For decades, scientists have worked to unravel the mystery of modern humans’ journey out of Africa. Three recent studies point to a possible answer: a single great migration out of the African continent.
The three studies looked at a combined 800 sequenced genomes from people around the world in more than 270 population groups. Scientists used the data to analyze similarities and differences between different populations, which helped point the way back to a common ancestor. The vast majority of non-African modern humans appear to be descended from a single group of pioneers who left Africa between 50,000 and 80,000 years ago.
One study, led by Harvard scientists David Reich and Swapan Mallick, sequenced 300 genomes across 142 small populations and saw a huge reduction in Eurasian genetic diversity stemming from 60,000 years ago, which points to a single group leaving Africa at once. Another study, led by Eske Willerslev of the University of Copenhagen found Australian aborigines diverged from Eurasians between 50,000 and 70,000 years ago, after a larger group first migrated out of Africa.
While the third study, led by Mait Metspalu of the Estonian Biocentre, found similar results, they also discovered that two percent of the genome of people from modern Papua New Guinea dates from even further back in time. That bit of genetic evidence might point to an even earlier dispersal of modern humans, as far back as 120,000 years ago.
“As population geneticists, we could spend the next decade arguing about that two percent, but in practical terms it doesn’t matter,” said Joshua Akey, an evolutionary geneticist at University of Washington who wasn’t involved with any of the research. This is because the remaining two percent doesn’t
detract from the evidence of one single, successful migration. The two theories could coexist. Early stone tools discovered in India and Arabia suggest modern humans left Africa earlier than the great expansion outward, but it seems likely that the majority of those lineages died out instead of passing on their genes to people today.
“The take-home message is that modern people today outside of Africa are descended from a single founding population almost completely,” said David Reich, who led the Harvard study.
The question that follows might be obvious: if modern humans evolved 200,000 years ago, why didn’t they migrate until much more recently?
Researchers Axel Timmerman and Tobias Friedrich of University of Hawaii, Manoa may have found an answer by reconstructing ancient climatic conditions. They studied wobbles in the Earth’s orbit and tilt known as Milankovitch cycles. These cycles, which occur approximately every 21,000 years, affect rainfall, how much sunlight different parts of the planet receive, temperature, sea levels, and vegetation. The cycles also created habitable green corridors in northern Africa, the Levant (a region on the eastern Mediterranean) and the Arabian peninsula.
The cycles included green corridors that lasted from 89,000 years ago to 73,000 years ago and another from 59,000 to 47,000 years ago – lining them up with the one great human migration timeline.
“Human history is this really fascinating and complex puzzle, and genetics can tell us about some of the pieces,” Akey said. But he added, “It’s really important to integrate information from as many other disciplines as possible.”