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Scientists in Europe believe they've uncovered a newcomer to the family tree. It lived and died in Siberia about 40,000 years ago, probably alongside modern humans and Neanderthals. Genetically, though, it was different. NPR's Christopher Joyce has more on what the scientists are calling the X-woman.

CHRISTOPHER JOYCE: Anthropologists didn't have much to work with on this paleontological puzzle - just a piece of pinky finger discovered in a cave in the Altai Mountains of Siberia.

What most surprised researchers at the Max Planck Institute in Germany was the DNA inside it - specifically, mitochondrial DNA, which is found in every human cell, but outside the cell nucleus.

It's rare to be able to isolate DNA that's 40,000 years old. When researcher Johannes Krause finally did, he called his colleague, anthropologist Svante Paabo.

Dr. SVANTE PAABO (Anthropologist): Johannes called me when I was in the U.S. and told me about this, and it was, of course, absolutely amazing. And I first really didn't believe him. I thought he was pulling my leg.

JOYCE: What the DNA showed is that whoever that finger belonged to was not a modern human like us, nor was it a Neanderthal, the only other member of the human line known to be living in Europe then.

Paabo calls the individual the X-woman, although he doesn't actually know its gender. The X represents the mystery. Was this a new species of human that scientists didn't know about? Paabo says they need DNA from the nuclei of the bone cells to determine that, and they're working on it.

But Paabo does say the DNA they already have does indicate that this Siberian stranger, along with humans and Neanderthals, evolved from some common ancestor that lived in Africa at least a million years ago.

Dr. PAABO: Whoever sort of carried this mitochondrial genome out of Africa about a million years ago is some new creature that has not been on our radar screens so far.

JOYCE: Scientists know generally when this new hominin, or hominid - as human ancestors are known - appeared on the scene by comparing the number of mutations in the DNA of humans, Neanderthals and the X-woman. Mutations occur in a population at a certain rate that can indicate how long they've evolved separately from a common ancestor.

And though Paabo can't say if this is a new species on the human line, he can say this migration out of Africa by the X-woman's ancestors occurred well before modern humans evolved. It would have been a migration that scientists didn't know about before.

Terry Brown, a geneticist at the University of Manchester in England, says the discovery suggests that our African ancestors were more restless than previously thought.

Professor TERRY BROWN (Geneticist, University of Manchester in England): I think that probably hominids were leaving Africa much more frequently than we imagined. There's no real reason why a hominid should stay in Africa if its populations grows and it needs more space, and obviously, it's logical to move into Eurasia.

JOYCE: Brown also says there's plenty more DNA to be found in fossils, especially from colder climates where it's better preserved.

Prof. BROWN: I suspect that there are going to be some more surprises around the corner, as well. If there are other bones we can get DNA from, then I think it's possible. We may find a greater variation amongst our early ancestors than we had previously realized. I think this is probably going to be the first step along that path.

JOYCE: The bone came from a cave called Denisova. Johannes Krause of the Max Planck Institute says one thing about the X-woman is clear: It would have been a rough life.

Dr. JOHANNES KRAUSE (Max Planck Institute): In the cave, we also find things like woolly mammoths, or also woolly rhino. And also in that region, it was, of course, affected by the ice age.

JOYCE: The research is described in the journal Nature.

Christopher Joyce, NPR News.

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