NEW YORK (AP) – While interactions between early humans and Neanderthals were not uncommon during periods when they inhabited the same geographical regions tens of thousands of years ago, the specifics of these encounters, including the individuals involved and the underlying motivations, remain largely obscure.
A novel genetic investigation has shed some light on this ancient social dynamic, suggesting that unions were more frequently formed between female Homo sapiens and male Neanderthals.
The precise mechanisms behind these interspecies pairings are still a significant enigma. Inquiries persist regarding whether human females proactively integrated into Neanderthal communities, or if Neanderthal males were drawn toward larger human settlements. Furthermore, the nature of these interactions — whether they were amicable, perplexing, clandestine, or even violent — is a subject of ongoing speculation.
“It’s uncertain if a definitive answer regarding the circumstances of these events will ever be attainable, given our inability to traverse through time,” commented population genetics specialist Xinjun Zhang from the University of Michigan, in reference to the recent analysis.
However, the research, which was published on Thursday in the esteemed journal Science, indicates that “whenever Neanderthals and modern humans engaged in interbreeding, there was a discernible inclination towards male Neanderthals and female modern humans, as opposed to the inverse scenario,” stated author Alexander Platt, a geneticist at the University of Pennsylvania.
The occurrence of matings between Neanderthals and humans is established by the presence of a small yet significant proportion of Neanderthal genetic material in the majority of contemporary humans. This inherited DNA encompasses genes that confer advantages in combating certain ailments, as well as those that predispose individuals to others.
Nevertheless, it has also been observed that Neanderthal genetic contributions are not uniformly distributed across the human genome.
Notably, an unexpected scarcity of Neanderthal DNA is present on the human X chromosome, one of the gene-carrying structures within cells designated as sex chromosomes, when contrasted with the quantity of Neanderthal DNA found on the other, non-sex chromosomes.
Initial hypotheses posited that the genes located in these specific chromosomal regions might have been disadvantageous or even detrimental. It was theorized that individuals possessing these genetic configurations may have had reduced survival rates, leading to their eventual elimination from the gene pool through evolutionary processes.
Alternatively, it was suggested that the disparity could be attributed to the dynamics of interspecies mingling.
In an effort to resolve this conundrum, Platt and his collaborators shifted their focus to the Neanderthal genome and the human DNA that was incorporated during a “hybridization event” approximately 250,000 years ago.
Upon examining these genetic segments, a more pronounced human influence was detected on the Neanderthal X chromosome – the very same chromosome that, in humans, exhibits a lower-than-expected prevalence of Neanderthal DNA.
The most plausible explanation for this symmetrical pattern is linked to mating behaviors, which is a consequence of the established mechanisms of sex chromosome inheritance from parents to offspring, as elucidated by Platt.
Given that genetic females possess two X chromosomes, while genetic males have one X and one Y chromosome, approximately two-thirds of all X chromosomes within a population are, on average, inherited from maternal lineage.
If a greater number of human females engaged in reproductive unions with Neanderthal males compared to the reverse scenario, an expected outcome over numerous millennia would be the observed pattern: an increased presence of human DNA on Neanderthal X chromosomes and a corresponding reduction of Neanderthal DNA on human X chromosomes.
“I believe they have made significant strides in illuminating the missing pieces of this complex puzzle,” remarked Joshua Akey, an expert in evolutionary genomics at Princeton University who was not involved in the current investigation.
The study, however, cannot completely dismiss alternative hypotheses. For instance, Zhang noted that it remains a possibility that offspring resulting from unions between human males and Neanderthal females experienced lower survival rates.
Nevertheless, the most straightforward and probable explanation identified by the research is also the most compelling: “This is not a product of a strictly Darwinian survival of the fittest,” Platt stated. “It is fundamentally a consequence of our interpersonal dynamics, and the nature of our culture, society, and behavior.”
