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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.

By Peter N .SpottsStaff writer / March 24, 2010

Participants in a 2005 archaeological conference crowd into Denisova cave in the Altai Mountains of Siberia. A pinkie bone found in the cave points to what could be a new human ancestor distinct from Neanderthals or modern humans. Researchers call this potential ancestor X Woman.

Johannes Krause/AP/Nature

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A mystery female known as X Woman may add a new chapter to the story of human ancestors leaving Africa to inhabit much of the planet.

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Genetic material from a pinkie bone discovered two years ago in Siberia is challenging scientists' understanding of when humans and their evolutionary brethren left Africa, and whether a distinct and previously unknown species might have existed.

Researchers estimate the age of the pinkie bone to be between 30,000 and 48,000 years old. At that time, Neanderthals and anatomically modern humans also lived in that region of Siberia, deep in the Altai Mountains.

But to the researchers' surprise, the bone's genetic signature contained many distinct features compared with those of the remains of Neanderthals or modern humans. Indeed, the team's DNA analysis suggested that the bone came from line of so-called hominins that last shared a common ancestor with Neanderthals and modern humans about 1 million years ago. The team dubbed this potential common ancestor X Woman.

By contrast, Neanderthals and modern humans last shared a common ancestor about 500,000 years ago, anthropologists say – making X Woman about twice as distant from humans on the evolutionary tree as from Neanderthals.

"This was absolutely amazing," says team member Svante Paabo, with the Max Planck Institute for Evolutionary Anthropology in Leipzig. Whoever this was that left "Africa 1 million years ago is some new creature that has not been on our radar screen so far."

Who is X Woman?

The findings raise questions about what, exactly, X Woman is. Moreover, it's not clear yet whether the pinkie's owner is a distinct species descended from that million-year-old lineage or whether it represents a hybrid whose ancestors had long since interbred with other hominins in the region.

They also throw into doubt the theory that hominins left Africa for Eurasia in three waves.

Up until this discovery, that was the widely accepted view, Dr. Paabo says. Homo erectus bailed out on the African continent roughly 2 million years ago. Ancestors of Neanderthals left between 500,000 and 300,000 years ago. And anatomically modern humans migrated north some 50,000 years ago.

"We now clearly see something coming out of Africa" between Homo erectus and ancestors of Neanderthals, Paabo says. "Now that we've seen this, perhaps we shouldn't be that surprised, in hindsight," given the relative ease with which hominins could move from northern Africa across the Middle East and into the rest of the world.

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