DNA from a 40,000-year-old human finger bone found in a Siberian cave points to a new lineage of ancient human, researchers report today. The find—the first made with genetic, not fossil evidence—suggests that Central Asia was occupied at that time not only by Neandertals and Homo sapiens but also by a third, previously unknown hominin lineage. "This is the most exciting discovery to come from the ancient DNA field so far," says Chris Tyler-Smith, a geneticist at the Wellcome Trust Sanger Institute in Hinxton, United Kingdom.
The work complicates the human story once again, much as the discovery of the controversial H. floresiensis—a.k.a. the hobbit —has upset earlier and simpler views of early human migrations around the globe. If four early humans including the hobbit were alive about 40,000 years ago, "the amount of [human] biodiversity ... was pretty remarkable," says geneticist Sarah Tishkoff of the University of Pennsylvania.
A team led by archaeologists Michael Shunkov and Anatoli Derevianko of the Russian Academy of Sciences in Novosibirsk found the finger bone in 2008 at Denisova Cave in Russia's Altai Mountains. The cave, which has many archaeological layers spanning 100,000 years, has yielded both Neandertal and modern human stone tools and a small collection of hominin bones too fragmentary to be identified. The finger bone came from a layer radiocarbon dated to between 48,000 and 30,000 years ago. Evolutionary geneticists Svante Pääbo, Johannes Krause, and colleagues at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, ground up a 30-milligram sample and extracted and sequenced all of the 16,569 base pairs of its mtDNA genome, using new techniques  Pääbo's group has successfully employed to sequence both Neandertal and prehistoric modern human DNA. The researchers compared the new mtDNA sequence with that of 54 living people from around the world, a roughly 30,000-year-old modern human from another Russian site, and six Neandertals.
They got a big surprise: Although Neandertals differ from modern humans at an average of 202 nucleotide positions in the mitochondrial genome, the Denisova hominin differed at an average of 385 positions from modern humans and 376 from Neandertals, the team reports online today in Nature. When mtDNA from chimpanzees and bonobos was added to the mix, the researchers were able to estimate that the new hominin had shared a common ancestor with Neandertals and modern humans about 1 million years ago.
But who was this mystery hominin? The team says the date is too late for Asian H. erectus, which first migrated out of Africa about 1.8 million years ago. And it's too early for H. heidelbergensis, which arose in Africa and Europe about 650,000 years ago and is thought by many researchers to be the common ancestor of humans and Neandertals. There's "no evidence" that these or other known species "persisted that late" in mainland Asia, says paleoanthropologist Russell Ciochon of the University of Iowa in Iowa City. Chris Stringer, a paleoanthropologist at the Natural History Museum in London, says the new species could represent "a pre-heidelbergensis, post-erectus dispersal" out of Africa "that we haven't picked up yet."
For now, Pääbo's team is not giving the new lineage a species name, at least until they know more about it. Next, the researchers plan to try to sequence nuclear DNA from the finger bone. If they succeed, they might discover the secret identity of Hominin X.
For full coverage, see the 26 March issue of Science.