Most people alive today carry traces of genes inherited from Neanderthals, indicating a complex history of interbreeding between modern humans and their now extinct cousins. But when exactly we first got together with our ‘cousins’ has proven elusive. Recent genome analysis now suggests that this genetic mingling occurred around 47,000 years ago. If it passes peer review, this research refines our understanding of when and where these interactions took place and their impact on human evolution.

The study, published as a bioRxiv preprint, utilized genomes from ancient and modern Homo sapiens to determine the timing of Neanderthal gene flow into modern human populations. By examining ancient DNA from individuals in Western Europe and Asia, researchers gained new insights into this critical juncture in human history.

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Ancient Genomes Reveal a Prolonged Period of Mixing

A recent article on Science explains how the research team, led by Priya Moorjani from the University of California, Berkeley, analyzed genomes from 59 ancient Homo sapiens dating from 45,000 to 2,200 years ago. These samples included DNA from notable ancient individuals such as the Ust’-Ishim man from western Siberia, the Zlatý kůň woman from the Czech Republic, and individuals from Bulgaria’s Bacho Kiro caves and Romania’s Peștera cu Oase caves.

According to the Science report, the study identified regions of Neanderthal DNA in these ancient genomes and compared them to genomes from 275 present-day individuals. Using sophisticated computer modeling, the researchers traced the evolution of Neanderthal genes over time. Their analysis suggested that Neanderthal gene flow into modern human populations began approximately 47,000 years ago, with a prolonged period of interbreeding lasting around 6,000 to 7,000 years.

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Exhibition in Bacho Kiro cave near Dryanovo, Bulgaria. (Fotokon)

The Implications for Human Migration

The findings indicate that Neanderthal and modern human interactions were likely not rare. For instance, individuals from Oase and Bacho Kiro showed recent Neanderthal ancestry within a dozen generations. These ancient humans carried segments of Neanderthal DNA that are not found in modern populations, suggesting additional encounters that did not result in surviving descendants.

“Clearly, humans were running into Neanderthals all over the place,” Fernando Villanea, a population geneticist at the University of Colorado Boulder notes, according to Science.

“Maybe some of these early interactions were in populations that didn’t leave descendants, but then [about 47,000 years ago] there’s this main event. That makes a lot of sense.”

The study also revealed that modern humans quickly lost some early Neanderthal DNA segments, which may have carried harmful mutations. However, certain Neanderthal genes, particularly those related to skin pigmentation, immune response, and metabolism, proved advantageous and persisted in modern human genomes.

The timing of these interbreeding events also has implications for understanding human migration. Indigenous Australians, who carry Neanderthal ancestry, must have reached Australia after the interbreeding period, no earlier than 47,000 years ago. This finding challenges some archaeological estimates that suggest modern humans arrived in Australia as early as 65,000 years ago.

“The implication is that those early dispersals either went extinct or were effectively replaced or swamped by larger later waves,” commented Chris Stringer, an anthropologist who studies human evolution at the Natural History Museum in London, to Science.

If accepted, the new analysis helps track the few ancient human groups that left descendants. Most ancient human groups did not survive, but the genetic legacy of those that did provides crucial insights into the evolution and migration of our species.

Top image: Humans and Neanderthals started breeding together 47,000 years ago.