A burst of innovation followed the expansion of modern humans out of Africa. Signs of that change existed in Africa before the expansion, but the pattern became much stronger in Europe some 20,000 years later, after anatomically modern humans had encountered and displaced the Neanderthals. That transition to full behavioral modernity—as seen in the archaeological record—occurred patchily and finished later in other parts of Eurasia. We argue that even limited gene flow from Neanderthals (and perhaps other archaic humans) would have allowed anatomically modern humans to acquire most of their favorable alleles. We believe that this sudden influx of adaptive alle-les contributed to the growth of the capabilities that made up the "human revolution," and we believe that this introgression from archaic human populations will prove central to the story of modern human origins.

So by 40,000 years ago, humans had become both anatomically and behaviorally modern (which is not to say they were exactly like people today). They had vastly greater powers of innovation than their ancestors, likely owing in part to genes stolen from their Neanderthal cousins. The speed of cultural change increased by tens of times, and when the glaciers retreated and new opportunities arose, it accelerated further.

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