Nature

Photo by Herbert Hale, via South Australian Museum Archives Norman Tindale Collection

There are two central mysteries about human history in Australia. First, when did people arrive on the world's southernmost inhabitable continent? And second, how did they colonize it? A paper in Nature offers new answers, based on an extensive analysis of decades-old DNA.

By studying the mitochondrial DNA of Aboriginal Australians from all across the continent, University of Adelaide biologist Alan Cooper and his team were able to trace the population back to its most recent common ancestor, a woman who lived between 43,000 and 47,000 years ago. Because mitochondrial DNA is passed from mothers to children virtually unchanged, it's often used to trace genetic histories over long time spans. Based on this finding and dates of the earliest archaeological sites in Australia, Cooper and colleagues write that the continent was likely colonized by a single group of people about 50,000 years ago.

At the time that this group was walking into Australia, the continent was joined to New Guinea in a larger landmass called Sahul. What's remarkable is that this group of explorers appears to have colonized the entire Australian continent—or at least its coasts—within about 2,000 years. Genetic evidence reveals that the original group split in two, one heading east and the other west. They met again in southern Australia just a couple of millennia later.

But as Aboriginals were colonizing Australia, something relatively unusual happened. While some people wandered, others stayed behind and founded communities. And these communities lasted for tens of thousands of years in roughly the same locations, relatively isolated from each other. The people in these communities developed distinct languages, cultures, and physical features, leading to an extremely diverse population whose traditions are often startlingly different. Indeed, these differences led many scientists to suggest that Australia was perhaps colonized by several unrelated groups over time.

That's not what the genetic data tells us, however. As Cooper put it in a statement:

Amazingly, it seems that from around this time [of colonization] the basic population patterns have persisted for the next 50,000 years—showing that communities have remained in discrete geographical regions. This is unlike people anywhere else in the world and provides compelling support for the remarkable Aboriginal cultural connection to country. We’re hoping this project leads to a rewriting of Australia’s history texts to include detailed Aboriginal history and what it means to have been on their land for 50,000 years—that’s around 10 times as long as all of the European history we’re commonly taught.

Haplogroups and the Stolen Generations

Cooper and his colleagues came to this conclusion by studying genetic differences between the people who were descended from that original group. Mitochondrial DNA mutates at a fairly regular rate over time, and this creates populations who share a common ancestor but have diverged into what are called "haplogroups," or people with a common mutation in their DNA. In Australia, these haplogroups are found in specific geographical regions, suggesting populations that settled down in one place and never left. The haplogroups known as P, S, and M42a are mostly found in the east, while O and R are found in the west. There is also an area in southern Australia where O and S are found together, perhaps the result of the two migrations meeting there after centuries of separation.

One of the biggest hurdles to studying the genetic history of Aboriginals in Australia is often called simply the "Stolen Generations." Government policies in the twentieth century forced Aboriginal groups to relocate, often forcibly removing children from their homes to teach them English in residential schools. Many children of the Stolen Generations today don't know who their ancestors are or where they lived. And that makes it very hard for geneticists to trace a connection between the deep history of these groups and geographical locations in Australia.

Luckily, the University of Adelaide has what the researchers describe as "a remarkable set of hair samples and detailed ethnographic metadata collected with permission from more than 5,000 Aboriginal Australians" between the 1920s and '70s, from groups who were not yet part of the Stolen Generations. Cooper and his team were able to get mitochondrial DNA from 111 people's hair samples, along with "extensive genealogical and geographical information" about them. The 111 people came from the Point Pearce community of South Australia, the Cherbourg of Queensland, and the Koonibba of South Australia. These samples, along with previous genetic studies, allowed the researchers to figure out the path those first Australians took 50,000 years ago and where they settled.

One of the surviving hair donors, Kaurna Elder Lewis O'Brien, participated in the study as an adviser. “Aboriginal people have always known that we have been on our land since the start of our time,” he said in a statement. “But it is important to have science show that to the rest of the world. This is an exciting project, and we hope it will help assist those of our people from the Stolen Generation and others to reunite with their families.”

Nomadic sedentism

O'Brien and the researchers agree that this genetic study offers an interesting scientific backstory for many Aboriginal communities' belief that they have a spiritual connection to the country and landscape. It's possible that this idea entered Aboriginal culture early, reflecting the fact that communities often stayed in one place for thousands of years, isolated from other groups. "The marked population structure of deeply diverged Aboriginal Australian mitogenomes appears to date back to the original arrival of people on the Australian part of Sahul," write the researchers in their paper. Many Aboriginal Australian communities may actually have been founded shortly after humans arrived in Australia 50,000 years ago.

That connection to the land may also have been the key to survival for these groups during terrifying and dramatic climate events, including the Last Glacial Maximum (AKA the Ice Age) about 21,000 years ago, a tropical warming event in the early Holocene between 9,000 and 6,000 years ago, and a period of extreme weather fluctuations from an intensification of El Niño between 4,000 and 2,000 years ago. These communities lived through centuries of drought and floods, as well as temperature changes, and the researchers believe that they did it without ever moving very far from their homes.

There aren't any traces of genetic bottlenecks that would mean the population crashed during rough times and expanded again when the climate was more favorable. Instead, suggest the researchers, Aboriginal groups may have settled in temperate areas that archaeologists have yet to discover.

Indeed, some climate changes appear to have been beneficial. There was a population expansion around 7,000 years ago in southern Australia, with people from the O haplogroup spreading outward and into the continent's interior. Not surprisingly, this also marked the mid-Holocene warm period, when the entire planet experienced about 2,000 years of relatively tropical weather. At that point, the Australian interior would have become wetter and more livable.

This was also a period of cultural transformation for Australians, with new trade relationships and cultural exchanges between previously isolated groups. Panaramittee rock art became popular in many parts of the continent, as did the Pama-Nyungan languages. Archaeologists have found evidence for an explosion in new technologies as well, including sophisticated toolkits for seed grinding, weapons like the boomerang, and message sticks for communicating across distances. Based on their genetic analysis, the researchers believe these changes were the result of meetings between formerly distinct groups. And yet, given the genetic diversity that endured until European colonization, it doesn't appear that one group conquered and replaced the others. The groups simply taught each other about new tools and ways of life.

This pattern fits with what we know of modern Aboriginal ways of life. Most Aboriginal communities at the time of European contact engaged in "nomadic sedentism," living nomadically but only within a bounded geographical area. Perhaps this system was established during periods when the climate was so harsh that wandering too far led to an arid wasteland. Certainly they were connected to land for both pragmatic and spiritual reasons.

With this study, a new picture has emerged of how Australia was colonized. Aboriginal Australians shaped the land, settling it rapidly, but their communities were also shaped by the continent's unique and changing environment. Many distinct societies grew out of a single origin, but they retained a belief that they have been on the land for a profoundly long time. Now it appears that scientific evidence backs up their claims.

Nature, 2017. DOI: 10.1038/nature21416

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