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Scientists said today that they have made a surprising discovery about prehistoric sex. Researchers say that they have found direct evidence that different types of human-like ancestors were mating with each other. NPR's Laurel Wamsley has the story.
LAUREL WAMSLEY, BYLINE: In the Altai mountains of southern Siberia, there's a cave that was inhabited for millennia. It overlooks a river valley good for hunting, and it's called Denisova. Svante Paabo has been there.
SVANTE PAABO: The main chamber is very high - 20, 25 meters - and have a little hole in the ceiling. So lights come in from above. It's almost church-like.
WAMSLEY: Paabo is a geneticist at the Max Planck Institute for Evolutionary Anthropology in Germany. Back in 2010, he and his colleagues took from the cave a bone fragment - a shard of a pinkie bone - and extracted and sequenced its DNA. What they discovered was a previously unknown branch of the human family tree, a group they dubbed Denisovans who lived in Asia until about 40,000 years ago. Denisovans weren't the only human relatives using this cave. Neanderthals were there, too. Neanderthals of course were our stocky, cave-dwelling cousins. So when researchers found another tiny bone fragment just an inch long that seemed vaguely human, they weren't quite sure who it belonged to.
PAABO: I was sort of convinced it would be either a Denisovan, a Neanderthal or a modern human. And the first part of the genome we looked at was the mitochondrial genome, which is a tiny part of the genome that we inherit exclusively from our mothers.
WAMSLEY: And that part was showing good evidence of this individual being a Neanderthal. But as they kept analyzing the bone...
PAABO: People in the lab then started to have this indication that it was equally close to Denisovans. I was initially convinced they had screwed something up in the lab or mixed something up in analysis.
WAMSLEY: But they hadn't. And as a paper published today in the journal Nature explains, they found something quite remarkable. The bone belonged to a young female at least 13 years old who lived about 90,000 years ago. What makes her special? She had a Neanderthal mother and a Denisovan father. From their previous sequencing of that pinkie bone in 2010, they already knew that Denisovans had mated with Neanderthals at some point in their past. But to find the remains of someone who was herself the first-generation offspring of these two groups is surprising.
PAABO: We then had very direct evidence of mixing with each other.
WAMSLEY: Sharon Browning, a statistical geneticist at the University of Washington, says sex between Neanderthals and Denisovans couldn't have happened very often, otherwise their genes would not be so distinct.
SHARON BROWNING: There can't have been too many of these admixed individuals. So being able to find this particular bone that is from this type of individual is pretty amazing. It's like catching something as it's happening.
WAMSLEY: Pabbo says that this finding means that when Neanderthals and Denisovans met, they had no problem mating with each other. And full disclosure - it wasn't just Denisovans and Neanderthals that were hooking up. Modern humans mixed with both of them, too.
PAABO: It's beginning to be a picture where all these three groups when they met mixed quite readily with each other.
WAMSLEY: And that's why he says many people today have Neanderthal or Denisovan DNA in their own genomes. Laurel Wamsley, NPR News.
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