The story of human evolution just got even more bizarre. The genome of an extinct hominin species, the Denisovans, contains unusual snippets of DNA that seem to have come from yet another group.
It could be evidence of an entirely new species of hominin, as yet unknown to science. Alternatively, it could be our first genetic record of one of the many species known only from their fossils.
The new hominin has left its traces in the genome of a Denisovan, an extinct hominin known to exist from a finger bone and two teeth found in a Siberian cave. Nobody knows what Denisovans looked like because there are so few fossils. But geneticists have managed to sequence their entire genome to a high degree of accuracy.
David Reich of Harvard Medical School in Boston, Massachusetts, has now taken a close look at the Denisovan genome and found that some stretches of it don't fit. He presented his findings at a Royal Society discussion meeting on ancient DNA in London, UK, on Monday.
The genome shows that Denisovans were cousins of the Neanderthals - this much was already known. Their lineage branched off from ours around 400,000 years ago, before splitting into the Neanderthals and Denisovans.
That should mean that Denisovans and Neanderthals look equally different from modern humans, but on closer inspection, Reich found that that wasn't the case. "Denisovans appear more distinct from modern humans than Neanderthals," he told the meeting. Specifically, scattered fragments amounting to 1 per cent of the Denisovan genome look much older than the rest of it.
The best explanation is that the Denisovans interbred with an unidentified species, and picked up some of their DNA. Or as Reich puts it: "Denisovans harbour ancestry from an unknown archaic population, unrelated to Neanderthals."
The data looks convincing, says geneticist Johannes Krause of the University of Tübingen in Germany. "There's a very strong signal, difficult to question."
Krause is one of several geneticists who have studied the Denisovan genome and wondered if it might show traces of past interbreeding. About the only thing we know from the sparse Denisovan fossils is that they had very large teeth, which look like those of a much more primitive species. If the Denisovans were breeding with an archaic species, that might explain their large gnashers ( via newscientist.com ).