13 January 2011
Ancient Denisovans and the human family tree
Last month scientists revealed remarkable evidence of a new group of ancient humans called Denisovans that interbred with our species and left behind a genetic trace in people living in south east Asia today.
An international team carried out a genetic study of a finger bone and a large molar tooth uncovered in Denisova Cave in the Altai Mountains, Siberia. They sequenced the genome and found that this ancient human shared 4-6% of its genetic material with some present-day Melanesians. In March, the team obtained a complete mitrochondrial DNA (mtDNA) sequence for the same finger bone, dated to about 40,000 years ago, showing that it was from neither a modern human nor Neanderthal.
Professor Chris Stringer, human origins expert at the Natural History Museum, comments on this research, "This new work showed that the fossil finger bone female was actually slightly closer genetically to Neanderthals than to modern humans, but something else even more remarkable was revealed: the Denisovan is also related to one group of living humans - Melanesians, who live on some of the islands of south east Asia. The most plausible explanation for this finding is that Denisovans were present further south as well as in Siberia, and pre-Melanesian [modern human] populations migrating from Africa through south east Asia must have interbred with some of these Denisovans, picking up an estimated 5% of their genes.'
Last May, scientists obtained genetic evidence that showed that modern humans outside of Africa share genetic information with Neanderthals which suggests that they must have interbred (Neanderthals died out about 30,000 years ago). And now Denisova genes have also been added to the human genetic mix.
"In terms of actual interbreeding events, on present data there might only have been two: one in the Middle East, perhaps 60,000 years ago, that input about 2.5% Neanderthal genes into recent humans outside Africa; and a subsequent one in south east Asia that added an additional 5% or so of Denisovan DNA into the ancestors of modern Melanesians," Stringer says.
Professor Stringer adds that the Denisovan lineage probably evolved in parallel with those of Neanderthals and modern humans and he suggested it could have been an early off-shoot of Homo heidelbergensis (whose fossils are known from about 500,000 years ago). This would also explain the mystery of some unidentified Asian skulls. "There are various Asian fossils that have been difficult to classify from sites in India (Narmada) and China (for example Dali, Jinniushan)," says Stringer. "They look different from H. erectus, but they also do not resemble Neanderthals or modern humans, and I have suggested some of them may be late-surviving examples of the species that gave rise to Neanderthals and H sapiens further to the west: H heidelbergensis. Following the first Denisova results, I speculated that this mtDNA might be from a late-surviving heidelbergensis living in Siberia."
So, in Europe, H. heidelbergensis gave rise to Neanderthals, in Africa they gave rise to us (modern humans), and in Asia, perhaps to the Denisovans. Should we reclassify Homo sapiens and include ancient humans in the same group? For now, Stringer says no, as other closely related mammal species such as wolves and jackals, and bonobos and chimpanzees, can hybridise (produce viable offspring) too. Stringer concludes, "Personally I think that the distinctiveness and separate evolutionary histories of groups like Neanderthals and modern humans warrant their continuing recognition at the species level, provided we remember that this may not preclude some hybridisation. However, if genetic data eventually show that such interbreeding events were common and widespread then it will certainly be time to revisit the way we classify human species."
Edited from The Natural History Museum (12 January 2011)