A multinational group of researchers has found strong genetic evidence of an interbreeding event between Neanderthals and anatomically modern Homo sapiens that occurred roughly 100,000 years ago – much earlier than scientists thought.
A Neanderthal man. Image credit: Mauro Cutrona.
The team, led by Dr. Sergi Castellano of the Max Planck Institute for Evolutionary Anthropology and Prof. Adam Siepel of Cornell University, Ithaca, analyzed the genomes of a Neanderthal (Homo neanderthalensis) and a Denisovan from the Altai Mountains in Siberia together with the sequences of chromosome 21 of two Neanderthals from Spain and Croatia.
“It’s been known for several years, following the first sequencing of the Neanderthal genome in 2010, that Neanderthals and humans must have interbred,” Prof. Siepel said. “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.”
“One very interesting thing about our finding is that it shows a signal of breeding in the ‘opposite’ direction from that already known,” Prof. Siepel noted.
“That is, we show human DNA in a Neanderthal genome, rather than Neanderthal DNA in human genomes.”
According to the scientists, people living today who are of European, Eurasian and Asian descent have 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.
The team’s evidence of “gene flow” from descendants of modern humans into the Neanderthal genome applies to one specific Neanderthal — the Altai Neanderthal.
The modern human ancestor who contributed genes to this particular Neanderthal individual must have migrated out of Africa long before the migration that led Africans into Europe and Asia 60,000 years ago.
In contrast, the two Neanderthals from European caves both lack DNA derived from ancestors of modern humans.
The Denisovan analyzed by the team did not have traces of modern human DNA, unlike the Neanderthal found in the same cave.
That doesn’t mean modern human ancestors never mated with Denisovans or European Neanderthals.
“What is does mean is that the signal we’re seeing in the Altai Neanderthal probably comes from an interbreeding event that occurred after this Neanderthal lineage diverged from its archaic cousins, a little more than 100,000 years ago,” Prof. Siepel said.
“The modern human sequences in the Altai Neanderthal appear to derive from a group of modern human ancestors from Africa that separated early from other humans, about the time present-day African populations diverged from one another, around 200,000 years ago.”
“Thus, there must have been a long lag between the time when this group branched off the modern human family tree, roughly 200,000 years ago, and the time they left their genetic mark in the Altai Neanderthal, about 100,000 years ago, before being lost to extinction themselves.”
The findings were published online today in the journal Nature.
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Martin Kuhlwilm et al. Ancient gene flow from early modern humans into Eastern Neanderthals. Nature, published online February 17, 2016; doi: 10.1038/nature16544
