A small amount of Neanderthal deoxyribonucleic acid (DNA) remains in the genomes of people who are not of African descent among modern humans. It is evidence that modern humans and Neanderthals met and mixed tens of thousands of years ago. However, while Neanderthal DNA remains throughout the genome across the body, traces of it are hardly seen on the X chromosome.
For a long time, the scientific community has explained the reason this way: some Neanderthal genes did not suit modern humans well, and genes on the X chromosome in particular were unfavorable for health or reproduction, so they naturally disappeared over time.
But a research team at the University of Pennsylvania recently reported findings that social interactions and mating patterns between groups at the time can explain the longstanding hypothesis. The study was published in Science on the 27th.
Neanderthals are a species genetically very close to the ancestors of modern humans, diverging from a common ancestor about 600,000 years ago. They had sturdy builds and large brain volumes, and are known to have used fire and tools and to have had language communication and funerary culture.
The ancestors of modern humans evolved in Africa, while the ancestors of Neanderthals evolved in Eurasia and adapted to that environment. Over hundreds of thousands of years, humans migrated into Neanderthal territory and then returned, and when the two groups met, they exchanged some DNA through mating.
When the team compared Neanderthal and modern human genomes, they found that, on the contrary, Neanderthal X chromosomes contained 62% more modern human DNA than other chromosomes. If the X chromosomes of the two groups were biologically incompatible, Neanderthal X chromosomes should also have had less human DNA. In reality, the opposite pattern appeared, meaning it is insufficient to explain the findings solely by saying X chromosome genes were toxic and disappeared.
The researchers focused on differences in how genes are transmitted by sex. The X chromosome is passed down differently to men and women. Women have two X chromosomes, and men have only one. Therefore, depending on which group's men and which group's women more often formed pairs, the composition of X chromosomes remaining in descendant groups can differ.
Based on this principle, they built a mathematical model and found that if pairings between Neanderthal men and modern human women occurred more frequently, the present genetic pattern could be explained. In this case, Neanderthal X chromosomes would remain relatively less on the modern human side, while modern human X chromosomes could enter the Neanderthal group in greater amounts.
The researchers said, "Other possibilities are not completely ruled out. For example, men and women may have had different migration routes, or the ways of mixing may have differed by region and period," adding, "However, such explanations require far more complex assumptions, and the simplest explanation for the pattern observed now is mating in a particular direction."
Alexander Platt, a senior researcher at the University of Pennsylvania, said, "These findings do not suggest that Neanderthal men were particularly attractive to modern human women or vice versa," adding, "Unions between the two groups were not identical in both directions, and it is possible that unions in one direction were relatively more permitted."
The team plans to examine the internal gender structure of Neanderthal society going forward. For example, whether women stayed in the groups where they were born or whether men moved to other groups could also be inferred from genetic clues.
References
Science (2026), DOI: https://doi.org/10.1126/science.aea6774