between a red and a yellow dwarf. This may be especially promising due to the fact that orange dwarfs, in accordance with their transitional status, also offer the "best" of both star types: they are more stable and less erratic, like yellow dwarfs, but also have much longer lifespans, such as red dwarfs, though, in their case, they might only reach tens of billions of years, instead of the speculated trillions of years for their smaller peers. Regarding the star system itself, it is composed of five, likely rocky planets, all larger than Earth to some degree. While the four inner planets are likely tidally locked and too hot for liquid water on their surfaces (with the possible exception of the fourth one, which might have liquid water in its portions less exposed to starlight, such as those close to or on the night side, and in cases of potential high reflectivity), the fifth one, a little larger than Earth, is very far from them, situated at the outer edge of Kepler-186's habitable zone.
At such a distance, with an orbital period of 130 days, this Super-Earth may or may not be tidally locked (even if it is, it might still have multipolar magnetic fields considering a composition overall similar to Earth's), but, even if it is not, it might have underwent significant slowing down of its rotational period as a result of the tidal effects of its star, potentially having it as several days or maybe even weeks (if the rotation period of the planet truly corresponds to weeks, it could have shorter, but dramatic seasons that affect most if not all latitudes, instead of here on Earth, where the most intense seasonal shifts only occur closer to or at the poles). More importantly, though, it receives only a small fraction of starlight when compared to Earth. Even so, considering it may have an atmosphere with large amounts of nitrogen as well as modest amounts of carbon dioxide, it could have temperatures high enough to sustain liquid water on its surface. This is especially plausible considering its potentially higher mass (accompanying the logic from its higher size in the context of it sharing a similar structure and composition to our planet) and great distance from its host star, two features that would have aided it in preserving its gas envelope.
As is the case for Proxima b, much is unknown regarding this world, and it may range all the way from a drier planet, akin to our own, all the way to an oceanic planet, such as TRAPPIST-1 f and g. Apparently, all of the planets in the Kepler-186 system might have considerable portions of volatiles, including water, and this may be especially the case for an astronomical body in the habitable zone, as is the case of Kepler-186 f. As such, it is plausible for this to be yet another oceanic planet, a scenario that likely would not be very conductive to life as a result of the formation of a layer of exotic ice around the world's interior portions, as cited many times before. However, it is also important to note, as was did in Proxima b, that super-Earths, due to their higher gravity, would be capable of having significantly more water than Earth before being completely flooded, which gives them a more ample range of volatile composition before becoming oceanic worlds (additionally, it is important to say that, even though a world might be completely enveloped by water, it is not necessarily a likely lifeless oceanic planet, as it may have a comparably thin hydrous covering, one not deep enough to lead to the formation of exotic ices).
A potentially important peculiarity regarding Kepler-186 is its higher amount of silicon when compared to the Sun. This could translate to a higher amount of silicon also in the planets, since they and the star are all formed from the same cloud of original material. This element, present in the crust and also in the mantle of rocky worlds (as will be mentioned again down the line), could, in excess, make the surface of the planet essentially too rigid for plate tectonics to successfully install themselves. At the same time, though, Kepler-186 f exhibits several attributes that would make plates more likely in it, such as being a Super-Earth and having cooler temperatures that prevent too much crust healing. Even if it does not have plate tectonics however, it would still be theoretically capable of hosting life (Mars, one of the main contenders for life outside Earth, lacks plate tectonics, possibly due to its small size), as, though these geological phenomena play key roles in promoting biospheres, these could potentially emerge without them as long as the planet itself is active in its interior, still engaging in processes that may lead to volcanism as well as hydrothermal vents (potential effects of volcanism without plate tectonics are mentioned further ahead).
Overall, Kepler-186 f, though with several nuances, still presents quite a significant bit of potential regarding its ability to house an extraterrestrial biosphere, one which may not be subject to atmospheric stripping as well as various intense and violent stellar events, allowing it to potentially exhibit a more Earth-like aspect should it be present in the first place. Regarding what this life might look like, we once again return to the photosynthesizers. If any are indeed present, they might be mostly reddish regarding the previous reasoning used for equivalent organisms on TRAPPIST-1 e, especially because Kepler-186 is a much brighter star and so any phototrophs would suffer less from way too low starlight. As mentioned for TRAPPIST-1 e as well, the lower amount of starlight this world receives puts it at a tendency of bearing a cooler climate and, once again, oxygenic photosynthesizers, if any develop, might represent significant trouble for the biosphere of a planet such as this one, since it may enter into snowball periods much more easily.
Moving even farther away, up to 1,200 light-years away from our Solar System, we arrive at the Kepler-62 system, one based on an actual orange dwarf, around 70% of the Sun's mass, and at an estimated similar age to TRAPPIST-1, at around 7 billion years of age. Once again, this system has five planets, most of them most likely rocky. The fourth, Kepler-62 e, and fifth, Kepler-62 f, planets are the most promising, both being significantly larger than Earth, with the fourth being around 60% larger than our planet and the fifth around 40% so, sizes that mean they may still be rocky in overall composition, though Kepler-62 e less so, as, above certain diameters, planets start having a higher chance of being small Neptune-like worlds. While Kepler-62 e is at the inner edge of a more optimistic habitable zone (with an orbital period of around 122 days), receiving more starlight than Earth, Kepler-62 f sits more comfortably on such a zone, being, like Kepler-186 f, at its outer edge, receiving less starlight than our planet and having an orbital period of about 267 days.
Seeing as both these astronomical bodies might be considerably more massive than Earth, assuming roughly similar compositions, they have plausibly held on to their atmospheres, which is something made even more probable due to the fact that their host star is not a violent red dwarf. In the case of Kepler 62-e, this likely means it is a considerably hotter planet than our own, though potentially habitable still. In the case of Kepler 62-f, this means that, should it have accumulated enough greenhouse gases, it could potentially host liquid water at its surface, though it would be, like TRAPPIST-1 e and Kepler-186 f, a cooler world than our own, with all the consequences that come with this, already mentioned for the two other planets. Like for Proxima b and Kepler-186 f, however, not much else is known about these two, and so they could range from drier places like Earth all the way to full oceanic planets with a considerable amount of volatiles and layers of exotic ice around their rocky interiors.
For Kepler-62 e, this would likely mean a balmy world with much water vapor in its atmosphere as a result of the higher temperatures and lots of water present. Since water vapor is a greenhouse gas and an oceanic planet would lack rock weathering (since there is no exposed surface whatsoever), it is conceivable how such a planet would undergo a runaway greenhouse effect, potentially rendering it completely uninhabitable (though, at the same time, water clouds increase the planet's reflectivity, with water vapor thus acting dually). In such a scenario, it may become like a much larger, much wetter, and perhaps even more extreme Venus, as its much higher mass means it could adequately accumulate a very substantial gas envelope that only grows as its extensive water reservoirs boil and reach the atmosphere (actually, temperatures might need to climb quite a bit higher than a hundred degrees Celsius for water to boil as a result of very high atmospheric pressures, a process that may have happened right here on our planet during its beginning). Consequently, Kepler-62 e might be much worse than just a sterile oceanic world, perhaps being a planet with an atmosphere exerting crushing pressures and keeping liquid a global ocean that is much above its usual boiling point.
For Kepler-62 f, significantly large amounts of greenhouse gases in its atmosphere would likely mean a temperate oceanic planet, one with quite amicable temperatures, but probably still sterile either way. Alternatively, should it lack such a reasonably powerful greenhouse effect, it would be frozen over, an extremely large icy world with several kilometers of ice upon ice upon more ice. Certainly a desolate and inhospitable landscape, but a possible one nevertheless. However, these two worlds are far from being confirmed as oceanic ones, and so more promising speculations about their nature can be made. One of these is that, as mentioned previously, Kepler-62 f has considerable chances of displaying plate tectonics, as not only is it a sizable super-Earth, but also a cooler one, which means its surface has maximized stresses and diminished healing. While Kepler-62 e is even larger, it is considerably warmer and as such its crust might heal way too fast for tectonics to get installed, quite a similar occurrence to Venus yet again. Another one is also regarding Kepler 62-f and is the fact that this planet, due to the significant distance from its host star, is plausibly not tidally locked and may actually exhibit a more Earth-like rotation period. This is quite important, as it would allow it to count on even fewer greenhouse gases to maintain sustainable temperatures, as there would be no eternal night side to be warmed up.
Besides this, there is another interesting, though maybe rarer possibility: even if this astronomical body has more Earth-like levels of greenhouse gases such as carbon dioxide, it could still potentially harbor liquid water, albeit in only a seasonal form. This could be achieved if it had a rather extreme axial tilt (somewhat like the planet Uranus), one which would give it one portion of the globe receiving much more starlight than the other portions. Coupled to this, it would also need a less circular orbit, one which, during some time of its orbital period, would cause it to pass much closer to its host star (though, in another time of its year, it would also pass much farther away). If this is indeed the case, it could create a potentially very interesting biosphere, in which organisms spend much of the world's year dormant or perhaps in reduced states of activity, only to suddenly blossom with the melting of their icy surroundings, going on a frenetic flurry of reproductive processes and conclusion of life cyles only to return to a desolate, frozen landscape once Kepler-62 f leaves the proximity of Kepler-62. Regarding the same rationale behind the appearance of photosynthetic life as explained for TRAPPIST-1 e and Kepler-186 f, maybe, from space, one could observe a grayish white surface of ice suddenly give space to various splotches of crimson as photosynthesizers emerge from their slumber and absorb the electromagnetic waves emanating from their host star.
One possible feature of Kepler-62 f that increases its chances of having a more extreme axial tilt is the presence of a moon. Unlike many other planets around smaller stars, in which possible moons would likely not stay with their host planets after 5 billion years, growing too distant, Kepler-62 f has been shown to be able to harbor a moon for timelines greater than these and under various different circumstances. Not only would a moon exacerbate its tides and potentially contribute to an even more Earth-like environment, but it would destabilize its axial tilt, allowing it to achieve more extreme values and also vary more, which could maybe lead to an alternation of areas where starlight falls upon more. This, interestingly, is quite the opposite of our Moon, which actually stabilizes Earth's tilt, impeding it from varying too much and making the variations that do occur (better explained here) be comparatively small and, while still influencing quite a bit on climate, be far from the dramatic effects speculated for Kepler-62 f, in which it, completely frozen, would experience seasonal deglaciations. Even when completely frozen, subsurface oceans could exist on this world, kept warm by its considerable geothermal activity.
Apart from the potential moon, yet undiscovered gas giant planets on the periphery of the Kepler-62 system are a real possibility, and their presence would probably further destabilize the tilt of the analyzed world (besides, these gas giants, as occurs in our own Solar System, might have several moons that potentially offer more unconventional habitats for life). It is important to mention that these extreme tilts are only necessary for habitability should Kepler-62 f have the same amount of carbon dioxide as Earth, but, especially due to its larger mass and increased ability to retain a thicker atmosphere, combined with its potentially increased tectonism, larger amounts of greenhouse gases such as carbon dioxide are certainly plausible. In such a case, these more extreme tilts would likely be a greater hindrance to life than the contrary, exactly due to the massive climatic shifts they could cause. Overall, though, Kepler-62 f offers another possibly promising habitat for an extraterrestrial biosphere, and, apart from its considerably larger size and discounting the possibility that it is an oceanic planet, it is probably the most Earth-like world seen so far.
As we leave the Kepler-62 system, we arrive at one located at a similar distance from our Solar System and home to the last exoplanet we shall address. It is the Kepler-442 system, based, once again, on an orange dwarf: Kepler-442, about 60% of the Sun's mass and thus standing at a middleground between Kepler-186 and Kepler-62. Unlike those two other stars, only one planet is definitely known around this system, that being the super-Earth Kepler-442 b, about 30% larger than our planet and orbiting at the star's habitable zone, with an orbital period of about 112 days. This leaves it in a position where it receives almost the same amount of starlight as Earth, just a little less. Consequently, this breaks the cooler world tendencies observed in the most promising planets seen so far (Proxima b, TRAPPIST-1 e, Kepler-186 f, and Kepler-62 f). Despite this more inward position at the habitable zone, apparently this super-Earth is not tidally locked, which further reinforces it Earth-like character and maybe allows it to sport a dipolar magnetic field, though, at its great expected mass (assuming a roughly Earth-like composition), it would have had a far easier time holding on to its atmosphere even without any magnetic protection (as mentioned previously, mass appears to be more important for atmosphere preservation than the presence of a strong enough magnetic field).
What is perhaps most promising about this planet is the fact that it receives sufficient starlight to power a biosphere the same size as Earth's, unlike the previous worlds just cited in the previous paragraph. However, it is important to say that this might not constitute as big a hindrance to those planets as one might initially believe, and the reason for that is that a great amount of the sunlight absorbed by photosynthesizers here on Earth is actually not used. Consequently, the photosynthetic organisms of such dim-lit planets could potentially evolve more efficient photosynthetic processes capable of better utilizing the available light, thus making them more capable of supporting biospheres of equivalent scale to that of our astronomical body. Like other super-Earths, Kepler-442 b is expected to have very active plate tectonics, as not only is it possibly quite massive, but also cooler, preventing excessive healing of the crust.
While the benefits of this geological feature have been previously cited, it is also important to consider some of the disruptive effects they have on already established lifeforms, as has been demonstrated during the chapters of this website. Not only can the assembly of supercontinents generate volcanism, but also potentially extensive masses of land with wide-spanning deserts, as was the case with Pangea. After this, the breakup of such landmasses can also cause extensive volcanism, as was also the case with Pangea. In the case of Kepler-442 b, which receives about as much starlight as Earth, it may undergo several greenhouse periods, and these potentially might be even more significant than those of our planet, for, though it receives slightly less starlight, its likely much larger mass has maybe caused it to sport a thicker atmosphere with more greenhouse gases.
As is the case for Proxima b, Kepler-186 f, Kepler-62 e, as well as Kepler-62 f, much is unknown about Kepler-442 b. As such, it may range from a world more like ours all the way to a potentially sterile oceanic planet with thick coverings of exotic ice around its inner parts. Overall, though, this world is quite promising and, despite its size and oceanic planet potential, it might actually be the one most similar to our planet seen so far as a whole, with its land surfaces perhaps painted red by thriving photosynthetic lifeforms adapted to the more red-shifted glow of their host orange dwarf, maybe with stocky, low-lying creatures prowling amongst the alien vegetation, adapted to the higher gravity of their planet. Such a vision is completely speculative and, even though it might not be the case for Kepler-442 b in particular, it could be for similar planets. The galaxy is too large and the universe is amazingly larger still, and all these analyzed exoplanets are only an incredibly small portion of what is out there, unfathomably vast and waiting to be discovered, or, perhaps, maybe not only waiting, maybe discovering too.
As we reach the conclusion of our great extraterrestrial exploration, we need to keep in mind that we have only focused on life as we know it, that being in the form of carbon-based organisms. Regarding alternative chemistries, silicon is often mentioned due to its somewhat carbon-like properties, the most important of which being its ability, like carbon, to form four chemical bonds, granting it plenty of versatility (this versatility in carbon can be seen by it being the basis of amino acids, nucleotides, and lipids). Curiously though, silicon is the second most abundant element on Earth's crust (just after oxygen), forming the most abundant compound, silica (composed of one silicon atom with four oxygen ones), and, despite this great abundance, it is not used by life as is carbon (though it is still used, as clearly mentioned here and here), making it clear, in an empirical manner, that, at least for Earth-like environments, carbon is clearly the most capable of serving as the molecular basis of lifeforms.
Perhaps one reason for this is that silicon, unlike carbon, usually does not form double bonds and, indeed, in its most abundant form, which is silica (this material forms many gems, such as quartzes and opals, and also sand), the silicon atom is bound not to two oxygen atoms in two double bonds, but rather to four oxygen atoms in four simple bonds. Despite this restriction, silica molecules are capable of bonding together to create vast, but extremely variable chains known as silicates, these being responsible for the various objects we observe macroscopically. Additionally, silica molecules can, instead of forming silicates with other silica molecules, actually stabilize their charges with metals, creating compounds known as nesosilicates and of which olivine, cited before, is an example. Apart from these, there are many other silicate types, forming various molecular structures based on the different arrangements silica molecules can adopt when binding to one another.
Apart from interacting with oxygen, silicon can also engage in chemical bonds with other silicon atoms and with hydrogen as well, forming materials known as silanes. However, unlike what happens with hydrocarbons, which, though with varying levels of stability are generally more stable, silanes can only grow to a length of about six silicon atoms due to the fact that silicon-silicon bonds are quite weak, leaving these compounds very prone to reactions with oxygen and other substances, making them unable to grow into larger structures. Despite this, silicon lattices, structures in which a silicon atom is bound to four other silicon atoms, are more stable and serve as the silicon analogs of things such as graphite and diamonds, which are also lattices of carbon atoms, all bound to four other carbon atoms.
All of this goes to show that silicon, despite not being as versatile as carbon, is still a pretty flexible element in what it can form, with a very great array of widely contrasting compounds and substances deriving from it. Not only this, but it is also extremely abundant and, as such, in specific contexts, it could perhaps serve as the molecular base of lifeforms just like carbon has done here and plausibly on most other life-containing planets as well as moons. Of course, this likely would not be something trivial, and so now we should dwell on where, if anywhere, these possibly silicon-based beings could be found and, more interestingly, how they could have come about. Due to the fact that the most stable silicon compounds are silicates and their many variations, it is not inconceivable to think of these as the main building block for silicon-based beings, as other arrangements, apart from the pure silicon lattices, are more unstable or synthetic (as is silicone, for example). However, their great stability is somewhat of a hindrance, for living beings require chemical reactions and thus a "sweet-spot" in which compounds are not too unstable to the point of quickly spontaneously degrading or too stable to the point of being completely inert.
For silicates to be more conducive to possibly life-giving chemical reactions, they might need to be subjected to extremely high temperatures, such as those found on very hot planets, which are often overlooked in discussions of extraterrestrial life. Silicates are formed under very high temperatures, above 800 degrees Celsius, and they crystallize into their solid forms from magma, such as happened here on Earth: initially, all was molten, but as the planet cooled down, some silicates crystallized first and others followed (olivine, for instance, is one of the first to crystallize), eventually leading to the solid crust of today. However, in Earth's interior, some silicates remained molten, such as those that flow out of volcanoes as magma. When thinking about mostly silicate-based lifeforms, a balance must be achieved. On one hand, the silicates cannot be completely liquified, for, in that state, the organisms would essentially just dissolve into one big goop (though, as just mentioned, different silicates have different temperatures at which they liquefy and, as such, a certain set of temperatures in which only a few silicates are molten is possible). On the other hand, they cannot be under such low temperatures as to be mostly inert, preventing the more dynamic reactions that characterize living beings.
Consequently, we may think of a narrow zone of silicate habitability, where planets cannot be too hot as to completely liquefy or even vaporize silicates, nor too cold as to inhibit more significant chemical reactions. This still leaves us with very hot planets, potentially several hundreds of degrees hotter than even Venus, in order for new silicates to form and take part in the constant processes of growth and reproduction that living beings engage in, the world must be at around some 800 degrees Celsius, as previously cited. Of course, there is room for plenty of natural temperature oscillations in planets that might leave this silicate "habitable zone" a bit more flexible, such as extreme volcanism and intense processes of tidal as well as of induction heating (as mentioned for TRAPPIST-1 b). In such contexts, volcanoes might serve as initial hotspots for the beginning of silicate life. Though spewing liquid silicates, they can create mixing of several silicate compounds and perhaps even offer the right temperatures for more radical reactions to occur. Additionally, if the organisms are composed of silicates with higher melting points, they might not even be threatened by magma. Such a picture may read like something totally incomprehensible and alien, but, indeed, these would be lifeforms like nothing here on our planet, operating on a completely alternate set of elements that act in radically contrasting ways to carbon ones.
Either way, how could such organisms even develop, and how would they be organized? While this is all completely in the realm of speculation, and wild speculation I might add, it is possible that silicate compounds, with all their varying arrangements and ability of associating to other elements, such as metals, might form enclosed particles, perhaps containing, in their interiors, other silicates with potentially differing properties. This would be somewhat of a mirror to what happened here on Earth, where fatty acid membranes enclosed around nucleotide-amino acid complexes. These particles could have porous walls and perhaps be localized on volcanic areas, as cited before. If they are composed of silicates of higher melting points than the magma they are bathed by, they could potentially use it as an energy source without dissolving themselves. For example, they could lay dormant while the volcano is also dormant, but once magma starts flowing again, it, working like a sort of cytoplasm, could fill the interior of the organisms with heat and new materials that fuel their metabolic processes, perhaps even allowing them to complete their life cycles. On planets that are hot enough, these extremely speculative organisms could perhaps go on to colonize other areas, considering there would be enough heat to make their silicate reactions possible.
Certainly, one of the biggest questions that arises regarding such beings is how they would store information. For Earthly lifeforms, such a feat is done through genetic material, such as RNA and DNA. For these lifeforms, silicate might also be the answer. Crystals, while usually following patterns, sometimes undergo irregularities called defects. These defects, due to being violations in what would be more uniform structures, might lead to different interactions of such a crystal with other silicates, such as those that might, for instance, function as peptide analogs. Through specific defects or through specific combinations, different interactions could emerge and, in the end, different organisms. During a potential process of crystal replication, other defects might arise, and thus natural selection may act, with this potentially being the basis for evolution in silicate life. This would be quite different from the genetic material here on Earth, which conveys information through chemical base-pairing. For these lifeforms, information would rather be conveyed through three-dimensional interactions between different silicate arrangements.
One may wonder if these crystals would still display heredity, and the answer is that they certainly could, especially in contexts of epitaxial growth, a widespread phenomenon for crystals in which the deposition of new crystalline material takes place over what was previously deposited. As the new crystal gets deposited, defects in the old crystal might propagate into the new one, a feature that would serve as the base of heredity for these peculiar beings. And, as previously said, new defects could also emerge, functioning as mutations. Additionally, such defects are also influenced by environmental conditions as well, and, as such, perhaps certain environments would drive silicate life evolution at greater speeds than others, potentially creating interesting contrasts in different areas of silicon-based biotas.
It is especially interesting to consider that some of these organisms, as they potentially move away from their volcanic sites of origin, might instead harness their energy from stellar light. Here on Earth, photovoltaic cells are made of silicon lattices that are modified with impurities. While pure silicon does not allow for the free movement of electrons, when such impurities are added, such movement becomes possible to some degree. Consequently, when sunlight hits the lattice, some electrons suffer excitations and become freer to move. Due to the design of the cell, they flow in a specific direction, creating an electric current. Silicate-based lifeforms could perhaps develop special tissues in which impure lattices of silicon are deposited, allowing electrons to be excited and form electrical currents that then might go on to fuel some of their metabolism. One important point to consider is that, for these organisms, food would be virtually infinite, for silicates probably also constitute the most abundant compound in the crust of many other astronomical bodies. The absorption of such material could take place in various ways: for the volcanic dwellers, the flux of magma would serve as a natural source of liquid silicates that could perhaps be stored in special compartments, and for the potentially photovoltaic lifeforms, they could perhaps produce silicon-based acids or other compounds that help degrade neighboring silicates, then absorbed and incorporated into their bodies. How reproduction would take place is especially hard to say. Perhaps it could occur as a result of repeated thermal, mechanical, or chemical stresses that, over time, lead to the fission of these crystallized organisms. Considering that the fractured part contained the information-encoding crystal, it could proceed with its growth, leading to the rise of a new individual.
Apart from all of these considerations, one more must be made, and it is regarding motility. Here on our planet, many organisms are motile, and there are several unifying reasons for this, which apply from bacteria all the way to tetrapods: escape from predators, escape from hazardous environmental conditions, search for food, and search for reproductive opportunities. Based on this, we can assume that even silicate-based life would also undergo selective pressures towards motility. Well, with that out of the way, comes another question: how would they move? Well, there are plenty of possible ways. Certainly one of the most interesting is through piezoelectric effects, which basically refers to how certain solid materials (such as crystals, various polymers, and even bones as well as proteins) develop a potential difference when subject to mechanical deformation, a phenomenon that also works in reverse. Basically, what this means is that tiny silicate organisms with a potential difference between their extremities would have their bodies deformed, and this, while looking like just plain vibration, could help them to move around more actively. Such an electrical effect could also perhaps be used for more sophisticated forms of movement by leading to contractions and extensions of flagella or cilia-like structures, which would then help propel their owners around in perhaps liquid mediums of different molten silicates and other substances.
Regarding the larger lifeforms, such as the photovoltaic ones, the piezoelectric effect could still be useful for movement. Imagine, for instance, an organism with several projections emanating from its underside that rhythmically retract and expand like the tube feet of echinoderms, gradually inching them over to where they seek to be. Once again taking inspiration from the tube feet of echinoderms, it is possible some organisms would utilize hydraulic-like mechanisms to move about, extending their appendages with the help of molten materials within their interiors. Such appendages could potentially take the form of well-developed limbs, allowing them to stride on the surface of their worlds. And while one might initially believe that the wide supply of food would perhaps make predation unlikely in such a biosphere, that might not be the case, as fellow silicate organisms could offer potentially more processed silicates that would lead be easier for integration into the consumer's tissues and, besides, they could potentially concentrate rarer, but still essential compounds, such as specific metals for instance. As a result, predation would probably still be present even in these utterly alien worlds, potentially leading to evolutionary arms races regarding several adaptations, such as increased movement, greater defenses, or greater offensive capabilities (such as more powerful appendages for drilling into tougher exteriors or more powerful degrading compounds).
These silicate lifeforms perhaps would be most common around red dwarfs and orange dwarfs for one possible reason: if their necessary chemistry, as speculated here, is too unlikely to occur, it likely would only lead to lifeforms over many billions of years, timespans that can be achieved around these stars, but not so much around yellow dwarfs, as our own Sun, for instance, is likely only to last close to 10 billion years in total, and much less so for even bigger stars. As unlikely or ludicrous as these speculative organisms might be, we once again must remember that the universe is incredibly vast and filled with all manner of possibilities. If something truly is possible, then it might as well occur, sooner or later and so maybe silicate forests of strange, crystal-like photovoltaic beings could indeed be out there, being inhabited by rocky creatures moving around a thousand steps at a time with tiny feet under their bodies, occasionally hunted by predators striding comparatively fast, pinning them with silicate hooks and digesting their insides with deadly cocktails.
Now comes the great question: what of the world that was left behind? Well, let us start with the basics: the future of the Earth lies in its continental arrangement. As we have seen many times before, the landmasses are fundamental to climate and weather patterns, apart from, in various circumstances, having been behind major volcanic as well as weathering events. With some, albeit modest, degree of precision, it is possible to speculate on what our planet's continents might be like millions of years into the future. Even before the nearest geological events take place, though, a significant change in Earth’s faunas might occur. As mentioned in the last chapter, humans were the main ones responsible for the extinction of several large creatures across the globe. As a result, many environments today, especially in the Americas, Eurasia, and Australia, are artificially empty, lacking large creatures found, for instance, in Africa, India, and Southeast Asia. Even before the aforementioned geological processes take place, it is possible these animals, from the cited holdouts, move on to reoccupy their former habitats.
This event of recolonization could be entirely natural, especially during glacial periods (considering humans do not take measures to avoid them, such as burning fossil fuels to intentionally increase the greenhouse effect, as mentioned previously), which, due to a decrease in sea levels, provide more land bridges. Alternatively, it could even be promoted by humans. Currently, there are already efforts to reintroduce many creatures into environments they formerly dwelled in, and this is not even taking into consideration the at least partial reengineering of animals formerly wiped out by humans (as has already happened with the Dire Wolf, Aenocyon dirus). Here, it is important to mention that these reengineered animals, even if not genetically pure (something very improbable to be achieved), may still play important ecological roles as long as their rough phenotype is reconstructed, acting as proxies for their long-gone counterparts. As a result, it is conceivable that, either by human action or simple inaction, a return to more Pleistocene-like faunas can occur, making these depopulated ecosystems more diverse and sustainable. But a caveat must be made. Many of the recolonizers, such as elephants and rhinoceroses, face grave existential challenges in the locations they still inhabit. If these continue or increase, there might be no recolonizers to speak of, but the increasing environmental concern across the world leads to the thought that these beasts might survive to reclaim their lost lands.
As they reclaim it, many may diversify into very familiar forms now extinct, a classic example of convergent evolution. Indian elephants (Elephas maximus), for instance, might, over many generations and as they move into colder regions, evolve into forms resembling Woolly Mammoths (Mammuthus primigenius), which, together with other mammoths, were their closest relatives. Sumatran rhinoceroses (Dicerorhinus sumatrensis) also might diversify into woolly forms as they move north (these rhinoceroses are, as well, the closest relatives of their extinct, woolly peers). Some lions (Panthera leo) might evolve to resemble Eurasian lions (Panthera spelaea), which, being some of their closest extinct relatives, were adapted to high latitudes and cold environments. These are just a few examples, and patterns like this might repeat for many other animals across Eurasia and the Americas, ushering in new, but quite old-looking megafauna.
The situation in Australia would be quite different due to the endemicity of its fauna. Though it was also left with a depauperate megafauna, there are no external reservoirs from which related animals can fill the vacant niches. Instead, these will be either filled internally or by invasive animals, such as the dingo (Canis lupus familiaris), a basal dog that has returned to its feral roots, currently serving as Australia’s only terrestrial apex predator, filling ecological roles once occupied by predatory marsupials, such as Thylacoleo (popularly known as the marsupial lion), gigantic monitor lizards, such as Varanus priscus (mentioned previously), and terrestrial crocodilians, such as Quinkana.
Another invasive creature that has established a robust presence in the landmass is the camel (Camelus), which, arriving less than two hundred years ago, has already grown to sizable populations. Adapted to arid locations, they have done well in the vast dry expanses that compose a major part of the Australian interior, causing ecological havoc in the areas they inhabit. It is possible that, over time, they could become better ingrained in the Australian biosphere, continuing to fill the niche of large herbivores and potentially even diversifying into other forms, eventually maybe diverging quite a bit from their founding populations in other parts of the world. However, the same way humanity may be responsible for reintroducing animals, it can also be responsible for wiping out those considered a nuisance, so the destiny of these and other invasive beasts remains completely up in the air.
Of course, the partial reengineering of extinct species cannot be ruled out here as well. The thylacine (Thylacinus cynocephalus), the largest remaining marsupial predator after the earlier extinction of Thylacoleo, is a target of “de-extinction” projects, and a proxy for it could conceivably be reintroduced, recreating a more native fauna and maybe helping limit the expansion of the foreign creatures. Not only this, but Tasmanian devils (Sarcophilus harrisii), formerly present in mainland Australia, are now being reintroduced, gradually expanding beyond their Tasmanian holdout. Additionally, there are proposals for the rewilding of Komodo dragons (Varanus komodensis), which not only would bring back an Australian native but also potentially serve as an ecological substitute for Varanus priscus.
Transitioning to greater, but still short geological timespans, a major tectonic event will likely reach its climax in a few million years: the separation of Eastern Africa from the rest of the continent. This process is ongoing as we speak and has already resulted in major geological landmarks across East Africa. Though we will not live to see the end result, we are already living through and being affected by the works at play. Importantly, this rift is associated with three hotspots and, as the landmasses grow apart and water rushes into the newly created seaway, significant volcanism might occur. It is impossible to say with certainty the intensity of these hypothetical eruptions, but it is possible they will be enough to usher in a warm spike. These hotspots, in the past, have already erupted, creating large areas of exposed magma.
Additionally, increases in temperatures such as this one have occurred before in the Cenozoic, such as during pronounced activity of the Yellowstone hotspot, both mentioned here. Besides, subduction zones across the Atlantic coasts of the Americas and of the Pacific Rim may generate, in about 25 million years, new mountain and island chains as well as more volcanoes, which might potentially contribute even more to outgassing and thus to the possible warm spell also brought about by the East African rift. Conversely, though, the East African separation might not last long, as Africa has been moving up towards Europe, and it may compress the newly nascent seaway, uniting both landmasses once again.
These rises in temperature might shift the Earth away from the cooling that gradually developed across the Cenozoic, ushering in a new greenhouse period. This would affect all sorts of ecosystems across the world, and it is hard to say how the “resurrected” megafauna would fare under these conditions. Certainly, their most vulnerable members would be hypercarnivores, which, due to their trophic position, occupy the most thermodynamically precarious spot in food chains: since they are the last consumers, they receive the lowest portion of the original energy fixed into carbon by the photosynthesizers. Marine environments, however, would likely be the most dramatically hit, potentially suffering a major reorganization as a result of the reversal of the cold, productive seas of the late Cenozoic, but it would also lead to many estuary/wetland ecosystems in areas such as the Amazon and Bengal basins, as former rivers and deltas get flooded by the advancing sea. Most dramatically, though, Antarctica could find itself once again free of ice caps, leading to even greater levels of sea rise and, consequently, of warming, as liquid water absorbs more sunlight, while ice tends to reflect it.
The impact of the oceans would plausibly lead to the downfall of large, filter-feeding whales (assuming they had not gone extinct before), which attained such large sizes as a result of the increasingly cold seas by the end of the Neogene and beginning of the Quaternary, as better explained in this tale. Diatoms could potentially be heavily hit as well, and maybe they would become less dominant members of the phytoplankton, with the ascension of other organisms better suited to hotter conditions, such as cyanobacteria and green algae. The expanded estuarine environments would potentially constitute an interesting interface for new marine dwellers, which would potentially assume vacant niches left behind by various cetaceans and other organisms unable to cope with the higher temperatures.
Crocodilians, which already dwell in these transitional habitats, could perhaps return to the seas, a feat that would be aided by the higher global temperatures, more amicable to their ectothermic metabolisms to some degree (as will be mentioned later). Over millions of years, they could potentially even become as specialized as the thalattosuchians of the Mesozoic, with smoother bodies and flippers, beings indissociable from the water. Additionally, squamates could also fill the niche, echoing the trajectory undergone by mosasaurs in the Mesozoic, which expanded into the ocean in the aftermath of the extinction of ichthyosaurians and many plesiosaurians, an extinction that, as explained here, took place amidst the context of an intense rise in global temperatures. Again, the warmer climate would probably prove amicable to these mostly ectothermic reptiles, which already have seafaring representatives, such as marine snakes.
Besides these conceivable reptilian pioneers, some marine dwellers would find themselves on better ground, especially jellyfish and cephalopods, which have already shown increased growth with the modern-day, anthropogenic global warming. This swelling of their numbers, potentialized by a destabilized aquatic biosphere, might lead them to diversify into new forms and even occupy new habitats, such as the frontier environments of the estuaries. These might offer the proper boundary conditions for cephalopods to colonize freshwater, for instance, which could conceivably lead to a great diversification on their part, potentially even with headways into partially terrestrial environments. Currently, octopuses already get out of the water under certain conditions, and, assuming some of them go on to live in freshwater habitats in much more contact with the land, it is natural to believe some might develop a progressively amphibious lifestyle.
Elsewhere, these hybrid locations might lead to even more intermingling of the terrestrial and aquatic domains. In the Amazon basin, for instance, sloths, which are already capable swimmers, might turn to aquatic lifestyles, consuming, for instance, underwater vegetation. This is by no means unheard of, as, remember, dear reader, the Thalassocnus were seafaring giant sloths. Capybaras (Hydrochoerus hydrochaeris), which in current times are already amphibious creatures, might see some individuals in their populations become ever more adapted to the water, potentially even turning manatee-like with time. Fire ants (Solenopsis), which are another Amazon native, can form living rafts, formed by their sheer collective power.
While these, in the modern day, are a survival strategy for these aggressive hymenopterans, the flooding of great parts of the Amazon could offer opportunities for some of these ants to become naturalized water farers, especially as the aquatic realm would offer these insects an area free of competition with fellow ants and with plenty of food opportunities. In a way, they could even resemble a Portuguese Man O'War (Physalia physalis), a cnidarian that is actually composed of many, smaller individuals known as zooids. In this sense, castes could grow even more specialized, as some could develop into rowers (perhaps with wide, row-like limbs), others into buoys (perhaps with inflated abdomens, rich in buoyant materials), and others even into divers, which, like diving beetles, might keep air bubbles that allow them to stay underwater for some time.
In the inundated Bengal basin, other creatures would certainly adapt to the new flooded conditions. Tigers (Panthera tigris), though hypercarnivores, exhibit a very ample distribution, from tropical rainforests to cold, conifer woodlands. Consequently, even in a warming world subject to many ecological perturbations, it is conceivable they could survive in some shape or form. In the context of great marine transgression into this Southeast Asian region, some of these big cats, which already have a reasonable affinity for water, may become semiaquatic in habit. With their robust bodies, they could begin to act somewhat like mammalian crocodilians, waiting in water to ambush unsuspecting prey, perhaps even going after crocodilians on a more frequent basis, as the jaguar (Panthera onca) already does with caimans.
Regarding other Bengali animals, the Indian elephants that choose to stay in the waterlogged habitats may become more familiarized with the aquatic conditions and, in a more conservative manner, perhaps become more akin to Sri Lankan swamp elephants, Indian elephants with wider feet, apt for crossing mushy ground, as well as longer legs, keeping them even over deep water. At the same time, though, especially considering 25 million years of evolution at play, more derived body plans could also develop, with some maybe becoming more hippo-like, adapted to cross the bodies of water on their bottom and thus granting them higher access to foodstuffs as well as a new environment with reduced competition. These more aquatic variants could even resemble early, basal proboscideans like Moeritherium, which were far shorter as well as stockier than their hulking, giant relatives of today.
Many already aquatic birds might change or have some of their traits exaggerated. The snake bird (Anhinga), a native of the Americas, already exhibits a hunting behavior that involves getting completely submerged, lacking waterproof feathers, which, by getting completely soaked, make it denser, something accomplished with the help of its also denser bones. While many populations of these birds might retain much of their current habits and flying ability, others, as their homes get flooded by advancing seawater, might lose the ability to fly altogether, as did penguins. Indeed, this would not be a brand-new adaptation for these birds. During the Neogene and the Quaternary, the snakebird genera Meganhinga, Macranhinga, and Giganhinga were all larger than their modern-day relatives, as well as likely flightless, showcasing how this evolutionary path might actually be quite likely. The Anseriformes, an order of birds known as waterfowl (including ducks, swans, and geese), already have some saltwater-dwelling representatives, but their numbers might increase dramatically as transitional environments develop.
Other marine-dwellers, apart from cephalopods and cnidarians, would likely be even more poised to take advantage of the expanding seas as they penetrate deep into continents (though the Amazon and Bengal basins were the ones subject to a more detailed view, seaways would also develop in other areas of the world, such as in the southeastern United States, southern Australia, and Northeastern China). Some of these are bull sharks (Carcharhinus leucas), fairly large sharks that often enter deep into freshwater habitats while also easily traversing saltwater ones. This is thanks to several adaptations these chondrichthyans have, with specialized kidneys apt for regulating the osmotic stresses associated with making these transitions. Not only do these make them exquisitely adapted to explore and colonize these new bodies of water, but these cartilaginous fishes also breed more often in warm climates.
Thus, they would be poised to thrive under a waterlogged and hot world. Not only would these fish benefit from more estuarine environments, as these habitats constitute great nurseries and offer plenty of hiding as well as feeding opportunities, being thus great spaces for fish closer to the bottom of food chains too, from lampreys to the more familiar ray-finned fishes. In less muddy waters than those of estuaries, corals, though initially suffering from the stresses associated with increased temperatures (mainly bleaching, as mentioned here), could ultimately benefit from a rise in shallow, well-lit seas, appropriate for nurturing their dinoflagellate symbionts with plenty of sunlight.
Of course, it would not be only animals influenced by these changes. On the contrary, plants would also see tremendous changes. While many would undoubtedly suffer great losses as the areas they occupy become inundated, many others would suddenly find various areas adequate for their growth. Certainly, some of the most benefited would be mangroves. These habitats, typical of estuaries, are formed by, in turn, very atypical plants, capable of dealing with conditions not usually tolerated by their peers. More common along warmer coasts, the higher global temperatures would likely additionally benefit these phototrophs, which possibly arose during the Early Cretaceous, later separating into Western and Eastern populations as the continents grew further apart. Curiously, not all mangrove species are angiosperms, as one might initially believe. Though the majority are indeed part of the flowering and fruiting plants, some are actually ferns!
Regarding the adaptations these eukaryotes have for their habitats, some of the most well-known are their roots. Instead of diving deep into the saturated and poorly oxygenated substrate, they instead grow in more horizontal positions, leaving them anchored to the soil, and vertical, allowing them to be in contact with the air and thus conduct gas exchanges. Their leaves are also especially adapted, being generally glossy and thick, apart from containing sheltered stomata (such as among trichomes or covered by horn-like protuberances for example) that help reduce transpiration and conserve water (while this may seem counterintuitive, for these plants live in extremely flooded environments, it is actually a necessity due to the liquid being very concentrated as a result of high amounts of salt as well as other minerals). The roots themselves also have mechanisms to deal with the high salt load, like ultrafiltration systems, or alternatively, water is not filtered by the roots, but the leaves possess salt glands that excrete the compound. There are even other mechanisms for dealing with saltwater, such as storing salt in specialized leaf vacuoles or simply in wood/bark or synthesizing other solutes to maintain osmotic balance within their tissues.
One of the most unique mangrove adaptations refers to their reproductive strategies. Many of these plants exhibit forms of viviparity, in which seedlings remain attached to the parent plant for months and even up to one year, during which time they develop tolerance to the increased salinity of their growth medium. After this period of maturation, the seedlings fall into the water and are capable of surviving for some time on the liquid before they get stranded and start setting down their roots. Not only do these plants constitute important shelters for many animals, be they fish, mollusks, or crustaceans, but they may also help other plants as well, such as seagrasses, analyzed in detail here. Such monocots are often found associated with mangrove forests, and part of this may be a result of the discharge of carbon dioxide as well as of other substances originating from the aforementioned mangroves, though this likely does not influence the growth of seagrasses that much. Either way, seagrasses, as corals, probably would also be favored by an increase in shallow and illuminated habitats, with underwater meadows becoming more common and widespread.
Australia would also likely undergo some significant changes, potentially reversing the drastic arid trend that it has been subject to for most of the Cenozoic. Such increased aridity was associated with two main factors. One of them was a decrease in sea levels accompanying cooling, as this causes better fluvial drainage with subsequent fewer swamps and floodplains. The other one was the cooling of waters around Australia, caused, in great part, by the isolation of the Antarctic continent and its subsequent refrigeration, as will be mentioned further in this entry and has been mentioned here. The colder waters evaporate less and thus less moist air masses penetrated into the Australian mainland, kickstarting an aridification process that progressed with many ups and downs along the following millions of years, especially during the Quaternary, when a constant alternation of glacial and interglacial periods frequently reshuffled sea levels, leading to an overall chaotic climate. Despite these alternations, the vegetations adapted to more humid conditions were gradually but surely squeezed into ever-tighter areas, regaining some ground when conditions became wetter but not enough to account for their losses. However, in a future where the planet is warmer and Australia has moved farther away from Antarctica, these conditions that set up the precedent for aridity would be mostly gone, with high sea levels causing inundations deep in the landmass and compromising the drainage of rivers, generating backflows that would translate into waterlogged areas, and with warmer waters surrounding the continent. Consequently, one could expect a great expansion of rainforests to levels seen much earlier in the Cenozoic, with corresponding changes in fauna, which, currently, is in great part adapted to open and dry areas, not so much to closed and humid forests.
Macropodids are a marsupial family containing kangaroos, wallabies, and tree kangaroos, as well as their fellow relatives. Curiously, these animals descended from arboreal ancestors and many, such as the kangaroos and wallabies, became increasingly adapted to the ever-drying climate of Australia, hopping across the vast grasslands. However, not all of them underwent this transition. Others, such as the tree kangaroos, became arboreal once again, elusive animals that live high on forest canopies. As the Australian climate shifts once again, not only may these arboreal mammals expand in distribution as well as population, but perhaps other macropodids would once again make the transition to trees. Alternatively, many may not, keeping their terrestrial lifestyles, but likely undergoing at least discrete physical changes to cope with environments far harder to maneuver in.
Wombats, fellow marsupials related to koalas (Phascolarctos cinereus), to the large Diprotodon (a now extinct member of the Australian megafauna, wombat-like, and the largest marsupial to have ever lived), and to the carnivorous marsupial lion, might also fare quite well. These rotund creatures are fossorial and have some of the slowest metabolisms of any mammal, being distributed around various environments in Australia, having few nutritional requirements, and thus adapted for environments in upheaval. The common wombat (Vombatus ursinus) in particular may be the most favored, as it has a preference for forested areas. Their koala relatives, however, might not have as much success due to their great dependence on Eucalyptus trees. These are more adapted to drier environments and might see some reduction as the landmass wettens and rainforests become more abundant. Even so, they will probably still be present and still constitute an important part of the Australian flora. About other successful herbivores in Australia, such as the invasive camels, they likely would not fare very well in the new climate, assuming they would not be exterminated by humans before that. If any are alive to see the Australian climatic shift, they will likely either retreat to remaining dry areas or simply vanish. Extinction is, after all, as normal a part of evolution as is adaptation.
Some primarily grassland-dwelling species would likely still be able to survive. One example is the emu (Dromaius novaehollandiae), a large, flightless bird that inhabits the entire Australian continent, having a nomadic lifestyle and frequently traveling between different biomes. As such, even in contexts of rainforest growth and expansion, these iconic omnivorous theropods would likely continue being an important member of the biota, though perhaps in not so expressive numbers due to the reduction of their preferred habitats. Other birds, such as the large kingfisher kookaburra (Dacelo), owner of a striking and well-known call, may face similar situations. While nesting mainly in forests adapted to drier conditions, such as ones made of Eucalyptus, these predatory birds with powerful beaks may be found basically anywhere where trees are large enough to build their nests on, justifying their likely continued existence.
Regarding predators, these are also highly dependent on humanity's actions on the continents, such as the possible reintroduction of komodo dragons as well as of tasmanian devils as part of rewilding programs or the reengineering of animals such as the thylacine. There are also invasive carnivores, such as the dingo and wild cats (Felis catus), which, actually, are domestic cats gone feral, introduced by European colonizers. The latter in particular has spread in all Australian environments, being extremely efficient predators of smaller animals. Even in a changing Australia, they may be in quite an advantageous position due to their broad range and adaptability, making them apt at thriving in disturbed areas. If also not controlled by humans, these might even diversify into different forms, perhaps some growing a lot larger and generating an endemic lineage of Australian big cats, prowling the jungles and waiting for the perfect moment to strike unwary victims. Dingos are also highly adaptable and will plausibly maintain their status as apex predators, though perhaps with more competition from reintroduced hunters as well as from cats increasing in size. Unlike cats, which are mostly solitary hunters, these dogs hunt in packs and so increases in size are possibly not as plausible, as taking down prey is not dependent on individual strength alone, but on cooperation as a whole.
When mentioning Australia, one cannot forget monotremes. These unusual mammals, like the wombats, also have slower metabolisms, but their other traits could not be more different, with they having diets composed exclusively of invertebrates. Platypuses (Ornithorhynchus anatinus), which are semiaquatic and distributed through freshwater environments, might be benefited by an increase in watery areas, but, at the same time, higher temperatures could also limit their ranges (while higher temperatures often make certain environments more amicable for ectothermic animals or those with lower endothermic metabolisms, they, when tipping over a certain point, can actually be dangerous by excessively increasing their metabolisms and surpassing their abilities to deal with overheating). Even so, these creatures possess sweat glands and so have an additional thermoregulating ability that, if increasingly developed, might help them deal with the climatic stresses, as well as other possible physical developments, such as a reduction in size.
Certainly, however, the Earthly continent that would undergo the most drastic transformation, with the largest environmental and biotic repercussions, would be Antarctica. Gripped by unreleting ice since roughly the middle of the Cenozoic, the frozen southern continent was formerly vegetated and with much more exuberant flora and fauna. By 25 million years into the future, it is possible that the warming climate could trigger a meltdown of the southern ice caps, a process that, as mentioned before, would further augment the greenhouse state by reducing the reflectivity of the Earth's surface. As ice melts, a desolate land would become ripe for colonization by all sorts of organisms. While the continent already has some resident lifeforms, many more would likely arrive and occupy the new lands made available. Probably, most of the immigrants would come from nearby regions, especially the Southern Cone of South America, since Australia would be in a more northern position, as just cited.
Before its glaciation around the Oligocene epoch, Antarctica had, at least in its coasts, a vegetation composed especially of plants such as Nothofagus (which, observed in this tale, currently has a rather limited distribution), conifers, like Podocarpaceae and Araucariaceae (the latter of which is much more restricted than the former), as well as ferns, and horsetails. Even photosynthesizers more typical of warm areas, like palm trees, could also be found. With time, though, as the land became ever colder, Nothofagus trees became more shrub-like and, instead of forests, tundras installed themselves, with a predominance of mosses. Eventually, however, even these landscapes were completely eliminated by advancing ice sheets. As these retreat, perhaps similar vegetated environments could emerge. Initially, the barren soils would be occupied by typical pioneer organisms, such as lichens and mosses. As these gradually degrade the substrate and enrich it with organic matter, other lifeforms would be able to establish themselves. Nothofagus, which tended to form vast, monospecific forests under temperate and humid climates, could perhaps reclaim some of its former importance. Similarly, araucarians could also find refuge in the austral landmass, rising in numbers and perhaps improving their current, rather precarious position.
Antarctica's more interior areas have higher elevations and, though the continent will probably be heavily infiltrated by seaways should its ice caps melt due to many areas below sea level (reducing the influence of continentality), these mountainous portions could create a rainshadow effect, leading to humid and cool temperate forests on the coasts, but a drier interior. At the same time, though, the heating of the continent would allow its air to hold significantly more moisture while at the same time dissipating the permanent high-pressure zone that has installed itself at the continent's center due to its extremely low temperatures. This high-pressure zone pushes air outwards and, coupled with the low moisture in the air as well as with continentality, and the region's high elevation, all contribute to the hyperarid conditions currently found at the center of the frigid landmass. Consequently, apart from drastically more hospitable temperatures, humidity would likely be significantly higher, though plausibly not as high as that of the coasts, which would be even more benefited by increased water vapour resulting from the warmer waters surrounding the land.
All of these great changes would also translate to equally great changes in terms of fauna. Without a doubt, the creatures most poised to take advantage of the newly available lands would be those already on the continent or easily capable of getting there. Those already on the continent include birds and several marine mammals, apart from the largest, purely terrestrial Antarctic animal: the tiny Antarctic midge (Belgica antarctica), spanning from 2 to 6 millimeters in length. Unlike many of their dipteran relatives, these insects are completely flightless and incredibly attuned to their extremely hostile habitat both through behavioral and physiological mechanisms. Their larvae, segmented, worm-like organisms, live under snow, where temperatures are actually milder due to the buffering provided by the ice covering. There, they consume a very opportunistic diet, ingesting bacteria, algae as well as fungi. However, during the harshest Antarctic periods, they become inactive and undergo metabolic changes that allow for their survival.
Some of these refer to cryoprotectants, especially sugar-alcohols, such as erythritol and glycerol. These dissolve in water and impede it from transforming into ice even below freezing temperatures. Not only do they avoid crystal formation, which can be disastrous by damaging cellular structures and thus tissues, but they also bind to proteins, preventing their denaturation (loss of normal protein structure under certain conditions), and to cell membranes, protecting them from damage. Additionally, these cryoprotectants help the dipteran survive against desiccation, another threat faced by these hardy arthropods in the arid Antarctic landscape. Apart from these compounds, B. antarctica also produces heat-shock proteins, which stabilize other structures in contexts of both freezing and warming. Counterintuitively, under snow, temperatures can rise significantly once there is more exposure to sunlight, and, as a result, adaptations, such as these proteins, to deal with this additional stress have also developed in these insects. Despite this, however, they are quite intolerant of higher temperatures, and constant exposure to 10 degrees Celsius for a week leads to larval death. In the cases where the larvae survive, they become skinny, black adults that do not feed at all. Instead, they live only for two weeks, during which they congregate in large groups to mate.
This goes to show that the warming of Antarctica would not be initially beneficial for these invertebrates. Instead, it is possible they would go completely extinct or progressively relocate to more upland habitats, where temperatures would be more similar to those they are currently accustomed. Alternatively, though, some might actually adapt to the warmer conditions and actually diversify, as the environment becomes less seasonal and overall more hospitable, with plenty more food opportunities. This could be the rise of a great Antarctic midge radiation, as they would get a head start over other insects. This could even lead them to more closely resemble their more typical midge relatives (all members of the Chironomidae family, more specifically known as the non-biting midges), which usually have aquatic larvae and winged adults. While the mature forms are also usually non-feeding or just consumers of nectar, the larvae have a wide variety of food habits. Some are filter-feeders (using silk nets to capture suspended food particles), others actively chew into vegetal matter, others even dwell along the substrate (scraping off bacterial and diatom growths from underwater surfaces), while some are predators (feeding on fellow midge larvae or on other small invertebrates). All of these various lifestyles could manifest from a single Antarctic midge common ancestor, leading to a purely austral lineage of these insects.
This lineage would likely prove pivotal in helping the nascent Antarctic ecosystems establish themselves, as these arthropods often constitute some of the most abundant invertebrates in the areas in which they occur, often in some unusual environments, not only in frozen Antarctica, but also in high-altitude environments and even saltwater ones. Of course, some midges might come in from outside the continent, further helping the nascent biota and incrementing the diversity of this dipteran family at the thawing landmass. Regarding the larger animals of Antarctica, it is possible the continent would emerge as a very much larger New Zealand, which, though spousing a possible native lineage of small, terrestrial mammals of Gondwanan origin that, nevertheless, went extinct during the Cenozoic, was only colonized by terrestrial members of Mammalia after humans arrived (with humans being terrestrial mammals themselves). Before then, theropod dinosaurs, in the form of birds, were the main ground-dwellers, with representatives such as the kiwi (Apteryx), the kakapo (Strigops habroptilus, a flightless parrot), and the giant moa (Dinornis), just to name a few. The only mammals present on the islands were either pinnipeds/cetaceans or bats, which, like birds, had easier access due to their ability to fly. Curiously, due to the lack of more typical ground-dwellers, an endemic family of bats, called Mystacinidae, actually became semi-terrestrial, foraging on the ground instead of flying, conserving important energy in the process.
This New Zealander example offers an interesting model regarding how a fairly inaccessible land mass could develop a very unique and unusual fauna, especially due to the lack of terrestrial animals normally found in other areas of the world. One of the reasons Antarctica might actually constitute an even bigger hurdle for non-flying colonizers are the ocean currents around it, which leave it largely isolated from the rest of the world (one of the reasons for it being so cool in the first place) and make the waters around it quite productive even under a warmer world (potentially making the southern pole one of the last refuges for baleen whales for instance). As a result, rafters would have a hard time getting to the thawed continent, leaving it open only for fliers or swimmers. With this in mind, it is plausible that penguins would become some of the first colonizers. Though these birds are exclusively carnivorous and very adapted for aquatic life, it is possible some would start dwelling ever more inland, especially in the search for shelter. Some, from there, could start consuming freshwater fish, sustained by the midges and brought by flying birds as eggs from distant areas (of course, some saltwater fish could also begin exploring freshwater areas as well). Some of these fluvial penguins could perhaps begin to consume the ever more abundant plants, perhaps those tied to aquatic environments initially, but, over time, shifting to terrestrial ones. As millions of years go by, some could have turned into completely terrestrial megaherbivores, akin to the moas of New Zealand or to the elephant birds (Aepyornis) of Madagascar. Others could become smaller, more opportunistic feeders, akin to the kiwis.
Of course, penguins would not necessarily follow this trajectory, and not necessarily only penguins, but they already are extremely abundant birds on the southern continent, apart from already being flightless, which increases, from the start, their habits as ground-dwellers. The transition from carnivory to herbivory might seem radical, and it truly is, but this transition has already been undertaken by several other creatures, including several theropod dinosaurs. Therizinosaurs and ornithomimosaurs, seen here, were descended from carnivorous ancestors. Animals like Elaphrosaurus, which displayed full herbivory once adults and seen in this tale, also had carnivorous ancestors. There are other examples, and one of the most dramatic may be that of the giant panda (Ailuropoda melanoleuca), a herbivorous carnivoran that, descended from carnivorous creatures, still exhibits a gut configuration highly typical of meat-eaters, though 99% of its diet is composed of bamboo.
Though some penguins as well as other avians might develop in the direction of herbivory, other birds might actually become more carnivorous and predatory, especially as bird numbers in Antarctica swell with new immigrants and upscaled reproduction from natives due to more hospitable conditions. Some of these are the skuas (Stercorarius), opportunistic birds that can also be found in the Northern Hemisphere. These animals, with downturned, sharp beaks as well as fairly robust bodies, are very aggressive and brutish in nature, often consuming the young of other nesting birds and even some adults as well, apart from readily scavenging and even stealing milk from lactating seals. Apart from these terrestrial foraging habits, they also catch prey at sea, especially during winter, when the weather in Antarctica becomes too harsh for continued habitation. Due to these habits, it is plausible that some of these theropods, which largely resemble seagulls with darker colors and more prominent beaks, may become more specialized for a predatory lifestyle on land, especially as prey there becomes ever more abundant. If some of that prey also gets bigger, such as the potential moa-like penguins, then they could also get bigger. Over time, they may lose their webbed feet and develop broader, but shorter wings that more adequately maneuver in the highly forested areas, apart from maybe even evolving strong talons to tackle prey while their powerful beaks land the killing blow.
Indeed, the Haast's eagle (Hieraaetus mooeri) occupied a very similar niche as the largest predator of New Zealand, pursuing and consuming giant moa amidst forested environments. Ironically, it, the largest eagle that ever lived, was part of a group of small eagles that include, for instance, the little eagle (Hieraaetus morphnoides), the smallest eagle known. While its relatives are adapted for transversing and hunting in open areas, they, after reaching the New Zealand islands from Australia, eventually changed their bodily proportions, not only growing much larger, but proportionally shortening their wings, aiding in the aforementioned maneuverability. Additionally, the ginormous eagle was not the only aerial hunter of the avian islands, for the Eyles' harrier (Circus teauteensis), the largest harrier to have ever lived, was also present. It, however, was far smaller and more lightly built than the eagle, likely hunting also smaller birds while in the air. This just goes to show how the lack of typical predators not only dramatically changes prey species, but also those that eventually arise to eat them. Consequently, Antarctica, in the future, is also setting up to be a land not only of strange ground-dwellers, but also of unusual hunters, like nowhere else in the world.
Apart from skuas, there is an even more aggressive and predatory bird on the frozen continent: the southern giant petrel (Macronectes giganteus). While consuming lots of carrion, even those of whales and seals, which it is able to dive deep inside thanks to a large, powerful beak, it often preys on fellow birds, including some the size of albatrosses. More apt at walking on land than its relatives, it constitutes an ominous presence for fellow nesting avians and even for those on the wing, as it can catch and kill them by violently ramming them onto the sea surface. Suffice to say, some of these animals, those that dwell progressively inland, may also undergo similar adaptations as those of the more terrestrial skuas, perhaps engaging in some sort of niche-partitioning or just competing. Probably, predatory birds on the thawed Antarctica would not be exclusively those already on the landmass, but these would certainly get a head start and, due to their current habits, already show some predisposition towards those lifestyles.
Though birds might become the dominant animals even in an unfrozen Antarctica, mammals would undoubtedly have some presence. Bats, as previously mentioned, would be capable of accessing the continent due to their volant capabilities. Once arrived, and with fewer ground predators, some could perhaps follow a similar path to that of the endemic New Zealander family Mystacinidae and become increasingly terrestrial. Due to much greater resources offered by Antarctica as well as an also much greater variety of landscapes, perhaps a bat radiation would occur, with, most interestingly, the rise of several, maybe fully ground-dwelling forms. These could be amazing in both diversity of body plans as well as lifestyles, perhaps even niche-partitioning with most birds by being only active at night or, in other cases, competing with or even predating on some of them.
Just as many other areas of the planet would be possibly enriched by transitional habitats as a result of intense advances of the sea into the continents, Antarctica would be no different, especially due to its previously mentioned low-lying areas, which would be very susceptible to infiltrating seawater. These habitats could offer, for instance, diversification opportunities for many cetaceans, particularly when considering that the reptilian seagoers cited before would likely not have a tolerance for colder waters should they not develop endothermy (a real possibility nevertheless). By providing several gradients between full sea and land, such inundated areas could lead to the formation of highly-specific communities, apart from facilitating access to freshwater and, thus, the evolution of some cetaceans in line with that of current river dolphins, for example. Additionally, pinnipeds would also be impacted by these changes and perhaps in various similar ways. Leopard seals (Hydrurga leptonyx), which are already top predators of the Antarctic, would maybe follow their prey deeper inland as well, adapting to freshwater and maybe acting somewhat like crocodilians by becoming riverside ambush predators.
By 50 million years into the future, more radical continental changes will have possibly taken place. As Africa continues moving north, the Mediterranean will probably slide under it as a great mountain chain, extending from Morocco to deep into Europe, forms, combining nascent orogenic processes as well as the intensification of older ones, such as the Alps, which already are a product of the ongoing collision between Africa and Eurasia (this process has already led to the closure of the Mediterranean before, during the Miocene, but this time it would be much more permanent). Not only will the Mediterranean be completely obliterated, but so will be the Red Sea, since Africa will potentially also push against Arabia, forming yet another mountain chain along what is today the western Arabian coast. These new mountains will possibly generate a similar effect to that of the Himalayas earlier in the Cenozoic, especially as they weather, leading to the absorption of greenhouse gases. This, coupled with a potential rainshadow effect increasing precipitation on Europe (Europe's coast already receives significant moisture and warmth through the Gulf Current, and, though it brings warmth, it actually contributed to the Quaternary glaciation by increasing the amount of snowfall in the North Pole), could be the start of a new ice age as the planet gradually cools down and ice caps return.
The whole world would be affected by this change, which likely would precipitate a string of smaller-scale extinction events as areas such as the Amazon and Bengal basins drained and returned to configurations more akin to the modern day. Oceans would start being more productive again and, perhaps, even among the reptilian newcomers, some would develop adaptations to explore the richer oceans, such as filter-feeding. Alternatively, remnant baleen whales in the southern waters of Antarctica could perhaps regain some of their former range. Either way, Antarctica itself would be the continent most dramatically affected once again, as it, just tens of millions of years before, thawed and would already be on its way to returning to its more than 50 million years of bitter frost. As the ice caps advanced, many forests would likely become tundra or steppes, probably triggering significant physical and physiological changes in the organisms that, until then, were adapted to the humid, temperate forests. In such a context, the more conservative creatures would be the ones to come out on top, since they, mostly retaining the phenotypes of their ancestors, adapted to the unforgiving cold of the continent, would be somewhat preadapted for the new glaciation to come. Regarding other continental changes, certainly the most significant would be the continued northward movement of Australia that, by this time, will have already culminated in a collision between it and Southern China. Undoubtedly, this would trigger a very significant biotic interchange, perhaps with the spread of marsupials into East Asia and the spread of more placentals to Australia.
As the planet advances into 75 million years from now, conditions may return, from a climatic standpoint, to roughly the current ones, meaning Earth would be immersed in yet another ice age. As already hinted at in the preceding paragraph, Antarctica would bear the brunt of the cooling, tragically returning to its frozen state and cutting short the amazing diversification of life that had blossomed from its shores. Elsewhere on the planet, the African-Eurasian collision would intensify, with mountains reaching even higher peaks, while the merging of Australia with East Asia would potentially proceed as well. Additionally, the continued westward movement of the Americas would possibly lead, in the far north, to the connection between Siberia and Alaska, bridging the Bering Strait through a direct land route. Also regarding Alaska, western California would, by around this time, be reaching its southern limits, a result of its constant sliding along the San Andreas fault.
In 100 million years' time, the major global shift would once again pertain to Antarctica, as it, moving northwards, will potentially make its way to higher latitudes, warming up and potentially ushering in the end of the ice age, as its melting ice caps and advancing sea levels will lower Earth’s reflectivity, leading it to absorb more of the Sun’s rays. This time, however, the colonization of Antarctica might not be exclusively by birds, bats, and cetaceans. Due to its more northward position, the Antarctic circumpolar current might be disrupted (another factor contributing to its warming), allowing rafters to land on its shores and thus possibly making it far from a New Zealand analog. As time advances, though, predictions become ever harder. Since evolution is cumulative, building on what has happened before, changes also scale up, making tiny transitions perhaps big events. Not only this, but the chances of great extinctions due to more unpredictable events increase as well, such as massive eruptions caused by hotspots or extraterrestrial cataclysms, such as asteroid impacts (though these might be stopped by future humans) and supernovas.
Through the next 150 million years after this point, Earth might remain in a greenhouse state, especially if the continents eventually converge to form Pangea Proxima, a new supercontinent formed as the Americas reverse their westward movement and collide with a combination of Eurasia, Africa, Australia, and Antarctica. As a result of this collision, volcanism would likely ensue, and the world’s landmasses would be concentrated on lower latitudes, forming a large inland sea surrounded by India, Southeast Asia, Eastern Africa, Eastern South America, and a portion of Antarctica. This new aggregation would likely be even more extreme than the late Paleozoic-early Mesozoic Pangea due to the Sun’s increased luminosity, triggering even more warming apart from the increased greenhouse gases. This, coupled to the great effects of continentality on the interior reaches of this supercontinent, might create deserts of unparalleled vastness, immersing the world in a scenario similar to that of the Great Dying, when temperatures turned hot enough to eliminate a staggering percentage of both terrestrial and aquatic life. The oceans, of course, would also be hardly hit, likely even harder. Waters would become hot, stagnant, and acidic, apart from possibly anoxic in some areas, as a result of significant decreases in dissolved oxygen. Consequently, the marine biota would be cleansed, undergoing massive upheaval and leaving the stage open for new organisms to fill vacant niches, though there might be far fewer to fill.
These new climatic and geographical conditions might, after all, prove incompatible with current, more complex forms of life. This is the case especially due to thermal limits on functional photosynthesis, which begins to fail at temperatures between 40 and 60 degrees Celsius. With plants out of the picture, many areas might only sport miniature biotas, in which the producers are those still capable of executing photosynthesis even at extremely high temperatures, such as some algae and cyanobacteria, potentially forming ecosystems very much like the biocrusts previously discussed here. In these miniature scales, faunas, protected by the water-retaining architecture of photosynthesizers and fungi, might be restricted to select, extremophile creatures, such as tardigrades, which already show various adaptations for survival in harsh conditions that are uninhabitable for most other animals.
Surprisingly, even tardigrades are not so thermally tolerant, and, actually, spider beetles (members of the subfamily Ptininae, being small coleopterans with very rotund abdomens, justifying their similarity to spiders) survive even higher temperatures, also being capable of undergoing desiccation and living. As such, they might represent a future phenotype of beetles and similar insects in these environments at least, since many of their physical characteristics, including their rotund shape, help them conserve water and live in exceedingly dry habitats.
Of course, this is considering modern-day physiological limits. With time, especially as temperatures gradually increase, a few plants might develop mechanisms to ensure the continuation of photosynthesis even under such climatic constraints, opening up the possibility for larger-scale ecosystems even in very hot areas. Despite this, animals, on their own, will face similar existential issues with such temperatures, with even endotherms, which are capable of maintaining temperatures different from those of the surrounding environment, reaching their physiological tipping points. As such, only very few might be able to “openly” dwell on the arid and heavily insulated expanses of Pangea Proxima, joining the limited ranks of the tardigrade-like extremophiles. Many might actually adopt only a partial tolerance to the extreme conditions by exhibiting specific behaviors, such as only going out at night or living their whole existence in burrows. Despite this desolate scenario for most areas of the supercontinent (which will likely constitute a mass extinction event rivaling and plausibly even surpassing the Great Dying), higher latitudes, as well as more mountainous locations, will plausibly offer more amicable temperatures, allowing for the continued existence of more familiar floras and faunas.
With time, though, the weathering of newly formed mountain chains as a result of the assembly of the supercontinent would start a gradual process of cooling, one that would allow, over millions of years, for the recolonization of lower latitudes by higher lifeforms, bringing the planet back to a more familiar state. However, things could once again take a turn for the worse with the breakup of Pangea Proxima, which would likely generate new and intense bouts of volcanism, probably leading to higher temperatures once again and a new mass extinction. More optimistically, however, an alternative model of supercontinent assembly predicts the formation of Amasia, a gigantic landmass centered on the North Pole, which, unlike Pangea Proxima, would lack Antarctica in its composition (it would stay over the South Pole). This boreal positioning would, even in the context of volcanism and increased Sun luminosity, possibly trigger an ice age that, though maybe very bitter, would preserve habitability much more than the scorching deserts of the alternative. Interestingly, the world was in an ice age when Pangea itself formed, and, thus, maybe some similar events could happen, such as the colonization of former sea shelves by vast forests and the formation of basins where plant parts are rapidly buried, events that, in the Carboniferous, led to spikes in oxygen and, eventually, to even more cooling.
After the eventual breakdown of these supercontinents, the world could go in a lot of different climatic directions. Maybe these would favor glaciation, as happened during the Cenozoic, or maybe they would just maintain a scorching greenhouse, though potentially more habitable as a result of less land being concentrated in the tropics. No matter what the result may be, the increase in solar luminosity will eventually lead to the same end result: a return or simply a continuation of an even more unbearably hot global climate. This will take place gradually and leave time for life to perhaps stretch its physiological limits, as, over the next hundreds of millions of years, the planet’s landscape will be irreversibly changed, as even oceans begin to evaporate under the rays of an ever-brighter Sun. As this takes place, the atmosphere will be filled with great amounts of water vapor, a greenhouse gas that might intensify the warm climate, leading to a runaway greenhouse effect. Concomitantly, though, the intense cloud cover that would form might actually mitigate some of these effects by increasing the planet’s reflectivity.
In a fairly strange turn of events, greenhouse gases such as carbon dioxide may actually be increasingly sucked out of the atmosphere, as unprecedentedly high temperatures increase the weathering of rocks. As such, any surviving plants, those that would develop some mechanism guaranteeing functional photosynthesis even in very warm climates, would also face the challenge of diminishing carbon, likely triggering a new wave of extinctions and, perhaps, the development of the hardiest plants of all, the last ones Earth will ever see. Despite this, other phototrophs, such as cyanobacteria and algae, are still capable of carrying out photosynthesis even under very low carbon dioxide concentrations, eventually leaving them as the last of the photosynthesizers in a dying world. Curiously, the atmosphere might remain one of the most hospitable habitats on this desolate planet. In the modern world, there is already plenty of microbial presence in the air. The Pseudomonas genus, for example, a diverse collection of rod-shaped gammaproteobacteria with polar flagella, has been found in rain, snow, and even samples directly from clouds. Not only are they present in these environments, but they also possibly play important roles in them, since proteins in their outer membranes provide a substrate on which ice crystals form. This, in turn, can influence processes such as rain and hail, as larger ice particles accumulate enough water to the point that it precipitates. Not only this, but they may also constitute important centers for the condensation of water vapor into liquid droplets in warmer clouds, making them of fundamental significance for these meteorological processes.
Even more amazingly, the stimulation of precipitation events may actually have been an evolutionary pressure, as clouds constitute hazardous habitats, subject not only to dangerously low temperatures but also to large amounts of UV radiation, apart from reactive oxygen species and other threats. Consequently, in order to return to land and proliferate, these bacteria might have developed these precipitation-stimulating mechanisms, being carried by what falls from clouds into more hospitable, terrestrial areas. In this scorched future Earth, however, temperatures in clouds might become much more hospitable, likely much better than the hot lands below. Additionally, the enrichment of the atmosphere with water vapor might make it significantly denser, allowing larger organisms to spend their whole lives airborne. With these conditions put in place, perhaps a diversification of aerial plankton could take place, maybe even with extremely derived plants that live as small forms containing materials of low density, like fat reserves or air bladders, apart from possible spiculated projections that increase drag and make it harder for them to fall downwards. Similar adaptations could also be present in eukaryotic algae, both autotrophs that, due to their bigger dimensions, would have more problems staying airborne than their prokaryotic counterparts.
Of course, UV exposure would be an even larger problem in these atmospheric habitats. As such, many of these aerial organisms would likely evolve highly developed mechanisms to survive even if highly irradiated. Indeed, some Earthly lifeforms already possess incredible forms of radiation resistance, which occur through various ways, such as sophisticated means of DNA repair or of highly efficient scavenging of reactive oxygen species. Even more amazingly, some lifeforms may actually even take advantage of higher exposure to radiation to fuel their growth. One organism that already does something of the sort is the fungus Cryptococcus neoformans, previously mentioned in this tale. This basidiomycete is found in some of Earth’s most irradiated environments, such as high up in the atmosphere and at the remnants of the nuclear reactor at Chernobyl.
With a cell wall containing melanin, it not only resists radiation but potentially extracts energy from it, with the pigment showing enhanced electron transfer properties after being irradiated. In this future world, perhaps yet another symbiotic arrangement between fungal and vegetal cells could emerge, some of which were already addressed here, here, and here. In this new symbiosis, the aforementioned aerial plants/algae could be covered by Cryptococcus-like fungi, which not only would protect them against radiation but further feed their photosynthetic apparatuses by contributing more electrons. In turn, the fungi would have access to the sugars produced by the autotrophs.
With a solid base of producers, animals would likely make their way to the air as well. It is practically impossible to say which creatures would be alive to make the transition into a life of potentially eternal flying, but these would likely be very aerodynamic in shape, perhaps with huge, long wings that minimize energy costs, taking full advantage of the surrounding air. Some might evolve into aerial filter feeders, perhaps with large maws to gobble up as many floating plankton as possible, or maybe with net-like mouthparts that could be retracted into their proper mouths after being filled with food. Predators, in turn, would likely be present as well, preying on these conceivable gentle fliers in all manners of possible hunting methods, maybe some with strong raptorial appendages or others simply with potent, piercing mouths. In a very unfamiliar future, life in the air might become much more exuberant than on land, as perhaps the skies become tinted with green in one of the last shows of life’s tenacity. In deeper parts of the oceans, things might be a bit better as well, despite the start of their evaporation as a result of the thermal buffering effects of water. Around hydrothermal vents, life might go on relatively unchanged, perhaps with some newcomers as a result of the broader biotic disturbances going on elsewhere. It is almost poetic to think that where life may have originally started, as discussed in detail in the first entry, is where it also might find some of its most secure holdouts.
Other bodies of water, though, will likely become exceedingly salty as water evaporation progresses and solutes become increasingly concentrated. All around the world, many locations bearing a striking similarity to the Dead Sea of today will develop, forming landscapes that might seem completely alien in a world that is today so lush and vibrant. However, even in these pools of extreme saltiness, some lifeforms might be doing more than just eking out a living and perhaps actively thriving. In the modern world, there are already various examples, ranging from archaea to bacteria to algae, all adopting different ways of dealing with such drastic stresses.
By this time, the most common strategy will likely be the one most commonly seen among halophilic archaea and extreme halophiles in general, that being of the accumulation of high amounts of inorganic solutes in their cytoplasm, such as potassium (in contrast, many bacterial and eukaryotic halophiles accumulate organic solutes within their cytoplasms). This accumulation ensures a balance between the solute concentrations within and outside the cell, preventing it from dehydrating, for instance. Apart from achieving osmotic balance, these beings also have several other adaptations, such as more hydrophobic, negatively charged proteins, robust cell walls with adjustable hydrophobicity, polar lipids that make them less permeable to ions, as well as, specifically in the case of archaea, peculiar lipids with special linkages that also render them more impermeable.
By around 1 billion years into the future, rapid ocean evaporation takes place (with it, tectonics halt as well due to lack of lubrication provided by water), and higher lifeforms go extinct once and for all, making Earth back into a purely microbial world. By then, the more complex organisms might have already been completely extinguished as a result of the decreasing carbon dioxide levels, which, by compromising the photosynthesizers, will eventually lead to ever-lower oxygen levels. The gas, apart from being essential to aerobic organisms, is also required for the production of ozone, and, as such, the ozone layer will gradually disappear, causing the planet to be bombarded by even more radiation, an event that will likely bring even more death to the remaining aerial biota. Even more tragically, the photodissociation of water molecules in the atmosphere may actually provide a last, final pulse of oxygenation that, though brief, might bring more destruction to the Earth’s remnant biospheres as levels of the reactive gas get too high, echoing what happened during the Great Oxygenation Event of way before the Phanerozoic. As the dust settles, though, life, even if extremely battered, plausibly survives in select habitats, now composed of hardy extremophilic microorganisms living at the edge of existence.
Some of these will likely be continuations of the halophiles described previously, eking out in high-altitude lakes (potentially hosted by hotspot volcanoes that, without tectonic movement, grow extremely tall, such as Olympus Mons present on Mars, which has a height of 21 kilometers), where temperatures might still be mild enough for some liquid water (it is also important to consider that this water, with a significant amount of solutes, would also have a higher boiling point). These, though, apart from high salt tolerance, will also need to have developed great thermal and radiation tolerance. One great example of microbial thermal affinity is the archaea Pyrococcus, which, not only a thermophile, is actually a hyperthermophile, thriving in hydrothermal vents at temperatures that exceed 100 degrees Celsius. This coccoid organism, containing several flagella that grant it rapid movement, is also a prolific reproducer and so offers a glimpse of what bizarre microbes might not only survive, but truly flourish in such a harsh world.
Cave systems might harbor more expressive ecosystems than alpine pools, being much more sheltered from excessive UV radiation (though salt crusts could also offer some degree of protection) and, given the right conditions, have more amiable temperatures. The lifeforms inhabiting areas below the continental surface or the former ocean floors, referred to as the deep biosphere, will likely be, however, the most abundant, as they conceivably already are today, since they might actually account for 50 to 80% of Earth’s microbial mass. Either way, both of these environments are and still will be far removed from the Sun’s influence, with their main producers being chemolithoautotrophs. These microorganisms, also widely present in hydrothermal vents (better explained here), utilize inorganic compounds as electron donors (these may range from ammonia all the way to pyrite, a substance formed from iron and sulfur atoms), powering electron chains that have various other elements as electron acceptors (here, the variety is great as well, with these varying from nitrogenated compounds to chlorinated ones). Organisms cannot sustain themselves only by generating energy, and these lifeforms also possess the means to fix carbon and thus grow. This shares the same source of their energy, as the electron chains are also utilized to form cofactors, which then participate in the fixation of carbon.
However, even these highly specialized and extremophile microbes will meet their end as well. By 2.8 billion years in the future, even these refugia will become uninhabitable as the planet’s temperatures continue to rise. In even more time, Earth itself might be completely destroyed as the Sun progresses in its natural evolution, turning from a yellow dwarf into a red giant, a process that will likely occur in 5 billion years, resulting from the exhaustion of hydrogen in its core. As the star runs out of fuel, gravity becomes predominant, as it is no longer counterbalanced by the immense amounts of pressure generated by the fusion process. Consequently, the Sun will contract, enough to the point that temperatures around and in the core will rise to such a high degree that a layer of until then unfused hydrogen begins to be processed, apart from the beginning of helium fusion in the core itself, leading to the formation of heavier elements, like carbon, oxygen, and nitrogen. This new round of fusion will lead to a rebound in energy generation and also a rebound in star size, which will expand to the point of potentially engulfing Earth, first completely eliminating Mercury and Venus from existence. This giant phase lasts little, though: only for 1 billion years. During this “brief” timespan, more distant planets and satellites will receive much greater radiation as a result of the Sun’s greatly increased size, likely increasing their temperatures to significant degrees, and Mars especially might be able to sustain liquid water to a more significant degree.
Through these 1 billion years, the helium will run out, and this time, there will be no more fusion, as the Sun lacks the mass to produce heavier elements. As gravity pushes down on the astronomical body, it will undergo a final flash of energy production as a layer of helium around the core also gets ignited. By this time, however, the Sun’s life is over. Its final expansion culminates in the shedding of its outer layers and in about half of its mass, generating a cloud of stellar material known as a planetary nebula. The end result will be a white dwarf, about the size of Earth but with half of the Sun’s mass, a body that was extremely compressed by gravity and left with an incredulously high density. The remaining planets will settle in much more distant orbits due to their host star’s loss of mass, turning much colder. Even so, the “corpse” of our former star will still be incredibly hot. Not because of nuclear fusion any longer, but just because of the heat it generated while still active, now to be gradually dissipated over timespans much longer than the age of the Solar System by then.
It truly is just left for pure speculation to think of what future humans might make of this. Will they have forgotten Earth by then, billions of years into the future? Will any of them still remain on its surface in extremely sophisticated habitats, resolute on accompanying our planet until its fiery end? Whatever it might be, it is comforting to think that Earthly life might far outlive Earth, seeded around the galaxy by our expansion. What is far less comforting is the fact that stellar formation will eventually cease, probably trillions of years from now, when even red dwarfs are becoming white dwarfs and when the first white dwarfs are becoming black dwarfs. It will possibly occur due to the halting of the recycling of star-forming material that occurs in galaxies. It begins when stellar material is expelled from the galaxy, likely due to a combination of factors, such as supernovae, powerful stellar winds from particularly massive stars, and beams of matter expelled from around black holes due to the extreme processes that occur near such objects. When such material is expelled and reaches the intergalactic space, it cools down and becomes denser, eventually flowing back to the galaxy, where it then serves to fuel the production of new stars. However, for not very well understood reasons, galaxies eventually become quenched, and this cycle is disrupted: star-forming material does not return, and so stars eventually stop forming altogether.
While all of this happens, another ongoing phenomenon further contributes to this rather gloomy destiny: the universe’s expansion, driven by a form of mysterious energy known as dark energy, which, unlike other forms of energy, does not dilute as space expands, instead apparently keeping constant (though this has been put into question, with dark energy potentially weakening over time), causing spacetime to grow ever more rapidly. As this happens, astronomical bodies such as galaxy clusters and eventually even individual galaxies become ever more distant, ever farther and farther, as spacetime itself stretches across all of the cosmos. This only decreases the chances for new star-forming events to occur, as increasingly farther objects have way fewer chances of interacting, further compromising, for instance, the flow of intergalactic star material. Skipping many trillions of years into this dark and cold future, even black holes, with eventually no more nearby mass to feed on, will cease to exist, a result of a constant yet extremely slow process of decay that happens due to their release of radiation (called Hawking radiation), itself resulting from quantum effects associated with the extreme curvatures spacetime is subject to around these extreme objects. This timescale is virtually incomprehensible, around a magnitude of 10 to the power of 100 for the largest blackholes, those found at the centers of galaxies
Will we even exist by then? It would seem impossible, but given so much time, it is impossible to predict how our technology and to what levels our technology could have progressed. Maybe we could find a way to extract power even from dark energy itself, securing our existence from what would be tearing the astronomical bodies of the universe apart. However, maybe none of this will actually happen. As cited before, it has been proposed that dark energy is not really constant and that it may have weakened over time. If it has indeed weakened, will it continue weakening, will it just stop at that point, or will it actually start growing? No one knows, but any of these would dramatically change the tragic picture painted above. Even if such a picture above is the correct one, which it very well might be, this dark universe might not be truly universal. Though inflation stopped and led to the Big Bang, as discussed in the first entry, it did not necessarily do so everywhere. Indeed, quantum fluctuations permeate existence, and in the same way these got stretched to significant variation densities during the inflationary period, they might have been the cause of the Big Bang in the first place: a variation in inflation that caused it to stop then. However, it might as well have continued elsewhere, and it might as well have suffered such variations elsewhere too, creating other Big Bangs, separated from each other by inflationary space, expanding much faster than light at an exponential pace. As such, it is certainly possible that, though our piece of universe may end in such darkness, simultaneously other pieces of universe are just starting to see their first rays of light.
Furthermore, check the sources for this entry here.
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