(Video Credit: COLD SPRING HARBOR LABORATORY)
Based on DNA analysis, an international research team has found strong evidence of an interbreeding event between Neanderthals and modern humans that occurred 100,000 years ago, almost 50,000 years earlier than what anthropologists previously thought.
The scientists provided the first genetic evidence of a scenario in which early modern humans left the African continent and mixed with archaic (now-extinct) members of the human family prior to the migration “out of Africa” of the ancestors of present-day non-Africans, less than 65,000 years ago.
“It’s been known for several years, following the first sequencing of the Neanderthal genome in 2010, that Neanderthals and humans must have interbred,” says Professor Adam Siepel, a co-team leader and Cold Spring Harbor Laboratory (CSHL) quantitative biologist. “But the data so far refers to an event dating to around 47,000-65,000 years ago, around the time that human populations emigrated from Africa. The event we found appears considerably older than that event.”
The interesting thing about the new finding is that it shows a signal of breeding in the ‘opposite’ direction that is showing human DNA in a Neanderthal genome, rather than Neanderthal DNA in human genomes.
This finding, based on advanced computer modeling algorithms comparing complete genomes of hundreds of contemporary humans with complete and partial genomes of four archaic humans, has revealed new human migration patterns.
People living today who are of European, Eurasian and Asian descent have well-identified Neanderthal-derived segments in their genome. These fragments are traces of interbreeding that followed the “out of Africa” human migration dating to about 60,000 years ago. They imply that children born of Neanderthal-modern human pairings outside of Africa were raised among the modern humans and ultimately bred with other humans, explaining how bits of Neanderthal DNA remain in human genomes.
Contemporary Africans, however, do not have detectable traces of Neanderthal DNA in their genomes. This indicates that whatever sexual contact occurred between modern humans and Neanderthals occurred among humans who left the African continent. “Ancestors of present-day African populations likely didn’t have the opportunity to interbreed with Neanderthals, who lived largely outside of Africa,” explains co-author Ilan Gronau.
The team’s evidence of “gene flow” from descendants of modern humans into the Neanderthal genome applies to one specific Neanderthal, whose remains were found some years ago in a cave in southwestern Siberia, in the Altai Mountains, near the Russia-Mongolia border.
The modern human ancestor who contributed genes to this particular Neanderthal individual – called the “Altai Neanderthal,” and known from a tiny toe bone fragment – must have migrated out of Africa long before the migration that led Africans into Europe and Asia 60,000 years ago, the scientists said.
“This is consistent with the scenario of gene flow from a population closely related to modern humans into the Altai Neanderthal. After ruling out contamination of DNA samples and other possible sources of error, we are not able to explain these observations in any other way,” Siepel says.