Kurzgesagt – In a Nutshell 

Sources – Big Bang Life

Thanks to our experts: 

• Prof. James Gurney

Georgia State University

• Prof. Bartolo Luque

Polytechnic University of Madrid

– Life has existed on one planet for about 4 billion years, for all we know. 

The Earth and the solar system were born about 4.5 billion years ago:

#NASA: “Earth: Our Home Planet”. Solar System Exploration – Planets (retrieved 2023)
https://solarsystem.nasa.gov/planets/earth/in-depth/
Quote: “When the solar system settled into its current layout about 4.5 billion years ago, Earth formed when gravity pulled swirling gas and dust in to become the third planet from the Sun. Like its fellow terrestrial planets, Earth has a central core, a rocky mantle, and a solid crust.”

The emergence of life on our planet is much more uncertain, but all direct and indirect evidence points to an origin at about 4 billion years ago, with an uncertainty of a few million hundred years:

#Knoll, A. et al. (2017): “The timetable of evolution”. Science Advances, vol 3, 5.
https://www.science.org/doi/full/10.1126/sciadv.1603076
Quote: “Life, then, appears to have been present when the oldest well-preserved sedimentary rocks were deposited (Fig. 1). How much earlier life might have evolved remains conjectural. Reduced carbon (graphite) in ancient metaturbidites from southwestern Greenland has a C-isotopic composition, consistent with autotrophy (24), and recently, upwardly convex, laminated structures interpreted (not without controversy) as microbialites have been reported as well (25); the age of these rocks is constrained by cross-cutting intrusions that cluster tightly around 3710 Ma (25). A still earlier origin for biological carbon fixation is suggested by a 13C-depleted organic inclusion in a zircon dated at 4100 ± 10 Ma (26), although it is hard to rule out abiological fractionation in this minute sample of Earth’s early interior.”

– To qualify as living things, even microbes need to eat, poop, grow and multiply. To do that, they need a genome, the biological instruction manual that sets the inner workings of an organism. 

A genome can be defined as the complete set of genetic information carried by an organism. This information is encoded in nucleic acids like DNA or RNA (long molecules present inside cells) and it contains all the information needed to build that organism, as well as all the inborn information that an organism needs to grow and interact with the environment. Genomes are hereditary (i.e. they are passed on from generation to generation) and they are common to every living being we know of. Although it is difficult to define from first principles what life is, all living organisms known on Earth have a genome:

#Royal Society of New Zealand: “What is a genome”. Gene Editing Technologies (retrieved 2023)

https://www.royalsociety.org.nz/what-we-do/our-expert-advice/all-expert-advice-papers/gene-editing-technologies/what-is-a-genome-2/ 

Quote: “The characteristics of all living organisms are determined by their genetic material and their interaction with the environment. An organism’s complete set of genetic material is called its genome which, in all plants, animals and microbes, is made of long molecules of DNA (deoxyribonucleic acid). The genome contains all the genetic information needed to build that organism and allow it to grow and develop.”

The genetic material of an organism is found inside every cell of every living being we know of. It’s composed of a very long strand of DNA that is then packaged (wrapped) many times into different levels:

#Iyer, B. V. S. et al (2011): “Hierarchies in eukaryotic genome organization: Insights from polymer theory and simulations”. BMC Biophysics, vol. 4, 8
https://bmcbiophys.biomedcentral.com/articles/10.1186/2046-1682-4-8 

Quote: “The hierarchical process by which eukaryotic double-stranded DNA (two meters long, in the case of humans) is packaged within the confines of a micrometers-sized cell. As shown schematically in the figure, this process encompasses three main organization levels classified as primary, secondary and tertiary [115, 116].”

– How dead things with no genome became living things with genomes is one of the biggest riddles of science. Simplifying a lot, the problem is that to have a functioning genome you need proteins, and to make those proteins you need a functioning genome. Both proteins and genomes are super long molecules made of pretty complex blocks that are extremely difficult to assemble by chance. It is a chicken-egg paradox with several chickens and eggs.

Given the fact that all living beings have a genome, the problem of the origin of life can be phrased in terms of how genomes arose from inanimate matter. But until now, basically the only step that has been conclusively demonstrated is how to convert inorganic molecules into basic organic constituents like amino acids, the building blocks of proteins. This was achieved for the first time in 1952, in a famous experiment that tried to reproduce in the lab the environmental conditions of the baby Earth:

#Koppes, S. (2022): “The origin of life on Earth, explained”. University of Chicago News.
https://news.uchicago.edu/explainer/origin-life-earth-explained
Quote: “As a University of Chicago graduate student in 1952, Stanley Miller performed a famous experiment with Harold Urey, a Nobel laureate in chemistry. Their results explored the idea that life formed in a primordial soup. Miller and Urey injected ammonia, methane and water vapor into an enclosed glass container to simulate what were then believed to be the conditions of Earth’s early atmosphere. Then they passed electrical sparks through the container to simulate lightning. Amino acids, the building blocks of proteins, soon formed. Miller and Urey realized that this process could have paved the way for the molecules needed to produce life.”

However, amino acids are just the basic building blocks of proteins, and they are still very far from a genome or a working organism. The main problem of getting life from scratch is that most organic chemical reactions taking place inside an organism (like the synthesis of proteins or nucleic acids) have a vanishingly small probability of happening spontaneously – they only take place efficiently thanks to enzymes, another kind of proteins that help those reactions happen quickly. But to synthesize these proteins, the organism needs the biological instructions encoded in the genome; while to synthesize the nucleic acids that make up the genome, the organism needs proteins. Moreover, both proteins and nucleic acids are very long chains of already complex building blocks, whose synthesis requires proteins and nucleic acids as well. The problem of how to get such a functioning intricate network of complex molecules from simple, inorganic matter has been frequently described as a very contrived chicken-egg problem:

#Trefil, J. et al. (2009): “​​The Origin of Life”. American Scientist, vol. 97, 3.

https://www.americanscientist.org/article/the-origin-of-life

Quote: “The essential problem is that in modern living systems, chemical reactions in cells are mediated by protein catalysts called enzymes. The information encoded in the nucleic acids DNA and RNA is required to make the proteins; yet the proteins are required to make the nucleic acids. Furthermore, both proteins and nucleic acids are large molecules consisting of strings of small component molecules whose synthesis is supervised by proteins and nucleic acids. We have two chickens, two eggs, and no answer to the old problem of which came first.”

– Once you have a finished cell, the whole system works efficiently. But starting from simple dead stuff and reaching that level of sophistication by pure chance should require an amazing amount of time for trial and error. So how did the first living things manage to cross that gap in just a few hundred million years?

Getting such complex molecules and reaction networks starting from purely random interactions in inorganic matter is thought to have a very small probability to occur:

#Trefil, J. et al. (2009): “​​The Origin of Life”. American Scientist, vol. 97, 3.
https://www.americanscientist.org/article/the-origin-of-life
Quote: “The RNA molecule is too complex, requiring assembly first of the monomeric constituents of RNA, then assembly of strings of monomers into polymers. As a random event without a highly structured chemical context, this sequence has a forbiddingly low probability and the process lacks a plausible chemical explanation, despite considerable effort to supply one.”

– Most theories about the origin of life try to explain that gap by theorizing how some primitive soup of prebiotic molecules could have efficiently produced the first self replicating entities. But we still don’t know how exactly this would have worked. 

The main theories about how life might have started on the primitive Earth differ in which kinds of constituents or networks of chemical reactions happened to occur first: RNA molecules, chains of metabolic reactions, etc. At any rate, they share in common that they try to explain how the chemical substrates of life arose from a combination of simpler inorganic molecules under the environmental conditions of the early Earth – what has generically been called a “primordial soup”. Until now, however, scientists have found no definite answer to this question:

#Walker, S. I. (2017): “Origins of life: a problem for physics, a key issues review”. Reports on Progress in Physics, vol. 80, 9
https://iopscience.iop.org/article/10.1088/1361-6633/aa7804/meta 

http://www.esalq.usp.br/lepse/imgs/conteudo_thumb/Origins-of-Life---A-Problem-for-Physics--A-Key-Issues-Review.pdf (open-access version)

Quote: “One might, for example, take a purely substrate-level definition for life and conjecture that life is defined by its constituent molecules, including amino acids, RNA, DNA, lipids etc as found in extant life. It then follows that the problem of life’s origin should reduce to identifying how the building blocks of life might be synthesized under abiotic conditions (which as it turns out is not-so-easy). This approach has dominated much of the research into life’s origins since the 1920’s when Oparin and Haldane first proposed the ‘primordial soup’ hypothesis, which posits that life arose in a reducing environment that abiotically synthesized simple organic compounds, concentrated them, and gradually complexified toward more complex chemistries and eventually life [40]. In 1953 Miller demonstrated that organic molecules, including amino acids, could be synthesized in a simple spark-discharge experiment under reducing conditions [41]. At the time, there was such optimism that the origin of life problem would soon be solved that there was some expectation that life would crawl out of a Miller-Urey experiment within a few years. This has not yet happened, and there seem to be continually re-newed estimates that artificial or synthetic life is just a few years away. This suggests a radical re-think of the problem of origins may be necessary [39].”

The two main hypotheses to explain the origin of life on Earth are known as “RNA world” and “Metabolism first”. The first idea posits that a primordial soup of prebiotic molecules gave rise to RNA – a molecule that is known to play a dual role of catalyzer (“protein”) and genetic material. The second idea posits that the primordial soup gave rise to a complex network of molecular reactions reminiscent of those that take place in organisms and that enable life. Still, until today no one knows for sure how any of these possibilities might have worked exactly:

#Trefil, J. et al. (2009): “​​The Origin of Life”. American Scientist, vol. 97, 3.

https://www.americanscientist.org/article/the-origin-of-life

Quote: “RNA World has been the prevailing theory for the origin of life since the 1980s. The emergence of a self-replicating catalytic molecule accounts for signature capabilities of living systems, but it doesn’t explain how the protobiological molecule itself arose. Metabolism First seeks the answer in primitive reaction networks that generate their own constituents, offering a substrate for chemical selection and a launchpad for life.”

–Think of genomes as a book telling the history of life. As time passed and life evolved, more characters were introduced: Amoebae, fish, amphibians, dinosaurs, mammals. Over billions of years, the story of life got more and more complex.

Life on Earth started with microbial, unicellular life. Over billion of years, those first organisms evolved and gave rise to a plethora of increasingly more complex species:

#Encyclopaedia Britannica: “Life on Earth” (retrieved 2023)
https://www.britannica.com/science/life/Life-on-Earth 

Quote: “Over hundreds of millions of years, life spread through the seas and over Earth's surface. The first life-forms were small and simple. Later forms were more complicated and diverse.”

–A genome can be viewed as a long string of letters with biological instructions.  And from microbes to us today, functional genomes seem to have been increasing in size at a fairly constant rate. The functional genome of fish is more than twice bigger than that of worms; our functional genome is more than twice bigger than that of fish and so on. It is a bit more complicated, but for now let’s run with this.

One way to analyze the evolution of life on Earth is to look at the evolution of genomes themselves. Can we track the increase in complexity of the different species by looking at the complexity of their genomes?

The genome of a species is the amount of genetic information encoded in its DNA. In turn, DNA can always be viewed as a very long string of “letters” taken from an alphabet made of four “signs”: A, G, C and T (taken from the names of the four chemical components, or “bases”, that make up DNA – adenine, guanine, cytosine and thymine):

#Encyclopaedia Britannica: “DNA” (retrieved 2023)
https://www.britannica.com/science/DNA  

Quote: “Each strand of a DNA molecule is composed of a long chain of monomer nucleotides. The nucleotides of DNA consist of a deoxyribose sugar molecule to which is attached a phosphate group and one of four nitrogenous bases: two purines (adenine and guanine) and two pyrimidines (cytosine and thymine).”

Over evolution, the genomes of the different species have generically become longer, so a first attempt to quantify the "complexity" of a given genome could be to identify it with its sheer length. However, if we think of a genome as a repository of biological information, such an identification won’t be enough. Over evolutionary history, genomes have accumulated whole regions that repeat themselves or that codify no useful information at all. For example, the genome of mammals has about 3 billion base pairs (“letters”) but its functional and non-redundant length is just 15% of that:

#Sharov, A. (2006): “Genome increase as a clock for the origin and evolution of life”. Biology Direct, vol. 1, 17.
https://biologydirect.biomedcentral.com/articles/10.1186/1745-6150-1-17

Quote: “Biological complexity was recently defined by Adami et al. [8] as a size of functional and non-redundant genome. This measure does not depend on duplications, insertions, or deletions of non-functional or redundant sequences, and therefore it is more stable in evolution than the total genome size.”
Quote: “Mammals (mouse, rat, and human), which appeared just recently in earth history, have a genome of ca. 3.2 × 109 bp, however only 5% of it is conserved between species [13]. Conserved regions are definitely functional but there may be additional functional regulatory regions that are species-specific. These regions can be identified based on the absence of transposons, because transposons that are inserted in functional regions would interfere with normal gene regulation and eventually disappear due to natural selection [14]. Transposon-free regions of 5 and 10 kb account for 20%, and 12% genome size, respectively [14]. If we take 15% as a rough estimate, then the size of functional and non-redundant genome in mammals is ca. 4.8 × 108 bp.”

If we now focus on the functional and non-redundant genomes of different kinds of organisms as they appeared over evolutionary history, a regular pattern emerges:

#Sharov, A. (2006): “Genome increase as a clock for the origin and evolution of life”. Biology Direct, vol. 1, 17.
https://biologydirect.biomedcentral.com/articles/10.1186/1745-6150-1-17

Quote: “Fish existed 0.5 billion years ago [15]. The genome size of the fugu fish is 4 × 108 bp and 1/3 of it is occupied by gene loci [16]. Worms existed at least for 1 billion years [17]. The genome of the worm Caenorhabditis elegans has size of 9.7 × 107 bp and ca. 75% of its length is functional [18].”

The “more than twice” increase in genome length mentioned in the script is a rounding off that doesn’t correspond with the exact figures cited here (which correspond to particular examples) but with the average increase in genome length that will be deduced from the regression explained below.

– When we put all these hints together, it seems that genomes have been doubling in size on average every 350 million years or so. As if evolution had been following an exponential inner clock.

When one plots the data for other animal groups (plants have not been included because they have a high degree of redundancy that it difficult to quantify) going back to the prokaryotes, the simplest life forms known on Earth, one gets a reasonably well adjusted linear regression on a logarithmic scale:

Given that the scale is logarithmic, the linear regression above implies that the size of the functional and non-redundant genomes on Earth follows an exponential law. The expression marked in red can be rewritten as

y = 4.4×108 · 2 x/0.34 ,

where y denotes the size of the functional, non redundant genome and x denotes the time from today into the past in billions of years (x = 0 is today, x = –1 is one billion years ago, etc). In turn, this means that genomes have been doubling in size every 0.34 billion years or so, which we’ve rounded off to 350 million years.

–But it gets even stranger. The very first microbes that emerged on Earth, even if they look simple, already seem to have had pretty long and complex genomes. But how could life have achieved that level of complexity in such a short time?

From the graph above we see that prokaryotes have functional genome sizes of the order of 100,000 base pairs, or “letters”. If this number really quantifies their complexity, a natural question is why life didn’t start with a much lower degree of complexity or, put differently, how life was able to “suddenly jump” to that degree of complexity.

–But there may be an interesting way to solve this riddle: We just take our exponential clock and extrapolate it back in time, to the simplest conceivable life form – something equivalent to a being with a genome containing just a few letters. But if we do that we end up 10 billion years in the past. More than twice the age of Earth, which means: If life actually evolved like this, it did not start here, but somewhere out there, in space.

If we assume that life somehow started with a complexity equivalent to that of a genome with just a few base pairs and extrapolate our exponential law back into the past, we get the surprising result that life should have started about 10 billion years ago:

#Sharov, A. et al. (2013): “Life Before Earth”. arXiv:1304.3381

https://arxiv.org/abs/1304.3381

Quote: “What is most interesting in this relationship is that it can be extrapolated back to the origin of life. Genome complexity reaches zero, which corresponds to just one base pair, at time ca. 9.7 billion years ago (Fig. 1). A sensitivity analysis gives a range for the extrapolation of ±2.5 billion years (Sharov, 2006). Because the age of Earth is only 4.5 billion years, life could not have originated on Earth even in the most favorable scenario (Fig. 2). Another complexity measure yielded an estimate for the origin of life date about 5 to 6 billion years ago, which is similarly not compatible with the origin of life on Earth (Jørgensen, 2007). Can we take these estimates as an approximate age of life in the universe?”

–This would explain why life started to thrive so quickly on our young planet. If it was already present in space like a seed, it just needed water and warm temperatures to wake up and go on evolving. And it would also explain the high degree of sophistication of the first life forms on Earth. They could have been complex already because they might have been evolving for billions of years somewhere else in the universe. 

Such an ancient origin of life plus the idea that life arrived on Earth from space would explain both the early emergence of life on our planet and the complexity of the first bacteria:

#Sharov, A. et al. (2013): “Life Before Earth”. arXiv:1304.3381
https://arxiv.org/abs/1304.3381
Quote: “Contamination with bacterial spores from space appears the most plausible hypothesis that explains the early appearance of life on Earth. Thus, despite the fact that we don’t have a final answer, it makes sense to explore the implications of a cosmic origin of life, before the Earth existed. First, we conclude that life took a long time, perhaps 5 billion years, to reach the complexity of bacteria.”

– But could life really be that old? Maybe yes. Actually, life could have started shortly after the universe itself was born.

Our exponential law implies that life should be at least 10 billion years old. However, this is assuming an uninterrupted evolution, so most probably the whole process took much longer than that. It seems difficult that, throughout the turbulent history of our universe, our pre-microbial forebears were always able to find a cozy place with nutrients, water and warm temperatures to continue evolving non-stop. Could life have started before? 

A Goldilocks Baby Universe

– At its most basic level life needs two things: the right chemical elements to form complex molecules and a liquid medium, like water, in which those molecules can move and interact. The liquid medium needs to stay warm enough to remain, well, liquid. So when we search for life in space, we focus on Earth-like planets at just the right distance from their star – warm enough to sustain liquid water. 

Life on Earth is carbon- and water-based. Carbon, as well as a few other chemical elements, are needed to form the complex molecules life is based on. Water acts as a universal solvent that allows the interaction between molecules. Over the years, scientists have considered the (hypothetical) possibility of life based on alternative biochemistries (i.e. non carbon-based) or alternative solvents (i.e. other than water). However, complex molecules interacting in a liquid medium is considered to be the most basic requisite for life in whatever form:

#Ballesteros, F. J. et al. (2019): “Diving into Exoplanets: Are Water Seas the Most Common?”. Astrobiology, vol. 19, 5.

https://www.liebertpub.com/doi/abs/10.1089/ast.2017.1720
https://digital.csic.es/bitstream/10261/213115/4/exoseas.pdf (open-access version)
Quote: “One of the basic tenets of exobiology is the need for a liquid substratum in which life can arise, evolve, and develop. The most common version of this idea involves the necessity of water to act as such a substratum, both because that is the case on Earth and because it seems to be the most viable liquid for chemical reactions that lead to life. Other liquid media that could harbor life, however, have occasionally been put forth.”

When astronomers search for habitable planets today, they focus on planets lying in the “habitable” or “Goldilocks” zone of their star: the distance at which the temperature of a rocky planet would be just right (not too hot, not too cold) to sustain liquid water on its surface:

#ESO: “The habitable zone” (retrieved 2023)
https://supernova.eso.org/exhibition/images/0310_habitable_zone_cc_8bit/ 

Quote: “This graphic visualises the "habitable zone" within our Solar System. This is a region of space which has conditions that could foster potential life. Of course, it is very difficult to know exactly what these conditions might be. For example, liquid water is generally assumed to be essential. The sweet spot within our Solar System is highlighted here by a green disk. You can see that Earth, unsurprisingly fall with the zone, whilst our neighbour Mars lingers on its edge.”

– But there was actually a time when almost all of the universe might have been habitable. Right after the Big Bang the universe was extremely hot. But as the cosmos expanded, it cooled. And between about 10 and 17 million years after the Big Bang, when the universe was a thousand times younger than today, it was between 100 ºC and 0 ºC – the temperature at which water is liquid. So for this window of time, more than 13.7 billion years ago, the whole universe, absolutely every inch of it, had the right temperature to support life.

This idea was put forward for the first time in this work:

#Loeb, A. (2014): “The habitable epoch of the early Universe”. International Journal of Astrobiology, vol. 13, 4.

https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/abs/habitable-epoch-of-the-early-universe/114595C6E860A5002A9B783875602106 

https://arxiv.org/abs/1312.0613 (open-access version)

Quote: “In the redshift range 100≲(1+z)≲137, the cosmic microwave background (CMB) had a temperature of 273–373 K (0–100°C), allowing early rocky planets (if any existed) to have liquid water chemistry on their surface and be habitable, irrespective of their distance from a star.”

and it is based on the following simple idea. Life needs a heat source, which is why when we search for alien life we look for planets near stars. However, not all the light in the universe comes from stars. There is a very particular kind of light which is intrinsic to the universe itself: the cosmic microwave background (CMB), also known as the “afterglow of the Big Bang”. This light was emitted right after the Big Bang and immediately filled the universe, and its photons have been traveling through space ever since. The temperature of this cosmic light has changed over cosmic history, but a few million years after the Big Bang it had just the right temperature to support liquid water. So, in a sense, there was a time during the early universe when all the cosmos recreated the conditions that today we find near stars.

The CMB was emitted when the universe was a few hundred thousand years old (an extremely short time in cosmic scales), when the particles of the hot plasma that had emerged from the Big Bang combined to form the first neutral atoms. When it was emitted, it had a temperature of about 3,000 kelvin:

#ESA: “Planck and the cosmic microwave background” (retrieved 2023)
https://www.esa.int/Science_Exploration/Space_Science/Planck/Planck_and_the_cosmic_microwave_background 

Quote: “The cosmic microwave background (or CMB) fills the entire Universe and is leftover radiation from the Big Bang. When the Universe was born, nearly 14 billion years ago, it was filled with hot plasma of particles (mostly protons, neutrons, and electrons) and photons (light). In particular, for roughly the first 380,000 years, the photons were constantly interacting with free electrons, meaning that they could not travel long distances. That means that the early Universe was opaque, like being in fog. However, the Universe was expanding and as it expanded, it cooled, as the fixed amount of energy within it was able to spread out over larger volumes. After about 380,000 years, it had cooled to around 3000 Kelvin (approximately 2700ºC) and at this point, electrons were able to combine with protons to form hydrogen atoms, and the temperature was too low to separate them again. In the absence of free electrons, the photons were able to move unhindered through the Universe: it became transparent.”

but then it gradually cooled as the universe expanded. This light is detected still today and it comes from all directions in the sky. Today its temperature is of just a few degrees over absolute zero:

#ESA: “Planck and the cosmic microwave background” (retrieved 2023)
https://www.esa.int/Science_Exploration/Space_Science/Planck/Planck_and_the_cosmic_microwave_background 

Quote: “Over the intervening billions of years, the Universe has expanded and cooled greatly. Due to the expansion of space, the wavelengths of the photons have grown (they have been ‘redshifted’) to roughly 1 millimetre and thus their effective temperature has decreased to just 2.7 Kelvin, or around -270ºC, just above absolute zero. These photons fill the Universe today (there are roughly 400 in every cubic centimetre of space) and create a background glow that can be detected by far-infrared and radio telescopes.”

Along this process of gradual cooling from 3,000 kelvin to 2.7 kelvin, there had to be a time in cosmic history when the temperature of the cosmic background was just right to sustain liquid water. This happened when the universe was between 10 and 17 million years old. And although this may look like a long time to us, it's actually a very short time in cosmic timescales. The current age of the universe is about 14 billion years old – if the universe today was a young adult, we’d be talking about a baby just a few days old.

During this time window, the temperature of the cosmic background was between 100 ºC and 0 ºC, i.e. the temperature at which water stays liquid in normal conditions of pressure:

#Loeb, A. (2014): “The habitable epoch of the early Universe”. International Journal of Astrobiology, vol. 13, 4.

https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/abs/habitable-epoch-of-the-early-universe/114595C6E860A5002A9B783875602106 

https://arxiv.org/abs/1312.0613 (open-access version)

Quote: “In the redshift range 100≲(1+z)≲137, the cosmic microwave background (CMB) had a temperature of 273–373 K (0–100°C), allowing early rocky planets (if any existed) to have liquid water chemistry on their surface and be habitable, irrespective of their distance from a star.

–Of course, just a warm temperature is not enough for life. We also need chemical elements like carbon and oxygen, which are forged in the cores of stars. But were there stars in super early cosmic times? Maybe yes – in regions of the universe where the matter was especially dense.

The results explained above just show the existence of a uniform source of warm temperature in the early universe. However, to have life –and water– we need chemical elements like carbon, oxygen, etc. Were they also present at these early cosmic times?

This is a crucial question because the Big Bang only created hydrogen, helium and a tiny bit of lithium. All other chemical elements (carbon, oxygen, nitrogen and the whole periodic table) were created later by the nuclear reactions that take place in the cores of stars or by other stellar processes:

#NASA (2020): “The Origin of Elements”. Nasa Science

https://science.nasa.gov/origin-elements

Quote: “The hydrogen in your body, present in every molecule of water, came from the Big Bang. There are no other appreciable sources of hydrogen in the universe. The carbon in your body was made by nuclear fusion in the interior of stars, as was the oxygen. Much of the iron in your body was made during supernovas of stars that occurred long ago and far away. The gold in your jewelry was likely made from neutron stars during collisions that may have been visible as short-duration gamma-ray bursts or gravitational wave events. Elements like phosphorus and copper are present in our bodies in only small amounts but are essential to the functioning of all known life. The featured periodic table is color coded to indicate humanity's best guess as to the nuclear origin of all known elements. The sites of nuclear creation of some elements, such as copper, are not really well known and are continuing topics of observational and computational research.”

The actual question is therefore if the very first stars were already shining when the universe was about 10 million years old.

The standard assumption is that the first stars were born later. Exact estimates vary, but models and simulations usually imply that the very first stars started to shine between a few dozen and a few hundred million years after the Big Bang:

#Barkana, R. (2016): “The rise of the first stars: Supersonic streaming, radiative feedback, and 21-cm cosmology”. Physics Reports, vol. 645.
https://www.sciencedirect.com/science/article/abs/pii/S0370157316301569 

https://arxiv.org/abs/1605.04357 (open-access version)

Quote: “For example, one of the first high-resolution “first star” simulations formed its first star only at redshift 18.2 [81], while analytical methods show that the first stars must have formed at z ∼ 65 [8, 9] within our past light cone (i.e., so that we can in principle see them as they formed), or a further ∆z ∼ 6 earlier [82] within the entire volume of the observable Universe (so that we can see them or their remnants after they formed). On this point, we note that there were some early, rough analytical estimates of the formation redshift of the very first stars [83, 84].”

(The “redshift” z mentioned in the quote above is a number sometimes used by astronomers to refer to the age of the universe. For an equivalence between z and years, see the graph below.)

However, this isn’t the only possibility. If the newborn universe was more lumpy than generally thought, the first stars could have formed when the universe was 10 million years old in regions where the matter was especially dense:

#Loeb, A. (2014): “The habitable epoch of the early Universe”. International Journal of Astrobiology, vol. 13, 4.

https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/abs/habitable-epoch-of-the-early-universe/114595C6E860A5002A9B783875602106 

https://arxiv.org/abs/1312.0613 (open-access version)

Quote: “In this brief paper, I highlighted a new regime of habitability made possible for ~ 6.6 Myr by the uniform CMB radiation at redshifts (1 + z) = 100–137, just when the first generation of star-forming halos (with a virial mass & 104M⊙) turned around in the standard cosmological model with Gaussian initial conditions. Deviations from Gaussianity in the far (8.5σ) tail of the probability distribution of initial density perturbations, could have led already at these redshifts to the birth of massive stars, whose heavy elements triggered the formation of rocky planets with liquid water on their surface.”

The “deviations from Gaussianity” (density conditions) required for stars to have formed at those early times are consistent with cosmological observations and are even predicted by some theoretical models of the early universe:

#Loeb, A. (2014): “The habitable epoch of the early Universe”. International Journal of Astrobiology, vol. 13, 4.

https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/abs/habitable-epoch-of-the-early-universe/114595C6E860A5002A9B783875602106 

https://arxiv.org/abs/1312.0613 (open-access version)

Quote: “The needed level of deviation from Gaussianity is not ruled out by existing data sets (Ade et al., 2013b). Non-Gaussianity below the current limits is expected in generic models of cosmic inflation (Maldacena, 2003) that are commonly used to explain the initial density perturbations in the Universe.”

– Such stars would have been very massive and gone supernova in just 3 million years – seeding the baby universe with the chemical elements needed to form dust, asteroids, planets and the ingredients of life.    

Today’s stars are born from clouds of dust and gas containing many different chemical elements. However, the first stars that formed in the universe were very different, since the only raw materials available at that time were hydrogen and helium. It is a well known general fact that, due to the lack of chemical elements other than hydrogen and helium, the first stars of the universe had to be extremely massive and short lived, exploding as supernovae at the end of their lives:

#ESA: “History of cosmic structure formation” (retrieved 2023)
https://www.esa.int/Science_Exploration/Space_Science/Planck/History_of_cosmic_structure_formation  

Quote: “The first stars were formed almost exclusively out of hydrogen and helium and are believed to have been extremely massive (about 100 times the mass of the Sun or more) and to have lived very short lives, exploding soon after their formation as supernovae and releasing their material in the surroundings, triggering the birth of new stellar generations.”

For massive and bright enough stars, their lifetimes can be calculated to be just 3 million years:

#Loeb, A. (2014): “The habitable epoch of the early Universe”. International Journal of Astrobiology, vol. 13, 4.

https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/abs/habitable-epoch-of-the-early-universe/114595C6E860A5002A9B783875602106 

https://arxiv.org/abs/1312.0613 (open-access version)

Quote: “For massive stars that are dominated by radiation pressure and shine near their Eddington luminosity LE = 1.3×1040 erg s−1 (M* / 100M⊙), the lifetime is independent of stellar mass M* and set by the 0.7% nuclear efficiency for converting rest mass to radiation, ~ (0.007 M* c2 )/LE = 3 Myr (El Eid et al., 1983; Bromm et al., 2001).”

After exploding, these stars would have seeded the universe with the chemical elements needed to form planets, water and life. 

It is important to notice that life still needs planets or rocky bodies like asteroids, at least for two reasons: 1) To remain liquid, water needs some ambient pressure apart from the right temperature; 2) It is generally acknowledged that life needs temperature gradients (local differences of temperature):

#Loeb, A. (2014): “The habitable epoch of the early Universe”. International Journal of Astrobiology, vol. 13, 4.

https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/abs/habitable-epoch-of-the-early-universe/114595C6E860A5002A9B783875602106 

https://arxiv.org/abs/1312.0613 (open-access version)

Quote: “Thermal gradients are needed for life. These can be supplied by geological variations on the surface of rocky planets. Examples for sources of free energy are geothermal energy powered by the planet’s gravitational binding energy at formation and radioactive energy from unstable elements produced by the earliest supernova.”

– Maybe the first ancestors of life were more exotic and didn’t even need water, but thrived in substances like ammonia or ethane that can stay liquid at temperatures far below 0 °C. They could have been sustained by the lingering warmth of the Big Bang for tens of millions of years longer, well into a time when we know for sure there were stars and all the chemical elements.

Life based on solvents other than water has been proposed several time as a viable alternative to water-based biochemistry:

#Ballesteros, F. J. et al. (2019): “Diving into Exoplanets: Are Water Seas the Most Common?”. Astrobiology, vol. 19, 5.

https://www.liebertpub.com/doi/abs/10.1089/ast.2017.1720
https://digital.csic.es/bitstream/10261/213115/4/exoseas.pdf (open-access version)
Quote: “The feasibility of other solvents for life has been often considered by many authors (Bains 2004, Benner et al. 2004, National Research Council 2007, Stevenson et al. 2015) with very suggestive outcomes, as the high viability of organic chemistry in hydrocarbon solvents, the possibility of membrane alternatives in nonpolar solvents, or even the feasibility of complex non-carbon chemistry. Although given the nature of the subject some results are debated, the main conclusion in all these investigations is that it is not possible to discard the possibility of life appearing in solvents other than water.”

In the context of the habitability of the early universe, the appeal of this possibility is that many of these solvents can remain liquid at temperatures far below 0 ºC. As such, they could have been sustained by the temperature of the cosmic background at later times, when the first stars are known to have arisen for sure and when the chemical richness of the universe was higher:

#Lingam, M. et al. (2021): “The Extended Habitable Epoch of the Universe for Liquids Other than Water”. arXiv:2101.10341

https://arxiv.org/abs/2101.10341 

Quote: “At high redshifts, the temperature of the cosmic microwave background (CMB) was higher than its value today. We explore the possibility that life may have arisen early because the higher CMB temperature would have supplied the requisite energy for the existence of different solvents on the surfaces of objects. At redshifts of z ≤ 70, after the first stars are predicted to have formed, a number of molecules (but not water) might have existed in liquid form over intervals of ~ 10 Myr to ~ 100 Myr.”

 – The real magic of this idea is that while the universe today is extremely deadly and hostile, back then the conditions for life might have been basically everywhere. For a period that may have lasted several dozen million years, a primordial life might have been allowed to emerge on any rock, entirely between the stars – sowing the universe with seeds of what, billions of years later, would become bacteria, trilobites, dinosaurs, and finally us. 

Today’s universe might be full of planets, but just a tiny fraction are habitable: those situated at just the 

right distance from their host star. In contrast, during this period of the early universe, all planets would have been potentially habitable, even rogue planets not being gravitationally bound to any star:

#Loeb, A. (2014): “The habitable epoch of the early Universe”. International Journal of Astrobiology, vol. 13, 4.

https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/abs/habitable-epoch-of-the-early-universe/114595C6E860A5002A9B783875602106 

https://arxiv.org/abs/1312.0613 (open-access version)

Quote: “In the redshift range 100≲(1+z)≲137, the cosmic microwave background (CMB) had a temperature of 273–373 K (0–100°C), allowing early rocky planets (if any existed) to have liquid water chemistry on their surface and be habitable, irrespective of their distance from a star.

#Lingam, M. et al. (2021): “The Extended Habitable Epoch of the Universe for Liquids Other than Water”. arXiv:2101.10341

https://arxiv.org/abs/2101.10341 

Quote: “Hence, if any planets existed at this time, they would have possessed surface temperatures conducive to liquid water due to the energy supplied by the CMB. In this scenario described by Loeb (2014), the distance from a star would be rendered irrelevant and virtually all planets would have clement temperatures.”

– At some point the universe cooled below the right temperature for life to thrive, but some of those ancestral life forms may have continued to exist in the internal warmth of the first planets, frozen in asteroids or hibernating in cosmic dust – tiny seeds roaming the cosmos waiting for new hospitable places to continue evolving. If they did, life now might be everywhere in the universe.

As the universe expanded, the cosmic background cooled and it has continued to do so until today, when its temperature has reached just a few degrees above absolute zero:

#ESA: “Planck and the cosmic microwave background” (retrieved 2023)
https://www.esa.int/Science_Exploration/Space_Science/Planck/Planck_and_the_cosmic_microwave_background 

Quote: “The cosmic microwave background (or CMB) fills the entire Universe and is leftover radiation from the Big Bang. When the Universe was born, nearly 14 billion years ago, it was filled with hot plasma of particles (mostly protons, neutrons, and electrons) and photons (light). In particular, for roughly the first 380,000 years, the photons were constantly interacting with free electrons, meaning that they could not travel long distances. That means that the early Universe was opaque, like being in fog.

However, the Universe was expanding and as it expanded, it cooled, as the fixed amount of energy within it was able to spread out over larger volumes. After about 380,000 years, it had cooled to around 3000 Kelvin (approximately 2700ºC) and at this point, electrons were able to combine with protons to form hydrogen atoms, and the temperature was too low to separate them again. In the absence of free electrons, the photons were able to move unhindered through the Universe: it became transparent.

Over the intervening billions of years, the Universe has expanded and cooled greatly. Due to the expansion of space, the wavelengths of the photons have grown (they have been ‘redshifted’) to roughly 1 millimetre and thus their effective temperature has decreased to just 2.7 Kelvin, or around -270ºC, just above absolute zero. These photons fill the Universe today (there are roughly 400 in every cubic centimetre of space) and create a background glow that can be detected by far-infrared and radio telescopes.”

Of course, this means that at some point it cooled below the needed temperature to sustain any liquid medium. The different cosmic times (in millions of years after the Big Bang) at which the CMB cooled below the freezing point of different liquids are denoted as τm in the table below:

#Lingam, M. et al. (2021): “The Extended Habitable Epoch of the Universe for Liquids Other than Water”. arXiv:2101.10341

https://arxiv.org/abs/2101.10341 

Beyond this point however, life could have survived in the internal warmth of the first planets or in subsurface oceans:

#Loeb, A. (2014): “The habitable epoch of the early Universe”. International Journal of Astrobiology, vol. 13, 4.

https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/abs/habitable-epoch-of-the-early-universe/114595C6E860A5002A9B783875602106 

https://arxiv.org/abs/1312.0613 (open-access version)

Quote: “These internal heat sources (in addition to possible heating by a nearby star), may have kept planets warm even without the CMB, extending the habitable epoch from z ~ 100 to later times. The lower CMB temperature at late times may have allowed ice to form on objects that delivered water to a planet’s surface, and helped to maintain the cold trap of water in the planet’s stratosphere.”

Will We Ever Know?

– All this makes for a nice story. And while both our exponential clock of life and the habitability of the baby universe are reasonable ideas – they are still speculative. One more possibility among many others, trying to explain our existence today.

The two main ideas explained in this video (the exponential clock of genomic evolution and its extrapolation to past times, and the habitable epoch of the early universe) are relatively new and for now purely hypothetical. Still, they provide new ways to look at the problem of the origin of life on our planet. And if true, they could have very easily left testable imprints in our solar system. These possible consequences are explained below.

– But IF life came to Earth from outer space, then it should have seeded other places in the solar system too. Maybe there are fossils in dry riverbeds on Mars. Maybe we’ll soon find life in the warm underground oceans of Enceladus or Europa.

It is true that the Earth is the only planet in the solar system with warm enough temperatures to sustain oceans of liquid water on its surface. However, it is not the only place in the solar system where liquid water may exist today or may have existed in the past. For all we know, Mars had oceans of liquid water in the past:

#NASA(2015): “NASA Research Suggests Mars Once Had More Water Than Earth’s Arctic Ocean” (retrieved 2022)
https://www.nasa.gov/press/2015/march/nasa-research-suggests-mars-once-had-more-water-than-earth-s-arctic-ocean
Quote: “Perhaps about 4.3 billion years ago, Mars would have had enough water to cover its entire surface in a liquid layer about 450 feet (137 meters) deep. More likely, the water would have formed an ocean occupying almost half of Mars’ northern hemisphere, in some regions reaching depths greater than a mile (1.6 kilometers).”

And today, underground oceans of liquid water are thought to exist under the icy surfaces of Enceladus, a moon of Saturn, and Europa, one of Jupiter’s moons:

#NASA: “Enceladus”. Solar System Exploration – Moons (retrieved 2023)
https://solarsystem.nasa.gov/moons/saturn-moons/enceladus/in-depth/ 

Quote: “Few worlds in our solar system are as compelling as Saturn’s icy ocean moon Enceladus. A handful of worlds are thought to have liquid water oceans beneath their frozen shell, but Enceladus sprays its ocean out into space where a spacecraft can sample it. From these samples, scientists have determined that Enceladus has most of the chemical ingredients needed for life, and likely has hydrothermal vents spewing out hot, mineral-rich water into its ocean.”

#NASA: “Europa: Ocean Moon”. Solar System Exploration – Moons (retrieved 2023)
https://solarsystem.nasa.gov/moons/jupiter-moons/europa/in-depth/
Quote: “Scientists think Europa’s ice shell is 10 to 15 miles (15 to 25 kilometers) thick, floating on an ocean 40 to 100 miles (60 to 150 kilometers) deep. So while Europa is only one-fourth the diameter of Earth, its ocean may contain twice as much water as all of Earth’s oceans combined. Europa’s vast and unfathomably deep ocean is widely considered the most promising place to look for life beyond Earth.”

If microbial life came from outer space because, for example, it was hibernating in the cloud that gave rise to the solar system, it would be very unlikely that it only settled on our planet – it should have settled on other habitable regions of the solar system, too. Moreover, it should be reasonably similar to the kind of life we know on Earth:

#Sharov, A. et al. (2013): “Life Before Earth”. arXiv:1304.3381
https://arxiv.org/abs/1304.3381
Quote: “Extrasolar life is likely to be present at least on some planets or satellites within our Solar System, because (1) all planets had comparable chances of being contaminated with microbial life, and (2) some planets and satellites (e.g., Mars, Europa, and Enceladus) provide niches where certain bacteria may survive and reproduce. If extraterrestrial life is present in the Solar System, it should have strong similarities to terrestrial microbes, which is a testable hypothesis. We expect that they have the same nucleic acids (DNA and RNA) and similar mechanisms of transcription and translation as in terrestrial bacteria.”


– Titan has seas, rivers and lakes of ethane and methane as warm as the universe when it was 90 million years old. So finding exotic life on Titan would support the idea that life could have originated in the weird baby universe.  

On the other hand, if life based on exotic solvents like ethane could have emerged in the early universe, then it could exist in Titan as well. This moon of Saturn has rivers and lakes of ethane and methane, and its temperature is that of the CMB when the universe was about 90 million years old:

#Loeb, A. (2022): “Life on Titan May Signal Early Life in the Universe”. Research Notes of the American Astronomical Society, vol. 6, 260

https://iopscience.iop.org/article/10.3847/2515-5172/aca909/meta

Quote: “The temperature of the cosmic microwave background was equal to the surface temperature of Saturn's moon Titan, 94 K, at a redshift z = 33.5, after the first galaxies formed. Titan-like objects would have maintained this surface temperature for tens of Myr irrespective of their distance from a star. Titan has the potential for the chemistry of familiar life in its subsurface water ocean, as well new forms of life in the rivers, lakes and seas of liquid methane and ethane on its surface. The potential future discovery of life on Titan would open the possibility that the earliest lifeforms emerged in metal-rich environments of the earliest galaxies in the universe, merely 100 Myr after the big bang.”

– So far, when we look out into the cosmos we don’t see anyone like us. But maybe that is because life needed ten billion years or more to reach the level of complexity that allows for a technological species. Maybe there are millions of worlds filled with microbes, oceans full of exotic fish, and continents of bizarre animals. And maybe even others like us, that just recently gained consciousness and are beginning to look at the sky, wondering if they are alone.

Finally, the exponential clock of evolution might have another interesting consequence if it proves to be true. If life really needed more than 10 billion years to reach our level of complexity, this means that humanity should be among the first technological civilizations in the universe. So this would explain the Fermi paradox: the puzzling fact, despite the abundance of habitable planets in the Milky Way, so far we’ve seen no signs of other technological civilizations:

#Sharov, A. et al. (2013): “Life Before Earth”. arXiv:1304.3381

https://arxiv.org/abs/1304.3381

Quote: “Fourth, the original Drake equation for guesstimating the number of civilizations in our galaxy (Wikipedia contributors, 2012c) may be wrong, as we conclude that intelligent life like us has just begun appearing in our universe. The Drake equation is a steady state model, and we may be at the beginning of a pulse of civilization. Emergence of civilizations is a non-ergodic process, and some parameters of the equation are therefore time-dependent. Because the cosmic transport of life is most likely limited to prokaryotes, young planets have not had enough time to develop intelligent life. Another time-dependent process is the probability of interstellar transfer of bacteria, which we expect to have become more frequent as the total pool of bacteria in the galaxy increased with time. There are many modifications of the Drake equation, but if civilizations have just begun to appear, any version is of limited use. The answer to the Fermi paradox (Wikipedia contributors, 2012d) may be that we are amongst the first, if not the only so far, civilization to emerge in our galaxy.”