ARI SHAPIRO, HOST:
Look at the DNA of most people living outside of Africa today, and you'll see that tens of thousands of years ago, early modern humans mated with Neanderthals and had babies. Exactly how and when that interbreeding happened has been kind fuzzy. Science reporter Ari Daniel says new research helps clarify that ancestral timeline.
ARI DANIEL, BYLINE: Roughly a hundred thousand years ago, humans started leaving Africa in waves. Arev Sumer is a paleogenetics Ph.D. student at the Max Planck Institute for Evolutionary Anthropology in Leipzig.
AREV SUMER: And sometime about 50,000 years or so, there was a group that migrated into Europe and Asia.
DANIEL: Where scientists know something crucial happened.
SUMER: They met Neanderthals there.
DANIEL: Well, more than just met. Prehistoric sparks flew.
BENJAMIN PETER: They interbred and have, like, common offspring.
DANIEL: Benjamin Peter is a population geneticist at the University of Rochester.
PETER: Most people that live today outside of Africa have about 1 to 2% of the genome inherited from a Neanderthal ancestor.
DANIEL: Now, the timing of this ancient interbreeding has been hazy. Specimens from that period are scarce and in rough shape.
PETER: If a fossil is in a cave or other site for tens of thousands of year, DNA tends to degrade a lot.
DANIEL: Peter and his colleagues got around the problem with a new computational approach. They took the beat-up DNA sequences from 59 ancient humans living thousands of years ago, primarily in Eurasia, and compared them to good-quality DNA sequences from a few Neanderthals and a bunch of people today with very little Neanderthal ancestry.
PETER: We looked at people that lived over the last 45,000 years and detected which parts of their genome come from a Neanderthal. So what we did that's novel is we traced Neanderthal ancestry through time.
DANIEL: Meanwhile, Arev Sumer and her team were also trying to clock when early humans and Neanderthals interbred. They worked with a rare set of well-preserved early human remains from Germany and the Czech Republic.
SUMER: So we were able to get high-quality genomes from two of them.
DANIEL: Sumer analyzed those two genomes, along with lower-quality DNA from five other individuals at the site in Germany. And she could draw a couple conclusions. First, these individuals lived at least 45,000 years ago, meaning...
SUMER: We just identified the oldest modern human family ever sequenced and ever known, actually.
DANIEL: Sumer also found that 3% of the DNA of these early humans arrayed in fairly long stretches came from Neanderthals, the result of that earlier interbreeding.
SUMER: In each generation, you can imagine the Neanderthal DNA getting broken into smaller pieces and getting shorter and shorter. So we can estimate how many generations must have passed since this event happened.
DANIEL: Adding that to the age of the specimens, Sumer could calculate a more refined timestamp for when the ancestors of this early group of humans likely interbred with Neanderthals over multiple generations. The answer - between 45 and 49,000 years ago.
PETER: Which is sort of on the later side from what people think.
DANIEL: Using their method, Benjamin Peter and his colleagues got pretty much the same time estimate, which means that when the descendants of those who interbred with the Neanderthals ultimately fanned out across Europe, Asia and eventually Oceania and the Americas, it would have been on the more recent side of what researchers have believed, adding a little more clarity to the complex story of human origins. The findings are published in the journals Science and Nature. For NPR News, I'm Ari Daniel.
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