Our own lineage began to spread within Africa starting at least two hundred thousand years ago, which may point to the advantages of collective learning.24 But in a world of small, scattered communities, most of them little larger than extended families, change was slow, erratic, and easily reversed. Whole groups could die out suddenly, along with the technologies, stories, and traditions they had built up over many centuries. The largest catastrophe of this kind occurred about seventy thousand years ago. Genetic evidence shows that the number of humans suddenly fell to just a few tens of
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