Male Neanderthals fathered most of the children born from interbreeding with Homo sapiens, according to a genetic analysis that examined patterns left on the X chromosome across both populations.
The study, led by Alexander Platt, Sarah Tishkoff, and Daniel Harris at the University of Pennsylvania, focused on a well-documented anomaly: the human X chromosome contains almost no Neanderthal DNA compared with other chromosomes. The team worked through four possible explanations before arriving at a sex-biased mating pattern as the most likely cause.
“One of the things that’s been pretty abundantly clear when looking at the human X chromosome is that it’s almost entirely a Neanderthal desert,” said Harris, according to the report.
Four Explanations, One Survivor
The researchers first tested whether hybrid incompatibility — the biological friction between two sufficiently different species — could account for the X chromosome gap. That explanation collapsed when the team found that the Neanderthal X chromosome actually contains more H. sapiens DNA than Neanderthal non-sex chromosomes do, suggesting the two species’ X chromosome DNA was compatible.
Natural selection was the second candidate. Neanderthals had small populations, making it harder to purge harmful mutations, while modern humans had larger populations with cleaner genetic libraries. But the modern human DNA found on the Neanderthal X chromosome sits mostly in non-functional regions — meaning it conferred no selective advantage and would not have spread for that reason.
A cultural explanation — specifically, modern human women relocating to live within Neanderthal groups — could produce an X chromosome bias, but the team calculated it would not be strong enough to explain the scale of the imbalance observed, even under a scenario where every interbreeding female was a modern human.
That left mating preference. Male Neanderthals may have favored female H. sapiens partners, female H. sapiens may have favored Neanderthal males, or the preference ran in both directions. “If they just like it that way, that explains everything,” Platt said.
Skepticism From the Field
Other researchers are not convinced the alternatives have been fully ruled out. Arev Sümer of the Max Planck Institute for Evolutionary Anthropology in Leipzig noted that an earlier episode of interbreeding — harder to date, possibly occurring 200,000 or more years ago — had profound effects on the Neanderthal genome, complicating any clean interpretation of current X chromosome data. “I think we need more evidence, because it’s a big claim about the behaviour,” Sümer said.
Interbreeding between the two species is known to have occurred in at least two periods: one roughly 50,000 to 43,000 years ago, and an earlier episode with less precise dating. All people of non-African descent today carry some Neanderthal DNA, a finding established since 2010.
The study makes no claim about whether any of the matings were consensual. On that question, Platt was direct: “I think we can say very little. The meaningful thing that we can say is that it was something that took place over generations.”
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