Having trained themselves to specialise and exchange between the sexes, having got into the habit of exchanging labour with others, the thoroughly modern Africans had then begun to extend the idea a little bit further and tentatively try a new and still more portentous trick, of specialising within the band and then between bands. This latter step was very hard to take, because of the homicidal relationships between tribes. Famously, no other species of ape can encounter strangers without trying to kill them, and the instinct still lurks in the human breast. But by 82,000 years ago, human
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