Neandertal genes in people today came from hook-ups around 47,000 years ago

The time frame in which Neandertals and Homo sapiens heavily intermingled just got a little clearer.

DNA analyses of ancient and modern H. sapiens reveal that Neandertals spread their genes to humans during a single epoch around 47,000 years ago, researchers report in two new studies. The findings narrow the time frame in which this interbreeding could have occurred; previous estimates dated the era to somewhere between 65,000 and 41,000 years ago.

It means that all living people without recent African ancestry descended from the same population of humans that mated with Neandertals in this newly identified period, says evolutionary geneticist Kay Prüfer of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who coauthored one of the papers.

Prior to this mating period, H. sapiens left Africa and encountered Neandertals residing in what’s now Europe and Asia. Neandertals went extinct about 40,000 years ago, but people alive today without recent African ancestry can trace about 1 to 3 percent of their genetic inheritance to Neandertals (SN: 12/18/13).

Modern-day people of African descent possess a smaller percentage of Neandertal DNA, possibly from people migrating back to Africa over the past 20,000 years (SN: 1/30/20).

To investigate humans’ Neandertal inheritance, one group of researchers built an evolutionary timeline with data from more than 300 H. sapiens individuals spanning the last 45,000 years. Changes to the Neandertal DNA in the H. sapiens samples over time hinted that most genes inherited from Neandertals came during a single period lasting from 50,500 to 43,500 years ago, according to work reported in the Dec. 13 Science.

“Our analysis shows that the out-of-Africa migration must have [been] completed 43,500 years ago, and earlier waves that occurred before 51,000 years ago may have been from individuals that have not contributed to living non-African individuals,” said evolutionary geneticist Priya Moorjani of the University of California, Berkeley during a December 11 news briefing.

Some genes inherited from Neandertals — including those involved with skin color, immunity and metabolism — became beneficial quickly, nestling into human DNA within about 100 generations, the team found, and have stuck around for millennia (SN: 10/2/20).

The other study, published online December 12 in Nature, echoes the first study’s interbreeding time frame and finding of a single event, although it looks at a population with no present-day descendants.

For that study, Prüfer and colleagues examined DNA from six ancient H. sapiens whose remains were found in Germany and one in the Czech Republic. Two individuals — one from each site — provided the earliest high-quality H. sapiens genomes sequenced to date, at about 45,000 years old. All seven were found to be part of a small population of early humans that mated with Neandertals around 49,000 to 45,000 years ago. Even though that group died out, the Neandertal DNA they carried traces back to the sole interbreeding event common to all modern-day people of non-African descent.

“I found [these papers] really exciting because they came at a similar set of questions from two different angles,” says evolutionary geneticist Tony Capra of the University of California, San Francisco, who was not involved with either study.

“Both really strongly agree that there was likely one main period of interbreeding between Neandertals and the ancestors of non-Africans,” he says. “There were likely other events that were happening in other human groups that were living at the same time, but just were not lucky enough to make it into the present.”

Other 44,000- to 40,000-year-old H. sapiens, whose remains were found in Bulgaria and Romania, experienced a second mating event with Neandertals (SN: 4/7/21). This more recent interbreeding was not seen in the population discovered in Germany and the Czech Republic, according to the Nature paper.

But the group discovered in southeastern Europe also lacks modern-day descendants, coauthor Johannes Krause, an archaeogeneticist also at the Max Planck Institute for Evolutionary Anthropology, said during the news briefing. “The human story is not always a story of success.”