When modern humans left Africa as far back as 70,000 years ago, they dispersed across the world, reaching Australia 50,000 to 40,000 years ago. From then until the 18th-century arrival of European colonists, aboriginal Australians did not mix their DNA with anyone else in the world — or so many scientists believed.
Now a study has turned up evidence of much more recent interbreeding between native Australians and people who came from India. The findings, based on a detailed examination of the DNA of aboriginal Australians and hundreds of people of other pedigrees, found that mixing occurred as recently as 4,200 years ago.
Reported in the journal Proceedings of the National Academy of Sciences, the results dovetail with interesting archaeological and fossil changes, said study leader Mark Stoneking, a molecular anthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Right around that time, new kinds of stone tools called microliths appeared in Australia, finer than earlier tools discovered there but similar to tools already in use elsewhere in the world.
Also at this point in time, Australia's wild dog, the dingo, shows up for the first time in the fossil records. Scientists know the dingo is not native to the Australian continent, where all indigenous mammals are marsupials that bear immature young and often carry them in pouches. The dingo, in contrast, is a placental mammal and a subspecies of the gray wolf, like the domestic dog.
"We don't know for sure that these events are connected, but the fact that all of these occur at the same time suggests that they may be," Stoneking said.
To reach their conclusions, Stoneking's team conducted a detailed scan of the genomes of 344 people, including Aborigines from the country's Northern Territory as well as people from Papua New Guinea, Southeast Asia, India, China and those of Western and Northern European ancestry. The scientists looked for places where the DNA code sometimes differed by a single DNA building block, or nucleotide, between members of their sample.
By noting to what extent individuals shared the roughly 1 million tiny variations that were found, the team could piece together trees that showed how each group of people was genetically related to the others and estimate how long ago the groups had become distinct.
They found, for example, that aboriginal Australians, Papua New Guinea highlanders and the Mamanwa people from the Philippines were genetically closest to each other and diverged about 36,000 years ago. This fit well with earlier genetic studies.
But the team was surprised to find — using four separate statistical methods — that a much more recent genetic mixing with people from India had occurred. They estimated that about 11% of the DNA of aboriginal Australians is derived from this event.
Earlier studies had hinted as much, but they were limited to smaller regions of the genome: the Y chromosome, which is only carried by males, and a type of DNA called mitochondrial DNA that is passed down from mothers to their children, Stoneking said.
The authors of the new study also estimated how far back this genetic mixing had occurred, via the following reasoning: A child born of an Aborigine and an Indian would carry in his or her genome an entire, unbroken stretch of each chromosome, one from each parent. But with each generation, those two chromosomes swap bits and pieces with each other. Down the generations, therefore, the pure Indian or pure Australian chromosome stretches will become increasingly shorter.
Using the size and number of DNA stretches in people alive today, the team ran computer simulations to calculate that 141 generations have passed since the initial interbreeding. With each generation assumed to be 30 years, that adds up to 4,230 years.
It isn't clear how the mixing took place. Though it might make sense that the gene flow occurred in Indonesia, no traces of Indian DNA could be found among the Indonesians in the sample and no Indonesian DNA could be found in the Australians, the authors said — perhaps suggesting the migrants came directly to Australia by water. Still, a more detailed analysis of Indonesian genomes would be needed to rule out that connection, Stoneking said.
Scientists can now pinpoint people's movements across the globe and their heady brews of ancestry with a precision that would have been unimaginable not long ago, said Stanford University geneticist Michael Snyder, who wasn't involved in the new study. In 2008, for example, an analysis of 3,000 Europeans found that their DNA could predict the places they came from to within several hundred kilometers.
Sometimes the results are surprising, he added. In 2010, for example, close inspection of a sample of hair from a man who lived in western Greenland 4,000 years ago revealed that his nearest living relatives were native Siberians — and thus that there had been migrations across the top of North America that had gone unappreciated by scientists before.