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One long-standing idea, which traces back to Darwin, is that the human lineage long ago became fundamentally less brutish and violent than apes. Unlike Rousseau, Darwin was no romantic, but he had a benevolent view of human nature. In his 1871 masterpiece, The Descent of Man, he reasoned (somewhat long-windedly) that reduced aggression was a key driving force early in human evolution: In regard to bodily size or strength…we cannot say whether man has become larger and stronger, or smaller and weaker, in comparison with his progenitors. We should, however, bear in mind that an animal possessing great size, strength, and ferocity, and which, like the gorilla, could defend itself from all enemies, would probably, though not necessarily, have failed to become social; and this would most effectually have checked the acquirement by man of his higher mental qualities, such as sympathy and the love of his fellow-creatures…. The slight corporeal strength of man, his little speed, his want of natural weapons, &c., are more than counterbalanced, firstly by his intellectual powers, through which he has, whilst still remaining in a barbarous state, formed for himself weapons, tools, &c., and secondly by his social qualities which lead him to give aid to his fellow-men and to receive it in return.17
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Daniel E. Lieberman (Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding)