Prehistoric humans and Neanderthals didn’t just interbreed, they did so with a consistent sex bias, as male Neanderthals and female modern humans mated more often, according to new research from the University of Pennsylvania. This ancient pattern could explain why Neanderthal DNA is nearly absent from the human X chromosome and reveal that social behavior, not just biology, influenced our genetic legacy.
Prehistoric mating preferences help explain why modern humans have small amounts of Neanderthal DNA almost everywhere in their genome except on the X chromosome. Image credit: Gemini AI.
“Along our X chromosomes, we have these missing swaths of Neanderthal DNA we call ‘Neanderthal deserts’,” said first author Dr. Alexander Platt, a researcher at the University of Pennsylvania.
“For years, we just assumed these deserts existed because certain Neanderthal genes were biologically ‘toxic’ to humans — as tends to be the case when species diverge — so we thought the genes may have caused health problems and were likely purged by natural selection.”
The new analysis of Neanderthal and modern human genomes suggests that long-standing mating preferences — rather than genetic incompatibility — shaped which Neanderthal DNA sequences persisted in modern humans and which were gradually lost.
The findings reveal the role social interactions in sculpting the human genome, challenging the idea that human evolution was driven solely by survival of the fittest.
“We found a pattern indicating a sex bias: gene flow occurred predominantly between Neanderthal males and anatomically modern human females, resulting in the loss of Neanderthal DNA X chromosomes of modern humans,” Dr. Platt said.
“Roughly 600,000 years ago, the ancestors of anatomically modern humans and their closest-related species, the Neanderthals, diverged, forming two distinct groups,” added University of Pennsylvania’s Professor Sarah Tishkoff, senior author of the study.
“Our ancestors evolved in Africa, while the ancestors of Neanderthals evolved in and adapted to life in Eurasia. But that separation was far from permanent.”
“Over hundreds of millennia, human populations migrated into Neanderthal territories and back again, and when these groups met, they mated, swapping segments of DNA.”
To determine whether Neanderthal X chromosomes contain alleles from anatomically modern humans, the researchers identified modern human DNA preserved in three Neanderthals from Altai, Chagyrskaya, and Vindija.
They then compared this dataset against one of diverse African genomes, a control group who had historically never encountered a Neanderthal.
“What we found was a striking imbalance,” said co-author Dr. Daniel Harris, a researcher at the University of Pennsylvania.
“While modern humans lack Neanderthal X chromosomes, Neanderthals had a 62% excess of modern human DNA on their X chromosomes compared to their other chromosomes.”
This mirrorlike reversal was their answer. If the two species were biologically incompatible, modern human DNA should have been missing from Neanderthal X chromosomes as well.
But because the scientists found an abundance of human DNA in Neanderthal X chromosomes, they were able to rule out reproductive incompatibility or toxic gene interactions as the barrier.
The remaining explanation lies in sex-biased interbreeding.
Because females carry two X chromosomes and males carry only one, mating direction matters.
If Neanderthal males partnered more often with modern human females, fewer Neanderthal X chromosomes would enter the human gene pool, and more human X chromosomes would enter Neanderthal populations.
Mathematical models confirmed that this bias could reproduce the observed genetic patterns.
Other possibilities, such as sex-biased migration, could theoretically produce similar results — but only through complex, shifting scenarios that varied across time and geography.
“Mating preferences provided the simplest explanation,” Dr. Platt concluded.
The results appear in the journal Science.
_____
Alexander Platt et al. 2026. Interbreeding between Neanderthals and modern humans was strongly sex biased. Science 391 (6788): 922-925; doi: 10.1126/science.aea6774
[Disclaimer: The content in this RSS feed is automatically fetched from external sources. All trademarks, images, and opinions belong to their respective owners. We are not responsible for the accuracy or reliability of third-party content.]
Source link
