Scientists finally know when humans and Neanderthals mixed DNA

Scientists have long agreed that early humans mated with Neanderthals, but a pair of recent studies have shed light on when exactly this DNA mixing occurred. Such a revelation could help geneticists learn more about our past — and crucially, our future.

The studies, published Dec. 12 in the journals Science and Nature, provide information about the timelines of Neanderthal and early human interactions, and reveal that ancient interbreeding left strands in modern DNA that can still be seen today. The fact that Neanderthals and early humans interbred has been known since the first Neanderthal genome was sequenced in 2010, but these studies suggest the interactions happened more recently than scientists once thought.

What did the studies find?

Researchers for the Science study "analyzed genomes from 275 present-day and 59 prehistoric humans who lived between 2,200 and 45,000 years ago," said The Washington Post, determining that "Neanderthals and humans interbred for 7,000 years starting about 50,500 years ago." At the same time, researchers for the Nature study discovered a "new line of evidence by sequencing the oldest human genomes yet, bringing to life a 45,000-year-old human family."

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By combining data from both studies, it was concluded that early humans and Neanderthals reached their peak interbreeding period around 47,000 years ago. The height of this interbreeding was not "exceptional trysts but a way of life," said the Post. Scientists "cannot time travel, but this data is allowing us to time travel and see what's happening 50,000 years ago in our history," Priya Moorjani, a senior author of the Science study and UC Berkeley professor, said to the Post.

During this peak, early humans "encountered Neanderthals, had sex and gave birth to children on a fairly regular basis," said CNN. It is estimated that 1% to 3% of people have DNA that can be linked to Neanderthals, and the studies also shed light on these genes. Some, like "those related to the immune system, were beneficial to humans as they lived through the last ice age, when temperatures were much cooler, and they continue to confer benefits today."

How is this information useful?

This updated timeline for Neanderthal-modern human interbreeding "shifts and narrows the possible range of time when humans spread to places like present-day China and Australia," said NBC News. These latest findings "line up with archaeological evidence that suggests Neanderthals and humans overlapped in geography as humans traveled out of Africa."

The new studies are also "much more of a direct estimate compared to the previous inference, which involved fitting a fairly complex statistical model that had a lot of uncertainty," Princeton University genomicist Joshua Akey said to NPR. These discoveries are "really important because it does constrain quite a few other things about human migration patterns," Benjamin Peter, a University of Rochester geneticist, said to the outlet.

The study also shows that we may have this interbreeding to thank for modern immunity. Neanderthal genes "may have been crucial to our success by protecting us from new diseases we hadn't previously encountered," said the BBC. This Neanderthal DNA might have given us "better adaptive capabilities outside of Africa," said Chris Stringer of London's Natural History Museum to the outlet. Early humans "had evolved in Africa, whereas the Neanderthals had evolved outside of Africa," and "by interbreeding with the Neanderthals, we got a quick fix to our immune systems."