Genetic analysis suggests that interbreeding between Neanderthals and Homo sapiens involved mostly male Neanderthals and female modern humans. Researchers examined sex chromosomes to uncover this pattern, which occurred during multiple periods after humans left Africa. The findings point to mating preferences as the likely explanation, though experts call for more evidence.
Interbreeding between Homo sapiens and Neanderthals took place after some modern humans migrated from Africa into Eurasia, with key episodes around 50,000 to 43,000 years ago and possibly earlier, over 200,000 years in the past. Today, individuals of non-African descent carry Neanderthal DNA in their genomes. A new study by Alexander Platt, Sarah Tishkoff, and Daniel Harris at the University of Pennsylvania analyzed the X chromosomes from both species to explore the dynamics of this mixing.
The human X chromosome shows almost no Neanderthal DNA, described by Harris as 'almost entirely a Neanderthal desert.' The researchers considered several explanations for this scarcity. Hybrid incompatibility, where DNA from the two species might not function well together, was ruled out because Neanderthal X chromosomes contain more Homo sapiens DNA than their other chromosomes, indicating compatibility.
Natural selection favoring modern human DNA was dismissed, as the retained human DNA on Neanderthal X chromosomes appears in non-functional regions. Cultural practices, such as females moving between groups, could contribute to a bias but not to the extent observed. This leaves mating preferences as the most plausible reason: male Neanderthals favoring female Homo sapiens, female modern humans preferring male Neanderthals, or both.
Platt noted, 'If they just like it that way, that explains everything.' However, other geneticists urge caution. Arev Sümer at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, said, 'I think we need more evidence, because it’s a big claim about the behaviour.' Moisès Coll Macià at the Institute of Evolutionary Biology in Barcelona, Spain, highlighted that hybrid incompatibility might not be symmetric and suggested considering meiotic drive, where genetic elements bias chromosome inheritance.
The study appears in Science (DOI: 10.1126/science.aea6774). Platt added, 'The meaningful thing that we can say is that it was something that took place over generations.' No details emerge on whether matings were consensual or forced.