Neanderthals, humans interbred earlier than thought

Scenario of interbreeding between modern humans and Neanderthals: Neanderthal DNA in present-day humans outside Africa originates from interbreeding that occurred 47,000-65,000 years ago (green arrow). Modern human DNA in Neanderthals is likely a consequence of earlier contact between the two groups, roughly 100,000 years ago (red arrow). Courtesy — Cold Spring Harbour Laboratory (US)

Neanderthals may have been mating with modern humans 100,000 years ago, tens of thousands of years earlier than previously thought, a new study based on several different methods of DNA analysis has found.

Scientists provide the first genetic evidence of a scenario in which early modern humans left the African continent and mixed with 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,” said Professor Adam Siepel, from Cold Spring Harbour Laborato-ry in US. “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 old-er than that event,” said Siepel. “One very interesting thing about our finding is that it shows a signal of breeding in the ‘opposite’ direction from that already known.”

“That is, we show human DNA in a Neanderthal genome, rather than Neanderthal DNA in human genomes,” said Siepel. This finding has implications for our knowledge of 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 Neander-thal-modern human pairings outside of Africa were raised among the modern humans and ultimately bred with other humans. Contemporary Africans, however, do not have detectable traces of Neanderthal DNA in their genomes. This indicates that whatever sexual contact occurred between mo-dern humans and Nean-derthals occurred among humans who left the Afri-can continent. “Ancestors of present-day African populations likely didn’t have the opportunity to interbreed with Neanderthals, who lived largely outside of Africa,” said co-author Ilan Gron-au, a former member of Siepel’s Lab who is now at the Herzliya Interdiscipli-nary Centre, Israel. 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 south-western Siberia, in the Altai Mountains, near the Russia-Mongolia border.