Neanderthal genes seen in modern humans may have entered our DNA through an interbreeding interval that began about 47,000 years ago and lasted nearly 7,000 years, a new study finds.
Neanderthals were among the closest extinct relatives of modern humans (Homo sapiens), with the ancestors of the two lineages diverging about 500,000 years ago. More than a decade ago, scientists revealed that Neanderthals interbred with the ancestors of modern humans who migrated out of Africa. Today, the genomes of modern human populations outside of Africa contain about 1% to 2% of Neanderthal DNA.
Researchers are still unsure when and where Neanderthal DNA made its way into the modern human genome. For example, did Neanderthals and modern humans mix at one particular place and time outside of Africa, or did they interbreed at many places and times?
To solve this mystery, the researchers analyzed more than 300 modern human genomes spanning the past 45,000 years. They included samples from 59 individuals who lived between 2,200 and 45,000 years ago, and 275 different present-day modern humans. The scientists published their findings on BioRxiv preprint database. (Because the study is currently under review for possible publication in a scientific journal, the study authors declined to comment.)
The scientists focused on how much Neanderthal DNA they could see in these modern human samples. By comparing how the level of Neanderthal ancestry varies in modern humans DNA at different places and times, they could estimate when Neanderthals and modern humans interbred and for how long.
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The best explanation for the majority of Neanderthal DNA seen in the modern human genome is a major interbreeding period about 47,000 years ago that lasted about 6,800 years, the researchers found.
Like modern humans began to leave Africa at least 194,000 years ago, the likely place for them to meet Neanderthals was western Asia, where Africa meets Eurasia, Chris Stringer, a paleoanthropologist at the Natural History Museum in London who was not involved in the new study, told Live Science. Modern humans of Neanderthal ancestry may have spread across the globe, he noted.
Scientists are also investigating how Neanderthal DNA is preserved in the modern human genome over time. The longer a piece of Neanderthal DNA has been around, the more likely it is to provide some evolutionary benefit to modern humans. Conversely, Neanderthal DNA that was quickly removed probably carried some sort of evolutionary disadvantage. Researchers found that Neanderthal genes that persisted were linked to skin color, metabolism and on immune systempossibly providing some immediate benefit to modern humans as they face new evolutionary pressures out of Africa.
Given the rate at which most of the Neanderthal DNA was eliminated from the modern human genome, the study estimates that when the newly discovered interbreeding period ended, more than 5% of the modern human genome was Neanderthal in origin. In other words, “about one in 20 parents in our ancestral population was Neanderthal,” Fernando Villaneaa population geneticist at the University of Colorado Boulder, who was not involved in this study, told Live Science.
Rajiv McCoy, a population geneticist at Johns Hopkins University in Baltimore who was not involved in this new work, told Live Science that interbreeding between Neanderthals and modern humans may have occurred at other times, but they did not leave any lasting traces in modern times. human gene pool. For example, modern human jaw from about 37,000 to 42,000 years ago, discovered in Romania in 2002, has Neanderthal DNA not seen in other modern human genomes, which may reflect an interbreeding event “that did not contribute to modern human diversity,” according to McCoy.
Stringer noted that previous research suggested that the interbreeding that introduced Neanderthal DNA into the modern human genome took place between 50,000 and 60,000 years ago. The new estimate of 47,000 years ago “has implications for Homo sapiens dispersals outside of Africa, since all extant [living] populations outside of Africa – Chinese, Amerindians, Indonesians, Indigenous Australians and so on – bear the hallmarks of this event, which therefore limits when their ancestors began to disperse to less than approximately 47,000 years ago,” Stringer said.
However, “there is archaeological evidence of human occupation in northern Australia around 65,000 years ago,” Stringer said. “So either this evidence is wrong; populations were Homo sapiens but they disappeared or were overwhelmed by a later dispersal; or the population was not actually Homo sapiens.” The latter possibility “seems much less likely given the complex behavior implied by the evidence, but it would be a huge bombshell, of course.”
Curiously, DNA exchange appears to have been one way—meaning that modern human DNA does not appear to have entered Neanderthal genomes. “Currently, there is little evidence of gene flow in the opposite direction – ie. Homo sapiens of a Neanderthal,” Stringer noted. “Maybe it did, but we haven’t found it yet. Or maybe that didn’t happen, with consequences for the behavior of the two populations.” Or maybe such hybrids were less successful for some reason, he noted — for example, maybe they were less healthy or less fertile.
