Sunday, May 23, 2021

#69. Storming South [Evolution]


Red, theory; black, fact.

This is a theory of the final stages of human evolution, when the large brain expansion occurred.
At least, we did.

Homo sapiens: created by ice

H. sapiens appears to have arisen from Homo erectus over the last 0.8 million years due to climate instability in the apparent origin area, namely East Africa. During this time, Europe was getting glaciated every 0.1 million years because of the astrophysical Milankovitch cycle, a rhythm in the amount of eccentricity in the Earth's orbit due to the influence of the planet Jupiter.
However, I am thinking of the poor slobs who had settled in Europe (or Asia, it doesn't matter for this argument) during the interglacial periods (remember that H. erectus was a great disperser) and when the ice began advancing again, were now facing much worse cooling and drying than in Africa, and thus much greater selection pressures. At least during the last continental glaciation, the ice cap only extended to the Baltic Sea at the maximum, but long before the ice arrives, the land is tundra, which can support only a very thin human population. In any given glaciation, the number of souls per hectare the land could support was relentlessly declining in northern Europe/Asia, and eventually the residents had to get out of Dodge City and settle on land further south, almost certainly over the dead bodies of the former owners. This would have selected Europeans or Asians for warlike tendencies and warfaring skills, which explains a lot of human history. 

Our large brains

However, our large brains seem to be great at something else besides concocting Games of Thrones: that is, environment modification. It's a no-brainer that the first thing someone living in the path of a 2-km wall of ice needs is to keep from freezing to death, and this would have been the first really good reason to modify environments. Unlike chipping a stone axe, environment modification involves fabricating something bigger than the fabricator. Even a parka has to be bigger than you or you can't get into it. This plausibly would have required a larger brain to enable a qualitatively new ability: making something you can't see all at once when it is at working distance.

Our rhythmic evolution

After parkas, early northerners might have evolved enough association cortex (maybe on the next glaciation cycle) to build something a little bigger, like a tent or a lean-to. On the next cycle, they might have been able to pull off a decent longhouse made of wattle. On the next, a jolly good castle surrounded by cultivated lands and drainage ditches. These structures would have delayed the moment of decision when you have to go and take on the Pleistocene-era Low-brows to the south. This will buy you time to build up your numbers, and I understand that winning battles is very much a numbers game. Therefore, environment modification skill would have been selected for in tandem with making like army ants.

Where is the fossil evidence for this theory?

Why do we not find fossil evidence of all this in Europe or Asia? <05-19-2022: Actually, we do: the Neanderthals and Denisovans, who have been difficult to account for in terms of previous theories of human origins.> My scenario can be defended against the inconvenient fossil evidence for a human origin in East Africa in general terms, by citing the well known incompleteness of the fossil record and its many biases, but, of course, I want details. Note, however, what else is in East Africa: the Suez, a land bridge to both Europe and Asia via the Arabian tectonic block, which was created by plate tectonics near the end of the Miocene, thus antedating both H. sapiens and H. erectus. Not only can hominins disperse through it to other continents during interglacials, but they can come back in, fiercer and brainier than before, when the ice is advancing again, to then deposit their fossil evidence in the Rift Valley region of East Africa. The Eurasian backflow event of 3000 years ago may be a relatively recent example of this. The Isthmus of Suez is low-lying and thus easily drowned by the sea, but the probability of this was minimal at times of continental glaciation, when sea levels are minimal. I assume that early hominins expanded like a gas into whatever continent they could access. Increasing glaciation/tundrafication of that continent would have recompressed the "gas" southward, causing it to retrace its path, partly back into Africa. 

Pleistocene selection pressures

To reiterate, this process would have been accompanied by great mortality and therefore, potentially, much selection. Moreover, during the period we are considering, temperatures were declining most of the time; the plot of temperature versus time has a saw-tooth pattern, with long declines alternating with short warming events, and it is the declines that would have been the times of natural selection of hominins living at high latitudes.

A limestone block in Canada showing scratches left by stones
embedded in the underside of a continental glacier.
The rock has also been ground nearly flat by the same process. Scary.

Glaciated boulder by night. Have a nice interglacial.

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