Genetic analysis of Neanderthal remains reveals a distinct pattern in interbreeding with early Homo sapiens: Neanderthal men were more likely to mate with human females than vice versa, according to a study published Thursday in the journal Science.
Researchers at the University of Pennsylvania arrived at this conclusion by examining the amount of modern human DNA present on the X chromosomes of Neanderthals. The study focused on the X chromosome because it is passed down differently between males and females. A lower-than-expected presence of Neanderthal DNA on the X chromosome in modern human populations suggested a bias in mating patterns.
“It allows us to say something about the social behaviour of these ancient populations,” said Sarah Tishkoff, professor of genetics and biology and head of the lab that conducted the analysis. The findings indicate that when interbreeding occurred, typically between 49,000 and 45,000 years ago, it was predominantly Neanderthal males who partnered with anatomically modern human females.
The research builds on existing knowledge that interbreeding between the two species did occur as Homo sapiens migrated out of Africa and into Eurasia, encountering established Neanderthal populations. Previous studies have established that most people today carry a small percentage of Neanderthal DNA. Still, the directionality of these encounters had remained largely unknown.
Scientists have previously theorized about the reasons for the reduced Neanderthal DNA on the X chromosome, including the possibility of evolutionary disadvantages associated with Neanderthal X chromosomes in humans. Tishkoff’s team was able to rule out these alternative explanations, strengthening the conclusion that mating preferences played a significant role.
The study compared ancient Neanderthal DNA with that of modern-day individuals from Africa, who have minimal Neanderthal ancestry, to establish a baseline for comparison. This approach helped isolate the Neanderthal genetic signal and determine its distribution across the genome.
The findings align with a separate study published on February 26, 2026, which also indicated a pattern of male Neanderthal and female human pairings. That research, as reported by Scientific American, noted the unusual distribution of Neanderthal DNA in the human genome as a key indicator of this mating bias.
Researchers continue to investigate the underlying reasons for this mating preference. The implications of these findings extend beyond simply understanding the biological interactions between Neanderthals and humans, offering insights into the social dynamics and reproductive behaviors of these ancient populations.
