Aboriginal DNA points to an earlier human exodus from Africa
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Humanity was born in Africa. But at some point, many of our ancestors left. The question of when and how they bid farewell to the continent remains a subject of some debate: A mass exodus occurred roughly 60,000 years ago, according to DNA and fossil evidence, but signs point to some back-and-forth migration much earlier than then.
In a trio of studies published Wednesday in Nature, researchers attempt to trace the origin of our species by looking to neglected DNA - populations that haven't been studied as extensively as large European and Asian ethnic groups. Together, the studies present 787 high-quality genomes from people around the world.
One of the studies, led by Eske Willerslev of Cambridge University, provides the most in-depth genomic analysis of native Australians ever published. The data backs up something the indigenous peoples already knew: Aboriginal people appear to be the oldest living civilization on the planet outside of Africa. After an exodus some 72,000 years ago, they split away from the larger genetic group (along with their future neighbors in Papua New Guinea) 58,000 years ago and arrived on the Australian continent around 8,000 years later. They've been there ever since, giving them the longest known ties of any culture to a single place.
''This story has been missing for a long time in science,'' Willerslev told the Guardian. ''Now we know their relatives are the guys who were the first real human explorers. Our ancestors were sitting being kind of scared of the world while they set out on this exceptional journey across Asia and across the sea.''
There's other genetic evidence for Aboriginal people's long relationship with their homeland: Since reaching the continent 50,000 years ago, the group has continued to divide into distinct ethnic lineages. Willerslev and his colleagues found that individual Aboriginals from different parts of Australia could be as genetically distinct from one another as Europeans are from East Asians. This points to a long, long period of separation - tens of thousands of years living on opposite sides of massive deserts.
''The genetic diversity among Aboriginal Australians is amazing,'' lead author Anna-Sapfo Malaspinas of the University of Copenhagen said in a statement. ''Perhaps because the continent has been inhabited for such a long time we find that groups from southwestern desert Australia are more genetically different from groups of northeastern Australia than are, for example, Native Americans and Siberians, and this is within a single continent.''
But what about the rest of us? The study that focused on DNA from Aboriginal Australians concluded that all humans can trace their ancestry back to that single, massive exodus from Africa some 72,000 years ago. Other migrations occurred, among modern humans of our own species and close relatives like Neanderthals, but none of those settlers survived long enough to become our ancestors. Another of the newly published Nature studies, this one led by David Reich of Harvard University, similarly suggests that all humans descend from a single wave of migration.
Intriguingly, their data showed little evidence of genetic shifts during the periods when modern humans are known to have picked up modern habits like art and burial rituals. In other words, the researchers don't think we have sudden, lucky mutations to thank for our cultural development.
''This genetics study sort of unseats genetics as a driving force behind the big changes'' that made us into modern humans, Reich told New Scientist.
In the third study, researchers led by Mait Metspalu of the Estonian Biocentre found evidence that some 2 percent of Papua New Guinean genes can be traced to a separate wave of migration from Africa - one that occurred as early as 120,000 years ago. The data from the other two studies can't totally rule out such a wave. But while the three studies have slightly different conclusions, they all support a single hypothesis: Modern, non-African humans got most of their DNA from one group of intrepid settlers.
''As population geneticists, we could spend the next decade arguing about that 2 percent, but in practical terms it doesn't matter,'' Joshua Akey of the University of Washington, who wasn't involved with the new studies, told Science magazine. The single, most recent migration still ''explains more than 90 percent of the ancestry of living people,'' he added.
Further study is needed to puzzle out whether modern humans have ancestors from earlier waves of migration. And scientists are only just starting to uncover our genetic ties to other species - extinct cousins like Neanderthals and Denisovans who interbred with Homo sapiens as the latter made their way out of Africa. But by conducting studies like these, where the latest genetic techniques are applied to understudied populations, researchers may soon be able to paint a complete picture of humanity's global dispersal.