December 22, 2010

Ancient Human Relative Interbred With Melanesia Ancestors

Scientists have recovered the DNA code of a human relative discovered recently in Siberia, which found that the relative roamed far from the cave that holds its only known remains.

Scientists found evidence that these "Denisovans" from over 30,000 years ago ranged all across Asia.  They apparently interbred with the ancestors of people that now live in Melanesia.

There is no sign that Denisovans mingled with ancestors of people living in Eurasia, which made the connection between Siberia and distant Melanesia quite a shock.

This is the second report in recent months of using a new tool to illuminate the evolutionary history of humankind.  Some of the same scientists found in May that Neanderthals interbred with ancestors of today's non-African populations.  Researchers say that might have happened in the Middle East after the ancestors left Africa but before they entered Eurasia.

One expert familiar with the situation said the new evidence is probably just the start of what can be learned from their genome.  Todd Disotell of New York University told The Associated Press that eventually, it should provide clues to traits like eye and skin color.

"We're going to be able to piece these people together in the next few years from this genome," he said.

Denisovans were first discovered nine months ago from a sampling of DNA recovered from a finger bone found in the Denisova Cave in southern Siberia.

Researchers said that there is not enough evidence to determine whether Denisovans are a distinct species.

The genome showed that Denisovans are more closely related to Neanderthals rather than modern humans.  That indicates that they both sprang from a common ancestor on a different branch of the evolutionary family tree other than the one leading to modern humans.

David Reich, a Harvard University researcher and an author of the new paper, wrote that scientists have no idea what Denisovans looked like.

Scientists found evidence that about 5 percent of the people now living in Melanesia could have their genomes traced back to Denisovans.

"We thought it was a mistake when we first saw it," Reich wrote. "But it's real."

He said this suggests that Denisovans once ranged widely across Asia. 

Disotell told AP that he and colleagues were "blown away" by the unexpected Melanesia finding, with its implication for where Denisovans lived.

"Clearly they had to have been very widespread in Asia," and DNA sampling of isolated Asian populations might turn up more of their genetic legacy, he said.

Rick Potts, director of the human origins program at the Smithsonian Institution, told AP that the new work greatly strengthens the case that Denisovans were different from Neanderthals and modern humans.

However, he said they still may not be a new species because they might represent a creature already known from fossils but which did not leave any DNA to compare.

Potts also told AP that the Melanesia finding could mean that the Melanesians and the Denisovans did not intermix, but retained ancestral DNA sequences that had been lost in other populations sampled in the study.

He said he does not know if that is a better explanation than the one offered by the structure.

"I am excited about this paper (because) it just throws so much out there for contemplation that is testable," Potts told AP. "And that's good science."

Reference: "Genetic history of an archaic hominin group from Denisova Cave in Siberia." Nature, 23. December 2010, doi:10.1038/nature09710


Image 1: The morphology of this Denisovan molar differs from the morphology of a Neandertal or a modern human molar. (Copyright: Max Planck Institute for Evolutionary Anthropology)

Image 2: Excavation works inside the East Gallery of the Denisova cave where archaeologists found part of a finger bone in 2008. (Copyright: Max Planck Institute for Evolutionary Anthropology)


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