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X Woman: Not human, not Neanderthal, what is she?

Scientists have found evidence of what might be a 'new creature' that is neither Neanderthal nor human. X Woman could revise theories about human ancestors and when they left Africa.

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Still, other researchers say they are impressed with the results, which appear in tomorrow's issue of the journal Nature.

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Though the lineage may not have remained in Siberia for a million years, some vestige of it still appears 50,000 years ago, when the area was populated with humans and Neanderthals as well. "The fact that you can find a million-year-old lineage in Siberia that dates to individuals alive about 50,000 years ago is really quite remarkable," says Theodore Schurr, a biological anthropologist at the University of Pennsylvania in Philadelphia.

A genetic treasure trove

The genetic material collected from the pinkie bone was mitochondrial DNA, which ordinarily is inherited only from mothers, researchers say. This type of DNA, found outside a cell's nucleus, is an evolutionary bonanza for anthropologists.

Mitochondrial DNA is present in larger quantities than nuclear DNA, which carries an organism's entire genetic blueprint. So it doesn't take a large sample of tissue or bone to perform the necessary analysis. Mitochondrial DNA also is simpler – forming only 37 out of a human's typical compliment of 20,000 to 25,000 genes. And because it doesn't recombine with male DNA, it tells a less garbled evolutionary story.

Mitochondrial DNA may be useful for tracking ancestry and as a biological clock for estimating the age of particular lineage, but the team acknowledges that it will take studies of nuclear DNA found in the pinkie bone to determine if the pinkie's owner represents a unique species.

What if it is a new species?

If it is a distinct species, it raises some interesting question, Paabo and other team members acknowledge. Successive migrations of humans and their ancestors into Europe appear to have led to one group replacing another. That does not appear to be the case in southern Siberia, especially if three lineages occupied the area at the same time, rather than two. What accounts for the difference?

Moreover, scientists at Denisova Cave, where the pinkie was unearthed, unearthed bracelets and other ornaments from the same layer in which they found the pinkie. "There are several elements in the layer that have been associated with modern humans," acknowledges Johannes Krause, who is the lead author on the Nature paper reporting the results.

It could be that material from higher layers somehow got mixed into the layer hosting the pinkie bone. But it also could indicate that the individual involved was somehow interacting with modern humans in the region.

"It's quite puzzling," Krause says.