Continued study of an approximately 40,000 year old finger bone from Siberia has identified a previously unknown type of human – one that may have interbred with the ancestors of modern-day Melanesian people.

The fossil scrap – just the tip of a juvenile female's finger – was discovered in 2008 during excavations of Denisova cave in Siberia's Altai Mountains. Anatomically, it looks like it could have belonged to a Neanderthal or a modern human. But, in an initial announcement published in April in Nature, a team of scientists led by geneticist Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology concluded the bone belonged to a distinct population of humans that last shared a common ancestor with Neanderthals and our species about a million years ago.

The new study, published by Pääbo and colleagues Dec. 22 in Nature, provides further evidence that Denisova cave was home to unique humans. The researchers analyzed genetic sequences recovered from the nuclei of cells, which offer better resolution of relationships than the mitochondrial samples used in the previous research. The Denisova DNA sequences were closest to the Neanderthals, indicating they shared a more recent common ancestor with Neanderthals than with us.

The new genetic data suggests the ancestors of the Neanderthals and Denisovans left Africa between 300,000 to 400,000 years ago and rapidly diverged. But this estimate is based on models of the rates that genes typically mutate and could be off the mark.

"The Neanderthal and Denisova population history may be roughly twice the length suggested in [the Nature] paper," said University of Wisconsin – Madison anthropologist John Hawks, who was not involved with this study. "The ancestors [of the Denisovans] might be the original "Homo erectus" dispersal from Africa."

The big question, however, is whether the Denisovans are a new species of human.

They were genetically distinct from other humans, and an upper molar tooth (above) found at the same excavation hints that these people were similar to earlier species like Homo erectus.

But this is not enough to declare a new species, especially since the same team of researchers recently found that Neanderthals likely interbred with populations of our species that moved outside Africa. Between 1 and 4 percent of the genes of non-Africans match those found in Neanderthals, making it difficult to draw the species line.

An unexpected discovery about the Denisovans further complicates the picture: Some modern-day people carry Denisovan genes. Through genetic comparisons Pääbo’s team found that some people from Melanesia – an assemblage of islands off Australia's east coast, including New Guinea – share 4 to 6 percent of their genomes with the Denisovans. This probably indicates that the Denisovans interbred with anatomically modern humans despite the split between our lineages over a million years ago.

The authors of the new paper didn't go as far as calling the Denisovans a new species, and "on a biological species concept," says Hawks, "there's really no reason to regard this as a different species."

Images: 1) The molar from the Denisova cave, as seen from above and the side. Credit: David Reich et al., Nature**. 2) A map of human migrations. The triangles and circles represent sampling locations of Neanderthal remains of present-day human genomes, respectively. The blue arrows trace major migration routes of anatomically modern humans out of Africa. The yellow box and star denote the correspondence between the Denisova DNA samples and the genomes of people from Melanesia. From Bustamante & Henn, Nature.

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