Research suggests mating direction bias between Neanderthals and humans | Neanderthals
Tens of thousands of years ago, as modern humans migrated to the northern territories inhabited by our ancient cousins, the Neanderthals, the two species met – and sometimes mated.
Now, genetic evidence has revealed a striking imbalance in these prehistoric dating, suggesting that interbreeding occurred primarily between Neanderthals and women.
They say this ancient mating pattern explains why Neanderthal DNA is largely absent from the human X chromosome.
“We found a trend indicating a sexual bias: gene flow occurred primarily between Neanderthals and anatomically modern human women,” said Dr. Alexander Platt, a senior research scientist at the University of Pennsylvania and first author of the research.
The ancestors of modern humans and their closest related species, the Neanderthals, diverged, forming two distinct groups, about 600,000 years ago.
“Our ancestors evolved in Africa, while Neanderthal ancestors evolved and adapted to life in Eurasia,” said Sarah Tishkoff, a David and Lyn Silfen University professor of genetics and biology at the University of Pennsylvania, who led the research. “But this separation was far from permanent.”
Over hundreds of millennia, human populations migrated to Neanderthal territories and vice versa, and when these groups met, they mated by exchanging segments of DNA. Today, individuals of non-African descent typically have a few percent of Neanderthal DNA, while those of African descent typically have a smaller proportion.
“It’s hard to say how many times these events have happened,” Tishkoff said. “But I just feel like it’s happening more than we initially assumed.”
However, the Neanderthal DNA carried by modern humans is not distributed evenly throughout the genome. In particular, areas of Neanderthal DNA, known as “Neanderthal deserts,” are missing along the X chromosome.
Platt said: “For years we simply assumed that these deserts existed because certain Neanderthal genes were biologically ‘toxic’ to humans – as tends to be the case when species diverge – so we thought that these genes may have caused health problems and had probably been purged by natural selection.
However, the latest research, published in the journal Science, offers an alternative explanation.
The researchers studied the presence of preserved modern human DNA in three Neanderthals – Altai, Chagyrskaya and Vindija – and compared this dataset with genetic data from specific sub-Saharan African populations lacking Neanderthal ancestry.
If the two species were biologically incompatible, modern human DNA should also have been missing from Neanderthal X chromosomes. However, the analysis revealed that Neanderthal X chromosomes had a 62% excess of modern human DNA compared to their other chromosomes – a mirroring reversal of the distribution of Neanderthal DNA in human populations.
Since females carry two X chromosomes and males only carry one, the direction of mating is important. If Neanderthals associated more often with modern human women, fewer Neanderthal X chromosomes would enter the human gene pool, and more human X chromosomes would enter Neanderthal populations.
“Mating preferences provided the simplest explanation,” Platt said.
The results do not suggest, Platt said, that Neanderthals were particularly attractive to modern human women or vice versa. “It could be that everyone considers interspecies mating to be gross – or attractive,” he said. “But it seems that one direction was considered better, or less worse, than the other.”
The strength of the effect suggested that it also went beyond the first mating event and that males of Neanderthal origin would be preferred over females of Neanderthal origin, within a predominantly modern human population. “This is something that was expected to continue within a population after the first mating,” Platt said.
