Single exodus from Africa gave rise to today’s non-Africans
Genetic data point to a date less than 72,000 years ago but climate scientists disagree
By Tina Hesman Saey
3:28pm, September 21, 2016
One wave of ancient human migrants out of Africa gave rise to all non-Africans alive today, three separate genetic studies conclude.
Those human explorers left Africa about 50,000 to 72,000 years ago, mixed with Neandertals and spread across the world, researchers report online September 21 in Nature. The studies present data from genetically diverse and previously unrepresented populations. Together they offer a detailed picture of deep human history and may settle some long-standing debates, but there is still room to quibble. All non-Africans stem from one major founding population, the studies agree, but earlier human migrations are also recorded in present-day people’s DNA, one study finds. And a fourth study in the same issue of Nature, this one focusing on ancient climate, also makes the case for an earlier exodus.
Scientists have long debated when anatomically modern humans first trekked out of Africa and how many waves of migration there were. Archaeological evidence indicates there were modern humans in Asia by at least 80,000 years ago. Human DNA in a Neandertal woman from Siberia indicates humans interbred with Neandertals outside Africa as long as 110,000 years ago (SN: 3/19/16, p. 6). But those people died out and didn’t contribute DNA to later generations, says Swapan Mallick, an evolutionary geneticist at Harvard Medical School and coauthor of a paper that traced the genetic history of 300 people from 142 populations around the world. The ancestors of today’s non-Africans probably left Africa about 50,000 years ago, Mallick and colleagues calculate.
Data in another study reveal remnants of a much earlier exodus from Africa that persist in the genomes of present-day Papuans, biological anthropologist Luca Pagani of the Estonian Biocentre in Tartu and colleagues report. About 2 percent of the genome of Papuans can be traced back to small bands of humans who left Africa 120,000 years ago. “This expansion was successful in leaving descendants today,” Pagani says. But a massive wave of migrants who left Africa after about 75,000 years ago probably overwhelmed that small trickle, swamping out their genetic signature.
A third study focusing on the genetic history of aboriginal Australians and Papuans from the New Guinea highlands didn’t find traces of a 120,000-year-old migration, but didn’t rule it out either, says study coauthor Eske Willerslev, an evolutionary geneticist at the University of Copenhagen.
Some previous studies have suggested that ancestors of Australians and Papuans came from an earlier wave of migration than other non-Africans did. “Australians and Papuans are descendants of some of the earliest modern human explorers,” Willerslev says. His group’s evidence suggests a single wave of migrants left Africa about 72,000 years ago and settled initially in the Middle East. Ancestors of Europeans and Asians stayed put for thousands of years before splitting into different groups. But Australian and Papuan ancestors kept going. “These guys were heading off on this marvelous journey across Asia,” ending up in Australia and Papua New Guinea about 50,000 years ago, Willerslev says.
Mallick and colleagues also found evidence of a main wave of migration into the Middle East that split into two groups after breeding with Neandertals. Those groups took different routes. One ended up in Europe, the other populated Asia. Instead of Australians and Papuans sprinting ahead of everyone else as an independent group, the researchers say they moved with the ancestors of East Asians and continued to the islands only later.
Pagani and colleagues used a method for analyzing their data that helped them pick out older chunks of DNA, says Mattias Jakobsson, an evolutionary geneticist at Uppsala University in Sweden. That method enabled them to see evidence of the older migration where the other studies couldn’t. But genetic dating methods are far from perfect; they can differ because of inaccurate mutation rates, skewed sampling, biased analyses or other reasons. Future genetic studies of present-day people added to new work on ancient DNA and archaeological evidence may help resolve some of the remaining discrepancies.
Even though results from Pagani’s study seem to disagree with the other two, “it’s a superficial disagreement,” says evolutionary geneticist Joshua Akey of the University of Washington in Seattle who was not involved in any of the studies. “One group is saying 98 percent” of DNA came from the main wave of migration, “while the other groups say it’s 100 percent. … The main conclusion is that the vast majority of ancestry in non-Africans can be traced to a single out-of-Africa dispersal.”
A study of ancient climates may create another discrepancy. It suggests the departure window geneticists propose was the absolute worst time to leave Africa. “Every 20,000 years or so, Earth’s axis wobbles caused massive shifts in climate and vegetation,” says Axel Timmermann, a climate scientist at the University of Hawaii at Manoa. Those fluctuations opened green corridors across northern Africa and the Arabian Peninsula, then turned those same areas to parched deserts.
Timmermann and University of Hawaii colleague Tobias Friedrich did computer simulations of climate and sea level changes over the last 125,000 years to predict when and where humans might have easily moved. By the scientists’ calculations, the timing of a mass human migration out of Africa 60,000 to 70,000 years ago “is the most unlikely scenario from a climate point of view, because … northeastern Africa was completely dry. It was one of the worst drought periods in the entire history, so the corridor was closed,” Timmermann says.
The researchers predict conditions were favorable for migration between 107,000 and 95,000 years ago and again 90,000 to 75,000 years ago. Another window didn’t open until 59,000 years ago, after humans were probably well on their way to Australia. “People could have chased antelope across and they wouldn’t even know they were on a different continent,” Timmermann says. That seamless landscape transition would have been mirrored in people’s mating habits, with populations moving in and out of Africa and mixing freely, he speculates.
Most genetic analyses don’t take that back-to-Africa movement into account, Timmermann says. Back-and-forth mating would make the Africans and non-Africans genetically indistinguishable from each other and obscure the real date at which people left Africa. Allowing for cross-continent mingling puts people’s exodus from Africa at about 80,000 to 100,000 years ago. Climate shifts that turned Arabia to desert 70,000 years ago would have cut off the connection, making people already outside Africa genetically distinct from Africans. That would show up in the genetic studies as the point at which people left Africa instead of the point-of-no-return.
Some previous genetic research has found evidence of back-to-Africa migrations starting about 45,000 years ago (SN: 6/25/16, p.14).
The climate study reinforces the idea that people had spread out of Africa much sooner than the new genetic evidence indicates, says archaeologist Michael Petraglia of the Max Planck Institute for the Science of Human History in Jena, Germany. He is a coauthor of the Pagani study, but says genetics alone won’t solve all the mysteries of early human history.
Debate over when people left Africa, where they came from inside Africa and who they interbred with as they spread around the globe are far from over, says Petraglia. “I expect some fireworks in the next few years.”