New York Times,
March 8, 2017Link
Human skeletons and archaeological remains in Australia can be traced back nearly 50,000 years before the trail disappears. Before then, apparently, Australia was free of humans.
So how did people get there, and when? Where did humans first arrive on the continent, and how did they spread across the entire landmass?
Answers to some of these questions are stored in the DNA of Aboriginal Australians. A genetic study of 111 Aboriginal Australians, published on Wednesday, offers an interesting — and, in some respects, unexpected — view of their remarkable story.
All living Aboriginal Australians descend from a single founding population that arrived about 50,000 years ago, the study shows. They swept around the continent, along the coasts, in a matter of centuries. And yet, for tens of thousands of years after, those populations remained isolated, rarely mixing.
The DNA used in the new study comes from aboriginal hair collected during a series of expeditions between 1926 and 1963. The Board for Anthropological Research at the University of Adelaide sent researchers to communities across Australia, where they collected vast amounts of information about aboriginal languages, ceremonies, artwork, cosmologies and genealogy.
Many Aboriginal Australians today no longer live where their ancestors did. During the 1900s, the country’s government forcibly removed many from their traditional lands and separated children from families. Many Aboriginal Australians moved to cities far from where they grew up.
Thanks to the subjects’ age and detailed records, scientists suspected the hair samples might offer a glimpse of the pre-colonial past. “It seemed obvious that this collection is perhaps the best way to reconstruct Australian history,” said Alan Cooper, a pioneer in ancient DNA studies at the University of Adelaide.
He and his colleagues first sought consent for the tests from the descendants of the people whose hair samples had been collected. They traveled to aboriginal communities, spending several days talking to family members to address their concerns. All but one of the families they visited gave them permission to run the study.
Dr. Cooper and his colleagues knew extracting DNA would not be easy. Over the decades that the hair had been in storage, the genetic traces may have broken down beyond recognition.
Making matters worse, the hair had been cut with scissors. The best way to get genetic material from a strand of hair is to pull it out at its DNA-rich root.
Given these uncertainties, the scientists decided to increase the odds of success by searching for abundant mitochondrial DNA, which is situated outside the cell nucleus and is inherited solely from the mother. Eventually, the scientists managed to piece together all the mitochondrial genes in each of the hair samples.
By comparing the aboriginal sequences to DNA from other parts of the world, the scientists determined that they all belonged to a single human lineage, indicating that all aborigines descended from a single migration to the continent.
Mitochondrial DNA gradually accumulates mutations at a roughly regular rate, ticking like a molecular clock. By adding up the mutations in the hair samples, the scientists also estimated that their owners all descended from a common ancestor who lived around 50,000 years ago. That finding fits nicely with the estimated ages of the oldest archaeological sites in Australia.
The mitochondrial tree also provided clues to how people spread through the continent.
Fifty thousand years ago, sea levels were so low that Australia and New Guinea formed a single continent. Humans moved from Southeast Asia onto this landmass, some settling in what is now New Guinea, others traveling farther south into Australia.
They kept to the coastlines until they reached southern Australia 49,000 years ago. But once this great migration was finished, the new study suggests, the ancestors of today’s aborigines hunkered down in their new homes — for tens of thousands of years.
The mitochondrial DNA contains no evidence that these populations mixed in any significant way, surprising researchers. “We were fully expecting a fully diverse mix of people in all places at all times,” Dr. Cooper said.
This is not the sort of migratory pattern documented by gene testing on other continents. In Europe, for instance, new populations have swept in every few thousand years, mixing with the societies they encountered.
Farming explains the difference, Dr. Cooper suggests. Unlike Africa, Asia and Europe, Australia did not experience the rise of agriculture several thousand years ago. “If you don’t have cheap carbohydrates, you don’t increase in population size,” he said.
Populations grew on other continents, but they often risked catastrophic crop failure. When that happened, Dr. Cooper said, “there’s only one response — mass migration.”
In Australia, however, aborigines did not depend on crops and lived as nomads in discrete regions. They never needed to move across the continent.
“This is really very surprising, but also hard to doubt,” said Stephan Schiffels, a population geneticist at the Max Planck Institute for the Science of Human History in Germany, who was not involved in the study. “The data is what it is.”
Peter Bellwood, an archaeologist at Australian National University who was not involved in the study, said much of the new data fit with archaeological findings. But he found it hard to see how Aboriginal Australians could have remained so sedentary for so long.
He pointed to tools shared by many aboriginal cultures across great distances, as well as to a family of languages spoken by many aboriginal groups. Dr. Bellwood doubted that they could have spread so far while individuals did not.
“If humans don’t move, why should languages and tools move?” he asked.
Dr. Schiffels and other researchers raised the possibility that the mitochondrial DNA was missing important details of Australian history.
DNA in the nucleus of each cell, coming from both parents, can offer clues to a wider range of ancestors.
It turns out, however, that Dr. Cooper and his colleagues were too pessimistic about the hair samples. Skin cells stuck to the hair shafts turn out to contain rich supplies of nuclear DNA.
“We can do the entire genome for each of these samples,” Dr. Cooper said. “So we’re returning to these communities to ask for permission to get a far more detailed look.”
Copyright 2017 The New York Times Company. Reproduced with permission.