Currently, the widely accepted story of human origins suggests that early members of our species left Africa in a single wave of migration about 50,000 years ago and interbred with Neanderthals in Europe and Asia.
But now, in a study published in July in the journal Science, researchers suggest Homo sapiens migrated from the African continent in several waves, interbreeding with Neanderthal populations as early as 250,000 years ago.
“This is the first time that geneticists have identified multiple waves of modern human-Neanderthal admixture,” first author Liming Li, an associate research scholar at Princeton University, says in a statement.
“It wasn’t a single out-of-Africa migration,” Sarah Tishkoff, a geneticist at the University of Pennsylvania who was not involved in the new study, adds to the New York Times’ Carl Zimmer. “There have been lots of migrations out of Africa at different time periods.”
According to Tishkoff, who has also studied interbreeding between early modern humans and Neanderthals, previous research largely didn’t recognize these earlier waves of migration, because they didn’t leave clear fossil records or DNA traces in living people.
Researchers have sequenced the genomes of hundreds of thousands of living humans, but they only have three known complete Neanderthal genomes, collected from one site in Croatia and two in Russia. As a result, scientists have an understanding of how interbreeding affected Homo sapiens, but a much murkier picture of how it affected Neanderthals.
We know much less about how these encounters impacted the genomes of Neanderthals,” Joshua Akey, a population geneticist at Princeton University and senior author of the study, tells Live Science’s Charles Q. Choi.
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This time, Akey’s team decided to look for human DNA in Neanderthal genomes, comparing the three available ones to those of 2,000 modern humans. They mapped the gene flow between the Neanderthal and modern human populations with a genetic tool called IBDmix, which uses machine learning to decode genomes. The researchers identified two separate instances of interbreeding: between 200,000 and 250,000 years ago and between 100,000 and 120,000 years ago.
Coupled with past research that supports a migration from Africa between 50,000 and 60,000 years ago, the study finds early modern humans left the continent several times and interbred with Neanderthals. Their results suggest the Neanderthal genome derived 2.5 percent to 3.7 percent of its DNA from Homo sapiens.
“Because we can now incorporate the Neanderthal component into our genetic studies, we are seeing these earlier dispersals in ways that we weren’t able to before,” Akey says in the statement.
This supports a separate study published in Current Biology last year in which another group of researchers, including Tishkoff, compared the genome of a Neanderthal fossil with the genomes of 180 humans from across Africa. This team also concluded that Homo sapiens and Neanderthals had contact 250,000 years ago.
Katerina Harvati, a paleoanthropologist at the University of Tübingen in Germany who was not involved in either study, tells the New York Times that “some mysterious human fossils from Europe and the Middle East might belong to these early waves.”
For one, skulls found in two Israeli caves seem to belong to modern humans, but they have a few confusing features, such as larger brows, that might have come from Neanderthal genes. These skulls date to about 100,000 years ago, during one of the newly proposed periods of migration. And another skull fragment found in Greece is roughly 210,000 years old—aligning with the first proposed exodus from Africa—and displays some human anatomical traits.
“We now know that for the vast majority of human history, we’ve had a history of contact between modern humans and Neanderthals,” Akey says in the statement.
His team’s research also hypothesizes, based on genetic data, that the Neanderthal population was even smaller than previously thought—on the order of 2,400 breeding individuals, rather than 3,400. As such, the team suggests Neanderthals’ mysterious disappearance had to do with them becoming absorbed into the population of modern humans through interbreeding.
“Modern humans were essentially like waves crashing on a beach, slowly but steadily eroding the beach away,” Akey says in the statement. “Eventually, we just demographically overwhelmed Neanderthals and incorporated them into modern human populations.”
