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, new research has found.
Neanderthals were among the closest extinct relatives of modern humans (Homo sapiens), with the ancestors of both lineages diverging about 500,000 years ago. More than a decade ago, scientists revealed that Neanderthals interbred with ancestors of modern humans who migrated out of Africa. Today, the genomes of modern human populations outside of Africa contain about 1% to 2% Neanderthal DNA.
Scientists are still unsure when and where Neanderthal DNA entered the modern human genome. For example, did Neanderthals and modern humans interbreed at one particular place and time outside of Africa, or did they interbreed at many places and times?
To solve this mystery, researchers analyzed more than 300 modern human genomes spanning the past 45,000 years. These included samples from 59 individuals who lived between 2,200 and 45,000 years ago and 275 different contemporary modern humans. The researchers published their findings on BioRxiv preprint database. (Since the study is currently under review for potential publication in a scientific journal, the study authors declined to comment.)
The researchers focused on how much Neanderthal DNA they could see in these modern human samples. By comparing how the level of Neanderthal ancestry varied in modern humans DNA in different places and at different times, they could estimate when Neanderthals and modern humans interbred and for how long.
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The researchers found that the best explanation for most of the Neanderthal DNA in the modern human genome was a single major interbreeding period about 47,000 years ago that lasted about 6,800 years.
As modern humans began to leave Africa at least 194,000 years ago, the likely place where they met Neanderthals was western Asia, where Africa joins 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 with Neanderthal ancestry could then disperse around the world, he noted.
The researchers also examined how Neanderthal DNA persisted in the modern human genome over time. The longer a piece of Neanderthal DNA lasted, the more likely it was to make some evolutionary contribution to modern humans. Conversely, Neanderthal DNA that was quickly removed probably provided some type of evolutionary disadvantage. Scientists have found that Neanderthal genes that persisted are linked to skin color, metabolism and immune systemprobably providing some kind of immediate benefit to modern humans when they encountered new evolutionary pressures outside of Africa.
Given the rate at which most Neanderthal DNA was eliminated from the modern human genome, the study estimated that when the newly identified interbreeding period ended, more than 5% of the modern human genome was of Neanderthal origin. In other words, “about one in 20 parents in our ancestral population was Neanderthal,” Fernando Villaneapopulation geneticist at the University of Colorado Boulder, who was not involved in the study, told Live Science.
Rajiv McCoy, a population geneticist at Johns Hopkins University in Baltimore, who was not involved in the new work, told Live Science that interbreeding between Neanderthals and modern humans may have occurred at other times, but that it left no lasting traces in the modern world. human gene pool. For example, modern human jaw from about 37,000 to 42,000 years ago found in Romania in 2002 has Neanderthal DNA not found in other modern human genomes, which McCoy says may reflect an interbreeding event “that did not contribute to current human diversity.”
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 dispersal out of Africa because they all exist [living] “Populations outside of Africa — Chinese, Native Americans, Indonesians, Aboriginal Australians and so on — bear the marks of this event, which therefore constrains when their ancestors began to diverge to less than about 47,000 years ago,” Stringer said.
However, “there is archaeological evidence of human occupation in northern Australia about 65,000 years ago,” Stringer said. “So either this evidence is wrong; the population was Homo sapiens but they became extinct or were overwhelmed by the later dispersal; or the population was not actually Homo sapiensThe latter option “seems much less likely given the complex behavior suggested by the evidence, but it would of course be a huge bombshell.”
Strangely, DNA swapping 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—i.e. Homo sapiens to the Neanderthal,” Stringer noted. “It might have happened, but we haven’t found out yet. Or maybe it didn’t happen, which has implications for the behavior of the two populations.” Or maybe such hybrids were less successful for some reason, he noted — for example, they might have been less healthy or less fertile.