February 26, 2026
2 min read
Sex between Neandertals and anatomically modern humans tended to follow a specific pattern
Interbreeding between Neandertals and ancient anatomically modern humans primarily occurred between male Neandertals and female humans, a new study suggests
Neanderthal man at the human evolution exhibit at the Natural History Museum in London, United Kingdom
Mike Kemp/In Pictures via Getty Images
Most Homo sapiens today have a little Neandertal DNA sprinkled through their genome. These genomic signals are the telltale signs that overlapping populations of ancient anatomically modern humans and Neandertals had sex. Exactly what these interactions looked like is a mystery, but a new study suggests that when our species and Neandertals did interbreed, it was primarily between male Neandertals and female humans.
Neandertal DNA tends to be absent on modern humans’ X chromosome. There were other theories as to why that might be, including the possibility that there was some evolutionary disadvantage to the Neandertal X chromosome in humans.
“Our study allowed us to distinguish between these possibilities,” says Sarah Tishkoff, a professor in genetics and biology at the University of Pennsylvania and a co-author on the study.
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The researchers compared ancient Neandertal DNA to that of people living in Africa today who have little or no Neandertal ancestry.
If mixing human and Neandertal DNA was disadvantageous, the scientists theorized that Neandertal genomes would show similarly large gaps devoid of ancient human DNA as modern humans do, explains Alexander Platt, a senior research scientist at University of Pennsylvania, and the study’s lead author. “But that wasn’t what we found.”
Instead, there was much more human ancestry present on Neandertal X chromosomes than the researchers had expected, including in regions that had nothing to do with fitness. This suggested that the conspicuous absence of Neandertal DNA in parts of the genomes of humans today is likely a result of a strong sex-bias in mating long ago.
The research published on Thursday in the journal Science.
Scientists already had an inkling that this might be the case, explains Tishkoff. But it’s unclear why male Neandertals might selectively mate with female humans, or vice versa. “One can only speculate!” Tishkoff says.
A major limitation of the study is that the team didn’t have a ton of Neandertal DNA to work with—there are only a handful of high-quality Neandertal genomes. And these provide just a snapshot of what human-Neanderthal sex may have looked like at a point in time.
As anthropologists work to uncover more Neandertal DNA in the fossil record, the human-Neandertal genetic picture will get clearer, Platt says.
In more of a philosophical sense, the study shows the value of looking outside of human DNA to understand our own ancestry, Platt says.
“We don’t just have to look in our own gene pool to find what happens to Neandertal alleles when they came into our population,” he says. By looking at the other half of these interactions, at Neandertals, “you get a much richer picture.”
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