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Now a peek inside a remote cave in Siberia. Scientists there have found a 50,000-year-old toe bone that tells a story about life and love among some of the earliest humans.

NPR's Christopher Joyce has this report on what the bone has revealed.

CHRISTOPHER JOYCE, BYLINE: Scientists stunned the world in 2010 by retrieving recognizable DNA from a Neanderthal bone. They compared it to modern human DNA, and what do you know? There's some Neanderthal DNA in most of us. Apparently we interbred with our primitive cousins. Since then, the plot has thickened.

The scientists then found a finger bone from a place called the Denisova Cave in Siberia. They thought Neanderthal. But no, it wasn't Neanderthal or modern human. It was some new but close relative, they called them the Denisovans - some third kind of early human and we know almost nothing about them. So now comes yet another bone, a toe from the same cave, a female Neanderthal this time and all its DNA is still there. And it shows that her ancestors also interbred with Denisovans.

Svante Paabo from Germany's Max Planck Institute leads the scientific team.

SVANTE PAABO: It was a general pattern. There were a lot of cases of mixing. Not only that modern humans had a tendency to mix with the people they met, but these now-extinct groups also mixed with each other.

JOYCE: So perhaps our ancestors, when it came to breeding, were not terribly particular.

PAABO: No, there was some level of promiscuity around.

JOYCE: Something else curious about this Neanderthal's DNA: it showed signs that in the deep past, Neanderthals bred with some other mystery species, perhaps Homo erectus. Now, Homo erectus evolved about two million years ago. As team member Montgomery Slatkin notes, it was a pretty primitive creature.

MONTGOMERY SLATKIN: As someone once said, if a Neanderthal came and sat next to you on the bus, you'd probably get up and change seats. But if a Homo erectus came and sat next to you on the bus, you'd probably get off the bus.

JOYCE: Slatkin, a geneticist at the University of California at Berkeley, says the pattern of DNA from this 50,000-year-old woman suggests that her family was inbred. Her parents were blood relations, half-siblings or something equivalent - perhaps an uncle and niece or an aunt and nephew.

SLATKIN: And that suggested to us that this was a rather small population, because that meant that close relatives had no choice but to mate with each other.

JOYCE: Writing in the journal Nature, the team says small groups of Neanderthals and Denisovans apparently met and reproduced in Eurasia. When modern humans arrived from Africa about 40,000 years ago, they joined the party.

Paabo says this DNA research isn't just about who slept with whom. He hopes to compile a genetic catalog.

PAABO: This catalog of all the genetic changes that makes modern humans unique, that's a very fascinating thing, I think, for the future to study.

JOYCE: One such future experiment, says Paabo: using ancient DNA to engineer Neanderthal-like cells to see how they differ from ours.

Christopher Joyce, NPR News.

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