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|Posté le: Ven 23 Sep - 04:27 (2011) Sujet du message: Humans colonised Asia in two waves
|Humans colonised Asia in two waves
22 September 2011 by Michael Marshall
If at first you don't succeed, have at least one more go. Early humans did, colonising Asia in two waves. The two migrations have left their mark in the genes of native people in south-east Asia, Polynesia and Australia.
Anthropologists have long debated whether there was more than one migration from Africa into Asia. Two studies published today aim to resolve the question.
A team of geneticists led by Eske Willerslev of the Natural History Museum of Denmark in Copenhagen has sequenced the genome of an Aboriginal Australian man, using a 100-year-old lock of hair.
They compared the genome with 1220 others from 79 populations around the world. It was most similar to those of highland Papua New Guineans. The analysis suggested that the two groups had each been isolated from other humans for about 30,000 years, and that Australian Aboriginals are descended from the first colonists of Australia. Archaeology suggests humans have been there for 50,000 years, so the Aboriginal Australians may be one of the oldest continuous populations in the world.
To find out how many migrations there were, Willerslev's team compared the Aboriginal Australian genome with Han Chinese, European and African genomes. They calculated that the Aboriginal Australians split from the other three between 75,000 and 62,000 years ago. The Chinese and European populations split much later, between 38,000 and 25,000 years ago. That suggests there were two migrations into east Asia.
It is remarkable that Willerslev was able to get so much information out of human hairs that had been stored for decades without any special conservation measures, says Chris Stringer of London's Natural History Museum.
But it is not clear where the first migration began, says Morten Rasmussen of the Natural History Museum of Denmark, who was also involved in the study. "We can't infer geographic origins from genetic data," he says. The genetics tells us when populations split, but not where.
One possibility is that the Aboriginal Australian migration left Africa before everyone else, but Stringer thinks that is unlikely. "If the ancestors of the Australian Aboriginals left Africa 70,000 years ago, where were they for 20,000 years before they made it to Australia and New Guinea?" he asks.
There's another explanation, says Mark Stoneking of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Stoneking thinks there was only one migration out of Africa, which got as far as the Middle East. From there, two successive migrations headed east. The first colonised Australia and the neighbouring islands, while the second colonised mainland Asia (see diagram).
Stoneking and colleagues scoured genomes from 33 populations from mainland Asia, Indonesia, the Philippines and Polynesia, as well as Australia and New Guinea. They looked for signs that their ancestors interbred with the Denisovans, a recently discovered human group known only from the DNA in tooth and bone fragments found in Siberia. The Denisovan genome has already revealed that the ancestors of modern New Guineans interbred with them.
Stoneking found more Denisovan DNA in populations further south and east of Siberia, especially in east Indonesia, Australia, New Guinea, Fiji and Polynesia. People from mainland Asia and west Indonesia didn't have any, suggesting they descend from a separate migration that did not interbreed with the Denisovans.
Between them the two studies are strong evidence for multiple dispersals, Stoneking says. "But one thing I've learned from being in anthropology for a long time is that data alone are never enough to settle an argument."
Journal references: Willerslev and Rasmussen's paper: Science, DOI: 10.1126/science.1211177; Stoneking's paper: American Journal of Human Genetics, DOI: 10.1016/j.ajhg.2011.09.005
Il n’y a point de génie sans un grain de folie