Research suggests that when our species, Homo sapiens, interbred with Neanderthals, most of the individuals involved may have been female Homo sapiens paired with Neanderthal males. This conclusion stems from analyses of genetic markers left in both populations due to this admixture.

The reasons behind this sex-biased mating behavior remain unclear. It is hypothesized that Neanderthal males may have favored female Homo sapiens over their own kind, or that modern human females were drawn to Neanderthal men, or possibly a combination of both. The question of whether these interactions were consensual is also unresolved.

“There’s limited insight we can draw,” states Alexander Pratt from the University of Pennsylvania in Philadelphia. “What we can confidently convey is that these events unfolded over many generations.”

Other geneticists find the evidence intriguing but not definitive. Areb Sumer from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, emphasizes, “We need further evidence as this stands as a significant claim regarding behavior.”

Since 2010, researchers have recognized that Homo sapiens, often called modern humans, interbred with Neanderthals following their migration from Africa to Eurasia. This interaction likely occurred during various periods, notably from approximately 50,000 to 43,000 years ago, and possibly more than 200,000 years ago. Presently, all non-African individuals carry some Neanderthal DNA.

However, there has been limited exploration regarding the implications of this interbreeding on sex chromosomes. Women typically possess two X chromosomes, while men have one X and one Y chromosome. Pratt and his team, including Sarah Tishkoff and Daniel Harris, also from the University of Pennsylvania, concentrated on the X chromosome in humans and Neanderthals.

“One significant observation regarding the human X chromosome is its relative lack of Neanderthal DNA,” Harris notes. Compared to other chromosomes, the human X chromosome has minimal Neanderthal genetic material. The research team proposed four possible explanations.

Firstly, it could be that Homo sapiens and Neanderthals were genetically incompatible, leading to hybrid incompatibility that resulted in health and reproductive challenges in hybrid offspring. However, the researchers found that the Neanderthal X chromosome contained significantly more Homo sapiens DNA compared to the non-sex chromosomes, indicating potential compatibility.

Secondly, natural selection may have favored modern human DNA. Given the smaller Neanderthal population, it would have been challenging for natural selection to eradicate harmful mutations. Conversely, modern humans had a larger population, allowing for the elimination of detrimental mutations, which could explain the proliferation of modern human X chromosome DNA within Neanderthal groups. Yet, the researchers argue this is negligible since the majority of the modern human DNA present on Neanderthal X chromosomes resides in non-functional regions.

Alternatively, cultural factors may play a role in mate selection. Different societies exhibit varying patterns of male and female migration. For instance, in certain cultures, females leave their familial groups to join male partners, while others may involve the opposite. If modern human females settled with Neanderthals, a bias in their X chromosomes might have emerged, but even if all the interbreeding females were modern humans, this could not sufficiently explain the pronounced bias identified by the researchers.

The researchers conclude that mating preferences are the most plausible explanation: Neanderthal men may have favored Homo sapiens women over their own partners, or Homo sapiens females may have preferred male Neanderthals to human partners, or perhaps both scenarios occurred. “If this is simply a matter of preference, it accounts for everything,” Pratt asserts.

However, other geneticists express caution about completely dismissing alternative explanations. Schumer points out that early interbreeding events had a pronounced effect on the Neanderthal genome, effectively replacing the ancient Y chromosome with a Homo sapiens Y chromosome. “This mixing must have involved a substantial number of modern human males,” she explains.

She cautions that hybrid incompatibility cannot be disregarded. Moises Col Macia at the Institute of Evolutionary Biology in Barcelona, Spain, notes that researchers have assumed Neanderthal DNA would function similarly when it integrated into modern human genomes, and vice versa. “This may not be the case,” he states.

Col Macia also suggests that another possibility, meiotic drive, warrants consideration. A rogue genetic element could skew inheritance patterns, causing one chromosome in a pair to be passed down more frequently than expected. His team has found preliminary evidence that this phenomenon also occurred in modern humans outside Africa, leading to the elimination of Neanderthal DNA from the X chromosome.