An analysis of the genome of a Siberian Neanderthal, published today in Nature, reveals for the first time that humans contributed DNA to the Neanderthal genome about 100,000 years ago; that’s 50,000 years earlier than the previous estimate. The finding points to an earlier departure from Africa for our human ancestors.

Between 1 and 7 percent of the Siberian Neanderthal’s genome was human — inherited from people who migrated out of Africa. That suggests humans and Neanderthals interbred several times. But it also alters our understanding of human history. Since Neanderthals didn’t make it to Africa, humans must have left about 50,000 years earlier than evolutionary biologists had previously estimated. And that’s big news, says Sergi Castellano, an evolutionary biologist at the Max Planck Institute for Evolutionary Anthropology in Germany and a co-author of the study. This is “the first piece of genetic evidence” that some modern humans “were already out of Africa” 100,000 years ago, he says.

Previous genetic analyses have revealed that humans interbred with Neanderthals less than 65,000 years ago, outside of Africa. As a result, Europeans and Asians inherited between 1 and 4 percent of their DNA from Neanderthals. And that DNA still has an effect on humans today; just last week scientists linked Neanderthal DNA to a wide range of human health conditions, including depression and nicotine addiction.

But until now, what researchers knew about Neanderthal-human interactions came from studying the flow of genes from Neanderthals to humans — and not the other way around. That’s mostly because researchers didn’t have the kinds of technologies or the appropriate Neanderthal DNA samples that would allow them to search in the opposite direction. This is the first time that scientists have been able to find evidence that humans left their genetic mark on Neanderthals as well, says Laurent Frantz, an evolutionary biologist at the University of Oxford who didn’t work on the study.

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