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Five million years ago in Africa, we humans last shared a common ancestor with the chimpanzees and bonobos. By four million years ago, Australopithecines were walking upright, though with an ape-sized brain. With the damming up of the Old Panama Canal about three million years, and the long detour imposed on the ocean currents that might tend to equilibrate the salinity differences between the Atlantic and the Pacific Oceans, a major climate transition began, characterized by marked instabilities in temperature on a number of time scales. By 2.5 Mya, the chimp lineage split into the bonobos (now solely on the left bank of the Congo River) and the chimpanzees (now found from the Rift Valley to westernmost Africa) stayed on the right bank. In Asia, the gibbons spun off the siamang at about the same time. And this is the time at which the Australopithecine lineage spun off the Homo branch, with their bigger and bigger brains (a three-fold enlargement in volume since the split). Archaeologists tracing the origins of stone toolmaking report a substantial increase in the numbers of simple split-pebble tools about 2.5 Mya (crudely crack a rock to get a sharp edge); by 1.5 Mya there were varied "toolkits."
No one knows when higher intellectual functions (structured language, planning ahead, multistage logical inference, a fondness for games with arbitrary rules, and music) developed, or in what order. There is some thought that the extensive "get set" planning needed for ballistic movements such as hammering and accurate throwing might have been important in evolving the foundations for syntax, helping develop the critical mass of neural machinery needed for the virtues of structured sentences and logical plans to begin earning their own way. (More).
Evolutionary theory often seems like economics; efficiency considerations might cause a lean mean machine to win, one stripped of any fat or excess capabilities, as suggested by the eyeless underground mammals. In practice, it is usually difficult to strip out no-longer-needed organs and behaviors without causing fatal errors in development; furthermore, climates often change before a selection regime can streamline things. Indeed, switches in climate may promote a jack-of-all-trades set of capabilities under some conditions. The rapidity of the climate change would appear to be more important than its magnitude.
Consider the difference between a large cooling (on the scale of the D-O cycles) that ramps down over 500 years (20 human generations) and an abrupt cooling that steps down in only 10-15 years (within one generation's lifespan). In both cases, this is the equivalent of jacking up the landscape by a thousand meters into a new climatic zone; it may well be accompanied by a substantial decline in rainfall; unlike ice sheets, this happens even in the tropics. If the temperature ramps down, the plant species gradually change, as does the mix of animal species, but hominids could still pretty much find and prepare their food in the manner taught them by their parents. If the temperature steps down, forests die by drought and fire; a new plant succession starts with grasses and, a few centuries later, there is a new environment. But for our ancestors, the abrupt transition would have meant that they had to find, within just several years, an entirely new way of making a living. A hundred generations earlier, something similar might have occurred, but culture likely did not retain this recipe for finding and preparing the substitute food over such a long interval when better foods were routinely available.
So the abrupt climate changes of the Pleistocene may have served to reward those variants who had retained the epigenetic rules (things such as "likes to hammer," "likes to practice throwing," "likes to dig") needed to exploit the foods of the bottleneck period. And what might those foods be? The earliest stage of a plant succession emphasizes the grasses. Grazing and browsing animals would have been able to expand their ranges. For hominids, unable to digest grass directly, one solution would have been to eat animals that eat grass. This is still a strategy for the high latitudes where winter causes a period of several months when plants are dormant and only the grass remains nutritious in any quantity. Something similar might have gotten our ancestors through the bottleneck period after an abrupt cooling.
Because there were so many of these events, only small incremental improvements might be seen after any one bottleneck. Following the transition, diet might revert to more of a mix. But slowly, the epigenetic rules that aid hunting of large grazing mammals would have been enhanced.
There is not a simple correltion between brain size and behavioral repertoire, but it is surely easier (just as in our expansionistic economy) to add on a new behavior if, at the same time, you don't have to eliminate something to make room for it. The bigger-than-average brains of a given generation may well have been more likely to preserve the features that, a hundred generations earlier, got their ancestors through a similar bottleneck. So downsteps in temperature could promote brain changes far more quickly than ramping down temperature.
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