The problem was compounded because hominines stayed the same size, with the result that their bigger brains could be achieved only by reducing the size of another organ, the stomach, a trade-off which in turn reduced the efficiency of the digestive tract, which in turn demanded a still better diet.引自 Human Origins from c. 5 million years ago
OUT OF AFRICA
Stone technology was not the only factor in the evolutionary pressures that led to larger brains. It was also to do with allowing hominines to remember, to manipulate, to support and to organize others in more complex ways. Perhaps paradoxically, as hominines developed these more sophisticated social structures, so they simultaneously became less reliant on one another and better adapted to living in smaller groups. This in turn allowed them to colonize harsher barrier habitats such as the Sahara at the margins of their homelands from where they could colonize new, more temperate areas beyond.
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