Genes From Australia Point to Several Waves of African Dispersal, Not Just One
New genetic evidence suggests aboriginal Australians may be directly descended from the earliest of several human groups that left Africa and colonized the world, a challenge to a widely held model that argues for a single dispersal from Africa.
In a notable technical feat, published in the journal Science, researchers assembled a genome using a lock of hair donated by a young aboriginal man from Australia 100 years ago. That DNA was compared with the full genomes of two Western Africans, three Han Chinese and two Europeans, and more broadly with other partial genomes. Genetic markers get passed down from one generation to the next, and studying gene flow can reveal migration patterns among different ethnic groups.
The study suggests a first wave of Africans traveled to Australia between 62,000 and 75,000 years ago and gave rise to the Aborigines, while a second dispersal—one that ultimately went northward and another eastward—some 25,000 to 38,000 years ago led to modern Europeans and Asians. Based on these findings, aboriginal Australians would represent one of the oldest continuous populations outside Africa.
Another conclusion: These early human explorers were surprisingly intrepid and capable as they moved from Africa, likely along the southern coast of what is now southeast Asia and, ultimately, to Australia.
SCIENCE
“They would have had to use boats even though the sea level was lower back then, and they wouldn’t have been able see the land mass of Australia” from the southeast Asian shore some 50 miles away, said Eske Willerslev, an evolutionary biologist at the University of Copenhagen and senior author of the paper. “We used to think they were just wandering around—we have underestimated them.”
The latest finding is unlikely to be the last word on how humans came to colonize the planet. Other DNA evidence supports the discipline’s conventional view that there was only a single dispersal from Africa, some 60,000 to 70,000 years ago. According to that model, the explorers traveled to Arabia where one branch split off and headed north—becoming Europeans—while a second group populated India and Australia. The rest of Eurasia, including modern-day China, was then populated by southeast Asians moving northward.
Martin Richards, professor of archeogenetics at the University of Leeds, England, who read the Science study but wasn’t involved in the research, is skeptical. “The argument for multiple dispersals put forward here comes entirely from a single newly developed statistical test based on just four complete genomes,” he said. “None of the other analyses support it, and many argue against it.”
The power and rapidly falling cost of gene-sequencing technology is transforming the way scientists delve into the story of how humans evolved and spread. In 2009, scientists announced the first draft of the genome of Neanderthals; it was later found that humans and Neanderthals likely interbred, with today’s humans owing some of their disease-fighting genes to those ancient trysts.
Scientists hope to use similar genetic data to reconstruct the history of peoples and cultures in more recent times.
The challenges are many—and they aren’t just scientific. Dr. Willerslev and his colleagues needed DNA from an Australian Aborigine who wouldn’t have carried genes from European settlers.
Dr. Willerslev found the DNA he needed in a 100-year-old hair sample from an Aborigine who lived in a relatively remote area of southwest Australia. After locating the sample in a collection maintained at Cambridge University, England, he traveled to southwest Australia and obtained permission from local aboriginal tribes to use the sample.
The human genome contains three billion DNA base pairs. But the DNA of the century-old aboriginal hair sample had degraded into millions of fragments, each an average of 69 base pairs in length. The scientists were able to reconstruct only 60% of the total genome.
By comparing this old genome with other people’s genomes, Dr. Willerslev and his colleagues concluded that there had been separate human dispersals from Africa into Asia.
That may not be the final story. “We need comparisons with a larger number of genome sequences” to conclude whether there was just one or several eastward dispersals” from Africa, said Dr. Richards.
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