The large brains of H. erectus are striking because, as we have seen, brains are costly evolutionary machines. Indeed, the rate of increase of brain size to body weight in hominins was faster than the rates in any other group of species in evolutionary history.9 Perhaps sociability was the driver. The importance of social calculations shows up clearly in the human brain structure, which devotes an exceptional number of neuronal pathways to social calculations. Perhaps more neurons meant more friends, more food, better health, and a better chance of reproducing. Certainly, larger brains allowed
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