Using a genetic blueprint contained in a nearly century-old lock of hair from an Aboriginal Australian man, scientists have found evidence the Aborigines are descendants of an early wave of people who left Africa and branched off on their own as long ago as 75,000 years, before Asians and Europeans became distinct groups.
This means Aborigines are likely one of the oldest continuous populations outside Africa, they write.
In 1921, the hair was donated to British anthropologist Alfred Cort Haddon when he was traveling through Golden Ridge near the town of Kalgoorlie in Western Australia, according to Morten Rasmussen, a postdoctoral researcher at the University of Copenhagen in Denmark and one of the international team researchers.
The researchers used this sample — which became the first genome sequenced from an Aboriginal Australian — to look far back into human history and clarify how our ancestors spread around the world from Africa, where they are believed to have emerged.
The lock of hair made good material for this kind of study because its owner was likely to have pure Aborigine heritage, not yet mixed with the European immigrants that settled Australia in relatively modern times. A genetic analysis of the hair sample confirmed this.
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The researchers sequenced the Aboriginal man's genome — his complete genetic blueprint — and compared it with those from Chinese, Europeans and Africans. By looking at differences, caused by mutations in the DNA code, the researchers were able to infer these populations' relationships to one another. (The more closely the groups are related the fewer differences their DNA should show.)
They found unique mutations in the Aboriginal man's DNA, indicating that his ancestors must have branched off from Europeans and Asians before these two groups split.
"So when Europeans and Asians were a single population, Aborigines' ancestors were already on their way to Australia," Rasmussen said.
However, all three groups showed roughly the same genetic distance from the Africans, indicating they all had split from Africans long ago, he said.
To check the accuracy of the results, the scientists used a total of three Han Chinese genomes, which they sequenced, as well as pre-sequenced genomes from two Europeans and two Africans belonging to the Yoruban people. They found that switching the individuals used in the comparison made little difference in the results, Rasmussen said.
"We are selecting a few individuals to represent whole populations. That does give some limitations — the more genomes we could add, the more certainty we could add and the more detail we could add," he said.
Small changes in our DNA code occur at a constant rate, so using this rate, the scientists were able to calculate an approximate time when the Aboriginal ancestors split off from the ancestral Eurasian population: somewhere between 62,000 and 75,000 years ago.
This calculation fits with the archeological evidence provided by Mungo Man, the name given to human remains found near Lake Mungo in Australia and dated to about 45,000 years ago, since the split would have occurred before the arrival of the Aborigines' ancestors in Australia, Rasmussen said. Based on genetic data, it is impossible to say where, geographically, the split occurred.
They also found evidence that the Aborigines' ancestors had mixed with archaic humans called Denisovans, whose remains were found in a cave in the Altai Mountains of Siberia.
The genomic analysis indicates that Aborigines are not descended from ancestral Asian populations, the authors write; rather, it implicates multiple waves of migration, with Aboriginal Australians descending from an early wave. Europeans and Asians appear to have emerged later, as the result of later waves. Thousands of years after that, ancestors of American Indians split from Asian populations when they crossed the Bering Strait.
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