Why did male Neanderthals pair with human females?

Genomes reveal a sex‑biased pattern in ancient interbreeding

Recent population-genomic studies show a consistent signal: ancestry from Neanderthals entered the gene pool of anatomically modern humans in a pattern that implies male Neanderthals mated with female Homo sapiens more often than the reverse. Modern human genomes carry traces of Neanderthal DNA unevenly distributed across chromosomes, and certain patterns—such as regions depleted of Neanderthal ancestry on the X chromosome—are most consistent with male-biased gene flow.

Scientists offer several non‑exclusive explanations. One is demographic imbalance: migrating Homo sapiens groups expanding out of Africa may have had more females available for mating with resident Neanderthal males, or vice versa, depending on local population sizes and movements. Another possibility is selection: hybrid offspring carrying Neanderthal DNA on some chromosomes, particularly sex chromosomes, may have suffered reduced fertility or viability, producing a genomic footprint that looks like sex‑biased mating.

What the findings imply

• Modern human genomes preserve an asymmetric record of ancient encounters, shaping the distribution of archaic alleles today.
• The observed pattern helps explain so‑called “Neanderthal deserts” of ancestry—large genomic regions with little archaic DNA.
• The result reframes archaeological interactions: contact between the groups probably involved repeated, complex social dynamics rather than single events.

Important caveats remain. The genetic signatures cannot determine whether encounters were consensual, nor can they fully disentangle mating bias from post‑mixing selection. Researchers continue to explore cultural, ecological, and biological factors that could produce the observed asymmetry. Still, the genomic evidence provides a clearer window into how two human groups shaped each other’s genomes during a pivotal chapter of our evolutionary history.