24 December 2010
New human ancestors found in Siberian cave
DNA taken from a pinkie bone at least 30,000 years old is hinting at the existence of a previously unknown population of ancient humans. The pinkie bone in question was unearthed in 2008 from what's called the Denisova Cave, in southern Siberia, and is the bone of a 6- to 7-year-old girl. David Reich, a geneticist at Harvard Medical School in Boston, and colleagues from the Max Planck Institute for Evolutionary Anthropology in Leipzig were able to extract DNA from the bone and sequence 70% of the nuclear genome. The researchers then compared this sequence with the genomes of Neandertals and modern humans and confirmed that the girl was neither human nor Neandertal.
This long-lost group of people, which researchers are calling 'Denisovans' after the Denisova cave in which the bone was found, lived at roughly the same time modern humans and Neandertals were in the region, and it appears to be more closely related to Neandertals than us. Although these Denisovans went extinct, they were widespread enough in Asia to interbreed with modern humans before they disappeared, leaving behind a ghostly legacy in the genomes of Melanesians.
Reich says there were several remarkable things about the group of people this girl is from. "On the one hand it's a sister group to Neanderthals, which means that it's more closely related to Neanderthals on average than it is to modern humans," he says. A Denisovan tooth found in the same cave shows a morphology that is distinct from Neanderthals and modern humans and resembles much older hominin forms. Bence Viola, a scientist at the Max Planck Institute of Evolutionary Anthropology comments, "The tooth is just amazing. It allows us to connect the morphological and genetic information."
The other remarkable finding was that Denisovans' genome was more closely related to humans currently living in New Guinea than it was to genomes of people in Europe or Asia. The researchers compared different parts of the Denisovan genome with the same segments of DNA in 53 populations of present-day humans. The data revealed that the Denisovans shared certain mutations with Melanesians from Papua New Guinea and Bougainville Island, mutations that are not found in Neandertals or other modern populations. Melanesians appear to have inherited between 4% and 6% of their DNA from these extinct Denisovans, the team reports.
"What it means is that there was gene exchange between relatives of this Denisovan and the ancestors of New Guineans," says Reich. In other words, as they left Africa, modern humans must have passed through the realm of the Denisovans on their way to Melanesia. And if you look at a map, the route from Africa to New Guinea does not go through Siberia, suggesting that the Denisovans may have lived over a quite a large swath of the globe. According to Svante Pääbo of the Max Planck Institute of Evolutionary Anthropology, "In combination with the Neanderthal genome sequence, the Denisovan genome suggests a complex picture of genetic interactions between our ancestors and different ancient hominin groups."
The best scenario to fit this data is that after Neandertals and Denisovans split, the Neandertals interbred with modern humans just after they left Africa but before they spread into Europe and Asia in the past 80,000 years. Later, Denisovans living in eastern Asia encountered a group of modern humans heading east from Africa toward Melanesia and interbred with them. As a result, Melanesians now carry DNA from both encounters with Neandertals and Denisovans.
Paleoanthropologists are also taking a new look at old fossils in Asia, trying to figure out which ones might be the Denisovans - if any. Along with the discovery in 2004 of the diminutive Homo floresiensis - a.k.a. the hobbit - that lived on the island of Flores as recently as 13,000 years ago, there are now at least three other types of humans who were alive at the same time as modern humans were taking over the world. Clearly, this means "the story [of the origins of modern humans] has undoubtedly got a lot more complicated," says paleoanthropologist Chris Stringer of the Natural History Museum, London.
Edited from Science, Nature News, Past Horizons (22 december 2010), NPR (23 December 2010)