Below are some thoughts about evolution, covering:
Was the extinction of the dinosaurs caused by an increase in gravity?
Why can insects jump so high, relative to their body size?
Why did humans migrate out of Africa during the ice age?
(with possible implications for climate-change modelling)
I also hypothesise about the evolutionary benefits of being gay (& some things that make women attractive to men) here.
1. Were dinosaurs killed by an increase in gravity?
Gravity & electromagnetism are linked and Earth's magnetic field 'randomly' changes from time to time, so maybe the earth's force of gravity suddenly increased when it was hit by a meteor that almost penetrated the earth's crust - if this disturbed the flow of ions in the earth's core that affect its magnetic field (& maybe we can't measure the "gravitational constant" accurately because it keeps changing!). And perhaps that's what killed the giant dinosaurs - they were too big to move around under the higher gravity, and giant pterosaurs like Quetzalcoatlus (with a wingspan of up to 16m!) became too heavy to fly (and modern birds haven't evolved to the same size, since weight increases with the cube but wing area only increases with the square of body dimensions).
Meanwhile smaller, winged dinosaurs survived (& then evolved to birds) as even if they couldn't fly they could hop around with the help of flapping, even though they were less able to maintain their body temperature when the planet suddenly cooled (since larger animals lose less body heat because they have a lower surface area to body mass ratio, which is also why megafauna like the mammoth got too hot and died out after the ice age, whereas whales survived as they have the sea to keep them cool).
Conversely, the massive volcanic eruptions at the end of the Triassic may have thrown enough dust into space (forming earth's dust moons) to reduce gravity and enable the initial growth of the giant dinosaurs (in the subsequent Jurassic period).
However, although dinosaurs became extinct, crocodiles survived, for reasons that scientists can't understand, and so did turtles (who also date back to the early dinosaurs) - both of whom live in shallow waters, which could partially support their weight when gravity increased, and would enable them to crawl up for air (whereas giant prehistoric sea creatures like plesiosaurs would have sunk and died on the sea bottom if gravity increased). They even look like their bodies suddenly got too heavy for them - forcing them to inefficiently drag it along the ground with their legs splayed out the side (whilst strangely, other earlier variants of crocodiles with the more efficient leg structure that is common to other animals - perpendicular to the ground - died out, as did other mammals whose limbs also started to change around this time). This might also explain why crocodiles developed an unusual ability to pull organs out the way of expanding lungs, in order to increase buoyancy - a muscle strength they could conceivably develop within months, or certainly faster than evolving other changes (like a larger chest cavity or an air bladder like fish) in response to a sudden increase in gravity.
Thus the higher gravity we have now may be why the only new giants to evolve since the dinosaurs are whales, with their weight supported by the sea.
And, if the earth's magnetic field suddenly flips again, maybe gravity will reduce and we will all be able to jump over houses? 😂
Alternatively, since the meteor hit Earth at an angle counter to its rotation (coming in from the northeast at an angle of about 60 degrees to the ground), this would suddenly slow its spin, which would slightly increase the net gravity due to a reduction in the effective centrifugal force (which currently offsets gravity by 0.346%), and this slower spin would also slow down the moon's orbit, which would push it into a bigger orbit with a reduced gravity pull on earth (offsetting earth's gravity), but again, probably with an even smaller effect, so this doesn't seem like a good explanation!
Or maybe the meteor strike simply caused global earthquakes which caused giant dinosaurs like T-Rex to fall over and break their bones.
Talking of which...
2. Why can insects jump so high? (relative to their body size)
I was thinking it must be due to how fast they can transmit electrical signals to activate their muscle cells, and was it just the shorter distance or - by my theory of gravity - does a lower gravity force in a less massive object increase the speed of transmission? (and did this explain Stegosaurus' supposed, but now dismissed, extra brain near it's hind legs - as enabling it to move faster or better coordinated than it otherwise could?)
But my further research suggests the latter is unlikely, because neuro signals are actually quite slow - being limited by chemical processes, not the speed of light (although that said, it may have an effect as everything should go faster in lower gravity, which could explain why insects perceive & can react to events faster than humans, and also age faster, which essentially means time seems to go slower for them as they pack more experiences into their short lives).
However, it seems jumping ability is ultimately because of atomic structure and gravity, as I explain following:
The strength of bones and muscles in tension is determined by atomic-scale electron bonding, which means the total force they can withstand increases with their cross-sectional area, or the square of body size measured along one dimension (e.g. length).
But body mass and total force required to achieve a certain acceleration (up to 'launch speed', to jump a certain height) increases with the cube of body length. So if an animal was simply scaled up in size, for example with its length & all other dimensions increased by a factor of four, then the maximum force that its muscles can withstand without breaking would increase by a factor of 16 (4 squared), but its mass would increase 64-fold (4 cubed) so the larger animal could only accelerate its limbs at one quarter of the rate of the smaller animal (16/64, from Newton's 2nd law, F=m.a).
Now with longer limbs, the acceleration could continue over 4 times the distance before the animal leaves the ground, but with one quarter of the limb acceleration, the launch speed will be the same as the smaller animal (by v^2 = u^2 + 2.a.s where u = 0), Then with the same gravity force slowing it as it rises in the air, the maximum jumping height should also be the same, not proportional to the body length.
However, when they fall down, all masses fall to earth at the same rate of acceleration (9.8m/s/s) and thus hit the ground at the same speed when falling from a given height (ignoring air resistance, which will slow smaller animals like cats to a greater extent). Hence the force impact of landing may increase with close to the cube of body length (proportional to mass), unless the animal can master a very controlled landing that allows the deceleration of ground impact to be slowed over the extra body length (like a 'parachute roll' landing). Hence, as the cross-sectional area that must withstand this force only increases with the square of body length, a larger animal will likely suffer almost proportionately bigger stresses and injuries on landing, especially with an uncontrolled fall. This must be why Walking with Dinosaurs (episode 6) says a T-Rex will kill itself if it simply falls over (& an elephant could have crippling injuries), although as this seems a rather fatal evolutionary design flaw, perhaps gravity was lower then than it is now, as I hypothesise above!
Some insects have evolved ways of jumping higher, such as hydraulic/catapult-like muscles and distributed nodal nerve systems that reduce the travel-time for muscle-switching signals, but I suggest they've only evolved these mechanisms because unlike larger animals, they can exploit them without killing themselves when they land!
And so we see that the size of animals on earth is fundamentally determined by the size of atoms (which dictates the strength of muscle & bones) and the size of the earth (which determines the magnitude of gravity and acceleration of all objects when falling to earth)! If the earth was bigger, gravity would be too great for larger animals to jump or fall and survive. And if the earth was much smaller and had less gravity, then essential life gasses like oxygen molecules would exceed the earth's escape velocity at room temperature and escape off into space.
Not only is this fascinating in itself, in explaining how diverse life on earth depends on the size of atoms and our planet being just right (amongst many other enabling factors), it reinforces just how unlikely it is that complex life will form on any other planet, as I discuss in relation to the supposed Fermi Paradox.
3. Why did humans migrate out of Africa during the ice age?
Finally, here's some tentative conclusions I reached whilst wondering why modern humans (Homo-sapiens) migrated north out of Africa about 100,000 years ago, just when a major ice age glacial period started (& related to that, why are black people's palms & feet-soles white?).
I think there must have been something bad happening in Africa then to force them north, because, as in marriage, people generally don't change unless they have to, & especially if you're going to have to move out to somewhere that seems a lot worse. But if there were glaciers expanding south, surely this suggests that an otherwise hot Africa was also cooling, and becoming more habitable?
So why did they head north to try and live in an inhospitable icy land?
The hypothesis I've come up with is that the expanded polar ice reflected & scattered light up to clouds & the earth's atmospheric edge where it was further scattered & reflected back down towards equatorial areas like Africa (more through scattering & bending than abrupt reflection, as the refractive index of the air gradually reduces at high altitudes due to lower air density & gravity). So even if the increased reflection into space from expanded polar ice cooled the planet overall (compared to absorbing this light in land previously not coated by ice), and winds at least partially spread this reduced temperature across the globe, areas like Africa, though perhaps no warmer on average (or even cooler), would experience more intense solar radiation, which would burn pale human skin and increase deaths from skin cancer. But if their skin was already dark, would it matter? Perhaps it wasn't dark enough - so I decided to research the evolution of dark human skin.
Besides being of historical academic interest, if my hypothesis is correct, then it begs the question of whether this scattered light reflection is adequately captured in modern climate-change models? If not, they may overstate global warming forecasts. (The reflection of scattered light back towards the equator should moderate the accelerated cooling caused by increased reflection from ice into space as the ice caps expand, or conversely, it may moderate accelerated warming as ice caps retreat.)
Anyway, to inform/support my hypothesis, here's what I found from some research on human evolution & historic climate change:
Between 4 & 2 million years ago, Australopithecus evolved (as a sub-tribe of Hominini and ancestor of the future sub-tribe Hominina (early humans) that consists solely of the genus Homo covering Homo-sapiens & our extinct relatives).
These bipedal ancestors probably started walking upright (about 4 million years ago, or maybe before this in earlier species) because it freed their hands to carry things (e.g. babies, food) or throw things (like stones/spears), &/or to look higher & further above grasslands to spot prey or predators, &/or to run faster than they could on all fours.
Although this early hominid walked upright, it was still rather hairy like an ape, so its skin would have been white underneath.
During this stage of human evolution (& before, whilst climbing in trees), they evolved hairless skin on their hands & feet with thick, callous layers that enhance grip and reduce abrasion & cuts, and, as a side-effect, scatter & absorb light (due to the thicker, rougher skin) and hence reduce exposure of lower skin layers to UV sunlight. As melanocytes (the cells that produce melanin pigment) are buried deeper below this thick skin, they make less melanin and don't create a tan like the rest of human skin does.
So this is why black people's palms & feet-soles are light - not so much because these parts of the body are less exposed to direct sunlight (noting that other relatively hidden parts of their skin are also black!), but because early human's palms & soles evolved thick, rough skin first, before our skin became dark....
About 3.4 million years ago, these hominin started using stone tools to carve animal carcasses. Besides assisting eating, this would also have enabled them to create skins for clothes.
About 3 to 2 million years ago bipedal hominin body hair started disappearing. So it seems humans started to lose their body hair because they learned how to make clothes! This meant they could stay warm when they needed to (e.g. in winter & at night) whilst retaining light skin (at least whilst living amongst shady trees), which provided an evolutionary advantage because lighter skin can generate more vitamin D (for which it needs to absorb sunlight). And without clothes, they could more easily keep cool through sweating, which could allow them to run faster & longer when chasing prey or running from predators. So there was an advantage from losing body hair, but climate change then forced it more rapidly...
About 2.6 million years ago the last ice age began (lasting until about 11,700 years ago), during which there were a series of glacial events, separated by warmer interglacial periods (cooling on average by about 4 degrees over the last 3 million years).
Whilst early humans may have already been gradually losing body hair, about 1.5 million years ago a mega-drought drove them into arid, open landscapes.
By this time (from about 1.9 million years ago), Homo-erectus had evolved, becoming probably the first hunter-gatherer species, with control of fire. They may have initially had relatively light skin, but moving into the open made it a necessity to lose most of their body hair and instead develop darker skin to protect against melanoma skin cancer & folate deficiency (relatively quickly, within 10-20,000 years, but potentially within only ≈2,500 years if there were strong evolutionary pressures).
With hairless skin, abundant sweat glands and skin rich in melanin, early humans could walk, run & forage for food for long periods of time under the hot sun without suffering brain damage from overheating. In a hot & open African landscape, this was necessary for survival.
Retaining head hair would also have helped avoid sunburn on the most important body area that is most exposed to direct sun, whilst also helping to retain body heat at night (along with animal skins). I imagine women lost more body hair because they have more body fat to keep them warm (which they need during pregnancy), but they developed longer & thicker head hair, so babies could cling to it while they were on the move (like baby animals do to their parents' fur).
While early humans developed dark skin in Africa, around the same time (about 1.2 million years ago) Homo-erectus evolved to Homo-antecessor, who it's believed were among the earliest waves of early human migration north to Europe (where some of their few fossil fragments have been discovered), although the first migrations out of Africa (by Homo-erectus) were around 1.9 million years ago, following routes through Saudi Arabia into Asia.
The colder, northern climates would have less intense sun and also required them to wear clothes, so if they weren't able to catch enough fish (as early humans did) in icy waters to maintain sufficient vitamin D levels, these migrants may well have retained the relatively fair skin of their hairy predecessors (whilst those that stayed in Africa darkened).
So although the current leading hypothesis is that archaic humans, including archaic Homo-sapiens, were dark-skinned from about 1.2 million years ago up until less than 100,000 years ago,* it seems this may only apply to those that stayed in Africa, because some early humans, like Homo-erectus, migrated north before the mega-drought of 1.5 million years ago that caused a loss of body hair and skin darkening in the open, African landscape.
About 850,000 years ago there occurred the first extensive glaciation of North America & Eurasia. It's believed that Homo-antecessor evolved to Homo-heidelbergensis not long after this - around 700,000 years ago (although some scientists consider them to be the same species) - and then evolved further to Neanderthals, who were better suited to cold weather (than another variant, Denisovans, which suffered a major "population bottleneck" drop) and migrated out of Africa some time earlier than 400,000 years ago.
The evolution of modern humans (homo-sapiens) from Homo-heidelbergensis or Homo-antecessor (with some admixing from Neanderthals and Denisovans) has previously been dated to about 350,000 years ago, but may have commenced much earlier, at a similar time to Neanderthals' arrival some 800,000 years ago (shortly after extensive glaciation).
The last glacial period (colloquially but incorrectly referred to as the "last ice age") started around 110,000 years ago (& continued to 11,700 years ago, with the greatest extension of ice occurring approximately 22,000 years ago), which broadly coincides with the first wave of recent modern human migrations out of Africa (around 130,000 to 100,000 years ago, although recent findings indicate over 150,000 years ago).
A second significant migration wave occurred through the "Southern Route" of Saudi Arabia into Asia around 70,000 to 50,000 years ago (with some earlier migration, 90,000 years ago, following a more northern route through Saudi Arabia), and then from there into Europe.
Multiple waves of migration out of Africa produced separate skin-lightening processes through partially different genetic changes (e.g. between light-skinned East Asians vs Western Europeans).
I think this could have been because earlier migrants started with lighter skin (than later migrants) before even leaving Africa...
So the above indicates both early & modern human migrations out of Africa started soon after extensive glaciation events some 800,000 and 100,000 years ago.
Is this coincidence? It's the opposite of what you'd expect - if the planet was cooling and glaciers were expanding south, why on earth would you move north to try and live in a much colder, icy land?
It seems there must be rather counter-intuitive causation involved - the expansion of glaciers somehow forced some early and modern humans to migrate out of Africa. And the only thing I can think of is that the increased level of light reflected from larger ice sheets in the north, reflected back again from the atmosphere down to Africa, causing more intense UV radiation. Then, whilst some humans in Africa may have been able to shelter from the worst of the sun, perhaps in coastal caves, and/or evolve extremely black skin (with the reduced Vitamin D production offset by eating a lot of fish - consistent with the theory of "aquatic apes/humans", who would then also benefit from brain development and increased body fat in place of hairy skin, thus evolving to more intelligent homo-sapiens), many humans inland either migrated to the icy north (as Neanderthals &, less successfully, Denisovans, in early human migration waves), or died of skin cancer. Similarly, very early migrations out of Africa into Asia by homo-erectus 1-2 million years ago followed a mega-drought that forced them into arid, sun-burning African landscapes.
The only other possible cause of these ice-age migrations that I can think of is that early humans needed to kill large animals for the meat that made their brains grow (unless they had plenty of fish), but this wasn't viable in a hot environment where meat would quickly go off, so moving into an icy environment provided the refrigeration required. However, whilst this might be useful & helped Neanderthals survive after migrating north, it wouldn't actually force migration out of Africa, and it is necessity (avoiding death), rather than desirable benefits (like the possibility of a bigger brain) that is the mother of invention & evolution.
* Note there also seem to be logical flaws in the Wikipedia explanations for the evolution of skin colour following loss of body hair, which relates this loss to bipedalism:
It explains that big quadrupedal savannah animals don't need body hair to stay warm if their body volume (relative to surface area) is large enough to retain sufficient heat, but this critical size is larger than a human, who one would therefore expect to retain their body hair in order to stay warm. It then points out that an upright person has less body area exposed to the sun compared to a similar sized quadrupedal animal, and therefore should get less hot in the sun, but this logic seems to indicate an even greater need for body hair. In any case, bipedalism developed 4 million years ago (or earlier), some 2.5 million years before the mega-drought that forced early humans out from the shade of trees into the open savannah, where they had to lose most of their body hair.