Neanderthal Men May Have Often Hooked Up With Human Women Thousands of Years Ago

Most people alive today carry a little Neanderthal DNA—except in a few spots. A new study might explain why

Neanderthal men may have had a thing for human women.

When the two species got together tens of thousands of years ago, the hookups may have often involved a male Neanderthal and a female human, according to a new study. The findings, described February 26 in the journal Science, might explain why Neanderthal ancestry is spread unevenly within the Homo sapiens genome.

Neanderthals, some of our closest relatives, vanished around 40,000 years ago. Within about 10,000 years before their demise, they interbred with Homo sapiens, leading to most humans alive today carrying a bit of Neanderthal DNA. But a mystery remained: Even in people with higher percentages of the extinct species’ ancestry, certain parts of the genetic instruction book, or genome—including most of the X sex chromosome—contain surprisingly little, if any, Neanderthal DNA. Scientists have long struggled to explain the existence of these strange stretches of the genome, called “Neanderthal deserts.”

“For the last ten years or so, we’ve had two families of explanations about what happened,” says study co-author Alexander Platt, an evolutionary geneticist at the University of Pennsylvania, to Katie Hunt at CNN.

One idea posits that the desert regions arose via natural selection. Perhaps Neanderthal genes were disadvantageous compared to human variants on the X chromosome, so humans slowly got rid of them as they evolved. Or maybe, Neanderthal DNA on the X chromosome simply didn’t transfer well to our species. Platt and his colleagues propose something else entirely.

To investigate the mysterious genetic regions, the researchers compared the genomes of three female Neanderthals with distant human ancestry to those of 73 human women with minimal to no Neanderthal ancestry. The extinct species showed the opposite X-chromosome pattern to that seen in Homo sapiens: Their X chromosomes contained, on average, 62 percent more human DNA than their non-sex chromosomes, the team found. Computer simulations of what population dynamics might have created this pattern suggested that the most likely culprit was sex bias.

Most female individuals have two X chromosomes, while male individuals usually have one X and one Y chromosome. So, a mother typically passes on her X chromosome to all her offspring, but a father passes his X chromosome only to his daughters. That means that less Neanderthal DNA on the human X chromosome, and vice versa, indicates a higher frequency of Neanderthal dads and human moms than the other way around.

“You almost couldn’t turn the bias up high enough in the models to get the patterns we were seeing,” Platt tells Annie Roth at National Geographic. “What we’re seeing here is not just ‘survival of the fittest’ in the classic Darwinian sense, but the imprint of very broad and very common sex biases.”

The work represents an incredible example of how ancient DNA can unveil behaviors from long ago, Joshua Akey, a geneticist at Princeton University who was not involved in the study, tells Carl Zimmer at the New York Times. “I wouldn’t have thought that was possible when I started doing this work in graduate school,” he says. “This study pushes us in a new direction.”

Other researchers call for more caution. “I would say it’s an intriguing idea, and it seems to fit the pattern,” says Leonardo Iasi, an evolutionary geneticist at the Max Planck Institute for Evolutionary Anthropology in Germany, who also was not involved in the work, to Freda Kreier at Nature. “But it’s very hard to conclusively show that this is what actually happened.”

Still, scientists tend to treat members of ancient populations as though “they just bump into each other and reproduce,” Iasi says. “Obviously, that’s not what happened.”