Evolution

Maybe this all explains 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 (causing the Earth's crust to spin slightly slower than its inner core, as it seemed to be doing until recently, although that's not certain) — which is known to have been faster 70 million years ago (rotating in 23.5 hours), just before the dinosaurs went extinct.  A slower spin 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 either!

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 early humans migrated north out of Africa about 100,000 years ago, just when a major ice age glacial period started (& related to that, why black people's palms & feet-soles are 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.  So even if the increased reflection into space from expanded polar ice cooled the planet further overall (compared to absorbing this light in land or sea previously not coated by ice), and winds at least partially spread this reduced temperature across the globe, areas like Africa – though no warmer & actually probably cooler on average – would experience more intense solar radiation (particularly blue/UV light, which is scattered the most), 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 somewhat overstate global warming forecasts (see discussion below).

Anyway, to inform/support my hypothesis, here's what I found from some research on human evolution & historic climate change:

• From 4 to 2 million years ago, the genus Homo, covering Homo-sapiens (modern humans) and our extinct "early/archaic human" relatives, evolved from Australopithecus (like the famous "Lucy" — a member of the sub-tribe Hominina within the tribe Hominini, which also includes chimps & bonobos).

  • These bipedal ancestors started walking upright about 4 million years ago (or maybe before this in earlier species) — probably because it freed their hands to carry things (e.g. babies, food) or throw things (like stones/spears), and/or to look higher & further above grasslands to spot prey or predators and to run faster than they could on all fours.

  • Although these early hominins walked upright, they were still rather hairy members of the "great ape" family (hominids), so their 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, along 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 seems to explain why black people have fair palms & feet-soles — 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...

• 3.3 million years ago, our ancestors used stone tools, which were used to carve animal carcasses for eating, but would also enable them to cut up skins for clothes.  In turn, the ability to make clothes to keep warm at night made it possible for early humans to lose their body hair (probably around 2-1.5 million years ago, consistent with genetic evidence of darker skin developing after this, around 1.2 million years ago), which allowed them to keep cool through sweating whilst hunting prey (or running from predators) in the heat of the day (more than compensating for the lack of insulation from having minimal hair, which in isolation would cause significant over-heating*).

  • During this transition, early humans may have retained fair 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).  Losing fur also provided a bonus benefit of reducing the prevalence of parasites (but this isn't a reason for hair loss, as otherwise it would lead to other apes and animals losing their fur too, and anyway, wearing clothes instead caused the more recent development of clothing lice — perhaps due to more consistent wearing of multiple layers).

  • From about 1.9 million years ago, Homo-erectus had evolved (becoming probably the first hunter-gatherer species, with control of fire), and from 1.8 million years ago they began the first migrations out of Africa, following routes through Saudi Arabia into Asia, and then into Western Europe and Spain around 1.5 million years ago.

• Although early humans may have already started to lose their body hair because they learned how to make clothes – which meant they could capture the advantages of having less hair without suffering when it was cold – it seems likely that hair loss and skin darkening was forced more rapidly by climate change about 1.5 million years ago, when a mega-drought drove them into arid, open landscapes.

• • Homo-erectus may have initially had relatively light skin, but moving into the open made it a necessity to lose most of their body hair and develop a more effective way of cooling through sweating,* along with 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. (This helps explain the persistence of babies' evolutionary grip reflex long after humans lost fur, and could also explain why men find long hair on women attractive.)

• From 1.2 million years ago, when the dark skin of modern Africans originally developed, Homo-erectus had evolved to Homo-antecessor, who migrated into Spain from 1.2 to 0.8 million years ago.  But whilst these early humans evolved dark, hairless skin to cope with heat & sun in Africa, a different challenge came from the north due to the ice age (lasting from about 2.6 million years ago until 11,700 years ago), during which there were a series of glacial events (separated by warmer interglacial periods) and an average cooling of about 4 degrees.

  • Significant glaciation had occurred by 1.8 million years ago, and there was an increasing intensity of glacial cycles in Europe from 1.2 million years ago, with a massive cooling event 1.12 million years ago (which lasted about 4,000 years and may have resulted in Homo-erectus being wiped out of, or forced to leave Europe).  This was followed by glacial cycles every 40,000 years until about 670,000 years ago, when glacial periods became longer & stronger, but less frequent (with distinct glacial and warm periods about every 100,000 years).

  • 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, the early migrants to Europe would benefit from lighter skin.

  • So although the current leading hypothesis is that archaic humans, including early 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 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.*  These early migrants – perhaps also including the very earliest of Homo-antecessor – may simply have retained the fair skin of their hairy predecessors, whilst those that stayed in Africa darkened.

• About 850,000 years ago there occurred the first extensive glaciation of North America & Eurasia and archaic humans suffered a major "population bottleneck" drop.  It was 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 further evolved to Neanderthals, who were better suited to cold weather (especially compared to another variant, Denisovans) and migrated out of Africa some time before 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 new analysis of Spanish fossils suggests modern humans started to diverge from Neanderthals much earlier — 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 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 and Australia (at least 65,000 years ago it is claimed, but maybe about 50,000 years ago).

  • At this time, glaciation reduced water levels, making it easier to cross the Red Sea into Saudi Arabia.

• 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 the repeated presence of major glaciation events around the same time as early human migrations out of Africa — first by a fair-skinned Homo-erectus around 1.8 million years ago (& 1.5 million years ago into Spain, to avoid being forced by the mega-drought into an arid, open, sun-burning African landscape), then by Homo-antecessor from 1.2 million years ago (possibly also fair-skinned then) to 800,000 years ago (when the cold-hardy Neanderthals followed) and 110,000 years ago by Homo-sapiens.

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 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 (i.e. indirect "snow glare", occurring even south of the extent of the ice caps).  Then, before Homo-sapiens had invented tailored clothes and sunscreen for protection (41k years ago), early humans in Africa either migrated to the icy north (where the intensity is modified by the sun's lower angle in the sky), or died of skin cancer — unless they were able to shelter from the worst of the sun, perhaps in coastal caves, until they'd evolved 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).

An alternative, much vaguer & speculative explanation for human migration out of Africa might be that it was prompted by other climate changes that occurred just before extended glaciation, with those climate changes then actually triggering glaciation through some kind of feedback mechanism — rather like the events in the movie, "The Day After Tomorrow"!  However, I'm not sure our dating of migration & glaciation events is accurate enough to test such a hypothesis.

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 that when I first reviewed it in April 2018 (although it looked like it had been updated by April 2024) there appeared to be logical flaws or incompleteness in Wikipedia's explanations for the evolution of skin colour following loss of body hair, which seemed to attribute hair loss to bipedalism, with bipedalism affecting thermoregulation as follows:

The article explained 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 since humans are smaller than this critical size, one would expect them to retain their body hair in order to maintain their body temperature (to keep warm when air temperatures are cool, in the absence of clothes, but also to insulate from hot air).  The article then highlighted that an upright person has less body area exposed to the direct sun compared to a similar-sized quadrupedal animal (at least around midday near the equator), and therefore should get less hot in the sun, and have less need for insulating body hair (implying bipedalism led to hair loss).  However, this logic is offset by the reduced reflection of radiation off fur (which helps to keep hairy quadrupeds cool), and by humans' greater need for insulation (compared to larger animals) when the sun is low and the air is cool.  Moreover, although modelling does indeed indicate that in the absence of body hair, a bipedal stance reduces midday heat loads, the excess heat is still much higher than with hair (which provides constant thermoregulation throughout the day regardless of stance, though not by enough to enable extensive activity in the peak sun).  However, the higher heat loads caused by a lack of hair can be dissipated by more effective & greatly increased sweating.

In any case, bipedalism developed 4 million years ago (or earlier), some 2.5 million years before the mega-drought that pushed early humans out from the shade of trees into the open savannah.  So hair loss was not a direct consequence of bipedalism, but rather a much later necessary development that enabled bipedal humans to stay cool when they were forced by drought to be active in the open heat of the sun.

Implications for climate modelling

I assume that current climate models would simplify the effect of ice reflection by modelling it as a parameter (e.g. the percent of light reflected back out into space), rather than a detailed, complex model of light rays reflecting, scattering at various angles and being directed back to Earth (the latter occurring more through further scattering or bending than abrupt reflection, with bending caused by the refractive index of the air gradually reducing at high altitudes due to lower air density & gravity).

So, if my hypothesis is correct about the effect of polar-ice scattering, what impact might that have on climate-change models & forecasts?

In a first approximation, light scattered by polar ice and then reflected back from the sky towards the equator should partially moderate the accelerated cooling that is otherwise caused by the increased direct reflection from ice into space as ice-caps expand (an otherwise unstable, runaway positive-feedback effect).  Conversely – of interest to current global warming predictions – light scattering may moderate accelerated warming as ice caps retreat (i.e. creating a partially stabilising, negative feedback effect – which would be good news!).

However, this moderating effect may only be relevant to the periphery of the polar ice caps, where a significant fraction of scattered light could be directed back by the sky to be absorbed by non ice-covered land or sea.  Within the inner part of ice caps, scattered light that comes back to Earth may simply hit another portion of the ice cap, where most of it may again be reflected back out into space.  Polar ice diffusely reflects 80% of light (scattered in all directions), but I can only speculate that some lesser fraction – let's generously say 30% – might be redirected back to Earth.  With these assumptions, only 7% of any light that is initially reflected from the ice caps will be returned to Earth after reflecting twice from the sky (30% x 80% x 30%), which may be neglected.

Then, for example, we may consider that light scattered from snow & ice at 30 degrees to horizontal land could travel laterally 346km if reflected there & back from the "edge of space" at a height of 100km.  This rather steep scattering angle may be considered a very crude, guesstimated average for all light that is reflected back from the sky to Earth, on the basis that a perfectly uniform scattering, diffuse white surface (which snow & ice aren't) will result in an average reflected angle of 45 degrees for normally incident light (at 90 degrees to the horizontal), but light reflected from the polar ice-caps will be closer to the horizontal because being at the poles, incident light will be much closer to the horizon, plus the fraction of light that is reflected more vertically will be less likely to get refracted or scattered by the sky back down to Earth (i.e. more likely to escape into space).

If this 30 degree scattered angle can be treated as representative of all light that is reflected from the polar ice-caps and back to Earth, then, given polar ice-caps had, until recently, a radius of about 2,200km (for both Antarctica and Arctica sea ice in winter), this would mean that only a 346km-wide peripheral ring around the ice caps, amounting to about 28% of their total area, would scatter some light back to be absorbed by Earth (partially moderating the runaway effect of receding ice-caps due to global warming).  However, as ice-caps retreat, the percentage area of this ring increases, and so the average reflectance into space from the ice caps reduces (a double-whammy, on top of the area of ice reducing).  This means, I think, that the current marginal impact of shrinking ice caps may not be quite as bad as assumed in current models, but as the melting continues, the effect on accelerated global warming may get progressively worse.  Forecast effects would, however, also be impacted by the need to recalibrate any revised model.

In summary, recognising this issue of light scattering off ice and back down from the sky could make a material although perhaps not dramatic difference to global warming predictions, and it may well be worth exploring with more rigour than my back-of-the-envelope calculations.