Neanderthals may not have truly gone extinct, but instead may have been absorbed into the modern human population. That’s one implication of a new study, which finds that modern human DNA may have made up 2.5% to 3.7% of Neanderthal the genome.
“This research really highlights that what we think of as a separate Neanderthal lineage was really more closely related to our ancestors.” Fernando Villanea, a population geneticist at the University of Colorado Boulder who was not involved in the study, told Live Science. Both modern human and Neanderthal populations “shared a long history of the exchange of individuals.”
Neanderthals were among the closest extinct relatives of modern humans, with our ancestry diverging about 500,000 years ago. More than a decade ago, scientists discovered that Neanderthals interbred with ancestors of modern humans who traveled out of Africa. Today, the genomes of modern human groups outside of Africa contain about 1% to 2% of Neanderthal DNA.
Connected: ‘More Neanderthal than human’: How your health may depend on our DNA long lost ancestors
However, researchers know less about how modern human DNA might have entered the Neanderthal genome. This is largely because there are currently only three known high-quality examples of a complete Neanderthal genome that have survived—from specimens discovered in Vindija Cave in Croatiawhich date from 50,000 to 65,000 years ago, and Chagyrskaya AND Denisova caves in Russia, which date to about 80,000 and 50,000 years ago, respectively.
In comparison, scientists have sequenced the genomes of hundreds of thousands of modern humans since the completion of the Human Genome Project in 2003.
“There has been a significant amount of research focused on how interbreeding between Neanderthals and modern humans affected our DNA and evolutionary history,” the study’s senior author. Joshua Akey, a population geneticist at Princeton University in New Jersey, told Live Science. “However, we know much less about how these encounters affected the Neanderthal genome.”
In the new study, the scientists relied on the fact that both modern humans and Neanderthals generally possess two versions of each gene, one inherited from the father, the other from the mother. Because the two groups were more different from each other than from others of their kind, interbreeding between Neanderthals and humans would lead to offspring that had a higher chance of possessing two different versions of each gene. – a situation known as heterozygosity – than children that did not result from such a cross.
The researchers compared the genomes of three Neanderthals with those of 2,000 modern humans. They found that the Neanderthal genome may consist of 2.5% to 3.7% modern human DNA. This is similar to 1 in 30 modern human parents in the ancestral Neanderthal population.
The research team’s analysis suggested that modern human DNA entered the Neanderthal genome during at least two distinct epochs of interbreeding — one around 200,000 to 250,000 years ago and the other around 100,000 to 120,000 years ago. Interbreeding may have occurred on other occasions, but such events may not have left any discernible imprint on the Neanderthal genome, Akey said.
or recent study, not yet peer-reviewed suggests that most of the Neanderthal DNA seen in the modern human genome resulted from a single major interbreeding period about 47,000 years ago that lasted about 6,800 years. Interbreeding that occurred at other times, such as earlier events that affected the Neanderthal genome, likely did not leave a discernible trace in our genome.
The skulls found in Skhul and Qafzeh Caves in Israel date back to around 100,000 years ago – around the same time as one of the major interbreeding events identified in the study. Those fossils appear to be modern human remains, but they still have relatively primitive features like larger eyebrows, which could be “signs of gene flow from Neanderthals.” Chris Stringera paleoanthropologist at the Natural History Museum in London, who was not involved in the new study, told Live Science.
By analyzing the level of genetic variation seen among the three Neanderthal genomes, the new study also suggested that the long-term average Neanderthal population was about 20% smaller than previously estimated. “That doesn’t sound like a huge difference, but given that Neanderthals were already estimated to have a fairly small population size, the fact that it was even smaller is an important insight,” Akey said.
These new smaller estimates of Neanderthal population size suggest that Neanderthals may have gone extinct because “they just got absorbed into the modern human population,” Akey said. “Repeated waves of modern human migrations out of Africa eventually overwhelmed Neanderthals’ ability to remain a distinct population, and they ultimately simply assimilated into the modern human gene pool.”
Future research could study the biological effects, good or bad, that modern human DNA may have had on Neanderthals, Akey said.
The scientists detailed their findings online Thursday (July 11) in the journal science.