Tuesday, May 08, 2007

Y-chromosomes of Australian Aborigines show that they came from Africa

Nicholas Wade:

Geneticists re-examining the first settlement of Australia and Papua-New Guinea by modern humans have concluded that the two islands were reached some 50,000 years ago by a single group of people who remained in substantial or total isolation until recent times. The finding, if upheld, would undermine assumptions that there have been subsequent waves of migration into Australia.

Analyzing old and new samples of Aborigine DNA, which are hard to obtain because of governmental restrictions, the geneticists developed a detailed picture of the aborigines’ ancestry, as reflected in their Y chromosomes, found just in men, and their mitochondrial DNA, a genetic element passed down just through the mother.

The results show that Aborigines and the people of Papua-New Guinea share several ancient genetic lineages, indicating that both are descended from a single founding population.

All Australian Aborigines, at least to judge by the genetic samples in hand, are descended from this founding population, meaning that no further immigrants reached Australia in numbers large enough to leave a genetic trace until the modern era.

The findings, by Toomas Kisivild and a group of geneticists and archaeologists situated mostly at the University of Cambridge in England, are reported today in The Proceedings of the National Academy of Sciences.

The oldest human remains from Australia, about 45,000 years in age, have quite thin or gracile bones, whereas fossils from 20,000 years ago are robust. The new findings suggest that the difference must stem from some internal process like adaptation to climatic change, and not to interbreeding with the archaic species Homo erectus, as some have suggested.

The new genetic analysis also undercuts the theory that the dingo, a dog that appeared in Australia 4,000 years ago, arrived with a new wave of settlers. Dr. Kisivild suggested that the dingo could have been an item of trade. But Richard G. Klein, a paleoanthropologist at Stanford University, said new immigrants were more likely to have introduced the dog, even though they were too few to have left a genetic trace.

Dr. Kisivild’s team has also been able to estimate, by counting mutations in DNA, the duration of the emigrants’ journey from India, an early point of settlement outside Africa, to the lost continent of Sahul that then included New Guinea and Australia.

The journey would have taken less than 5,200 years, the geneticists calculate, as a growing population gradually spread along the coastlines of southeastern Asia. The emigrants would have had to cross open sea only at the final stage of their journey.

The genetic estimates for the duration of the journey are “intriguing, but not compelling,” Dr. Klein said. “I think we need lots more archaeological evidence to resolve the question of how long it took modern humans to spread from Africa to Australia or anywhere else.”

The oldest inhabitants of New Guinea, who speak languages of the Papuan family, would have changed under the evolutionary forces of selection and genetic drift. Still, modern Papuan speakers may offer “a pretty good idea of what our ancestors might have looked like coming out of Africa 2,000 generations ago,” said Peter Forster of Anglia Ruskin University in Cambridge, an author of the new report.

Dr. Klein, however, said that aboriginal Australians and Papuans differed in appearance from that expected for the earliest fossil remains in Australia and that their morphology had to have emerged later.

“I don’t think genetics implies that aboriginal Australians and Papuans resemble the original modern African emigrants more closely than anyone else,” he said.

Aborigines came out of Africa, study shows

Aborigines, Europeans Share African Roots, DNA Suggests

Australian Aborignals, a people that dwelt alone?

Y-chromosome evidence suggests a common paternal heritage of Austro-Asiatic populations

DNA Links Aborigines To African Walkabout

Genetic tests on Australian fossils suggest Aborigines did migrate out of Africa

How Europeans Got to Europe


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