The human family tree may be in for a dramatic rewrite. DNA collected from a fossilised finger bone from Siberia shows it belonged to a mysterious ancient hominid – perhaps a new species.
"X-woman", as the creature has been named, last shared an ancestor with humans and Neanderthals about 1 million years ago but is probably different from both species. She lived 30,000 to 50,000 years ago [in a cave like the one pictured above, where the fossil was found].
"This is the tip of the iceberg," says Chris Stringer, a palaeoanthropologist at the Natural History Museum in London who was not involved in the find. More hominids that are neither Neanderthal nor human are likely to be discovered in coming years, particularly in central and eastern Asia, he says.
Previously, anthropologists thought that Neanderthals and humans were the only hominids roaming Europe and Asia during the late Pleistocene. The discovery of 17,000-year-old Homo floresiensis – the "hobbit" – dispelled that notion, but many anthropologists look on H. floresiensis as an anomaly, isolated from the human–Neanderthal hegemony on the mainland.
The newly discovered creature, which probably lived in close proximity to humans and Neanderthals, suggests that things were not that simple. "The picture that's going to emerge in the next years is a much more complex one," says Svante Pääbo, a palaeogeneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
Pääbo and colleague Johannes Krause discovered the specimen in the Denisova cave in southern Siberia, and sequenced DNA from its mitochondria. It is impossible to say what the creature would have looked like based on a single pinkie bone, so Pääbo and Krause are hesitant to call it a new species.
Though the creature's sex is not known, they are for now referring to her as X-woman because mitochondria are inherited maternally. "No one really knows what she would look like," Pääbo says.
X-woman's mitochondria differ from a human's at nearly 400 DNA letters; Neanderthals show only half as many differences.
This suggests that X-woman shared an African ancestor with the two other species somewhere between 780,000 and 1.3 million years ago, before striking north and east. This expansion is distinct from the one that occurred around 500,000 years ago that gave rise to Neanderthals, and from our own species' peregrinations from about 50,000 years ago.
The split seems too recent for X-woman to be related to Homo erectus, which began moving out of Africa around 2 million years ago.
However, Clive Finlayson, a palaeoanthropologist at the Gibraltar Museum, says the idea that there were just a handful of hominid migrations out of Africa is a vast oversimplification that ignores how other species expand their range over time. "To talk about one or two expansions from a particular region doesn't make any biological sense," he says. "There were probably hundreds, thousands of migrations out of Africa."
Though there is no complete skeleton for X-woman, her lineage could mean she is related to any number of more complete specimens recovered in Asia that don't neatly fit human or Neanderthal body patterns, says Stringer. "This new DNA work provides an entirely new way of looking at the still poorly understood evolution of humans in central and eastern Asia."
Pääbo and his team are hesitant to speculate too much about X-woman's nature until they obtain DNA sequences from the nuclear genome's 3.1 billion letters. That project is already under way, and the first results should come within months. Pääbo's team will likely want X-woman's genome to answer the same questions they are asking of the Neanderthal genome, which is due for publication soon.
For instance, humans and Neanderthals share unique mutations in a gene linked to speech and language called FOXP2. If X-woman's sequence is complete enough, they will be able to determine if it possesses the same change – and potentially the capability for language.
There is no sign in X-woman's mitochondrial genome that her kind interbred with humans or Neanderthals, but the nuclear genome will offer a far better chance of finding out.
Given the close proximity of Neanderthal remains dated to the same time and artefacts that appear to be human, interbreeding is not unlikely, Pääbo says. "Having in about the same time window three different forms [of hominids], increases the potential of all types of interactions, including genetic."
X-woman's mitochondrial DNA begins to paint a picture of what she was like, if only a blurry one. The protein-coding genes do not contain any surprising mutations that would cause disease.
Finlayson would love to link X-woman to other bones, and even stone technologies, though the chances of doing this may be slim. "Ideally we would like to have all that information, but we don't. The fact that we've got this genetic result is important, it's very important."
Pääbo hopes that such a connection will come through sequencing DNA from other Asian hominid fossils. But he, too, is prepared for the possibility that such bones may never turn up.
He sees in X-woman the beginning of a new way of understanding human history. "It gives another picture of our past, a molecular picture of the evolution of our genome" which he says is in some aspects even more conclusive than fossils.
Journal reference: Nature, DOI: 10.1038/nature08976