A new genetic study published in PubMedCentral, led by Alexander Platt, examines the distribution of Neanderthal ancestry in modern human genomes.
Researchers utilized population-level simulations to analyze why the X chromosome contains significantly less Neanderthal material than autosomal DNA, shedding light on ancient interbreeding patterns.
Modern humans outside Africa retain about 2% of their DNA from Neanderthals, the result of contact recorded tens of thousands of years ago. However, the X chromosome shows a significantly lower presence of such genetic material. While this situation was initially attributed solely to negative natural selection – that is, the disappearance of genetic variants that were incompatible with modern humans – the new analysis leads to different estimates.
The researchers observed differences in the distribution of DNA in regions that do not contain instructions for making proteins compared to those that do. Through simulations, they found that Neanderthal men may have bred more often with women who had a greater genetic ancestry than Homo sapiens, rather than the other way around. According to the findings, this pattern was repeated in at least two different time periods, about 200.000 years apart.
The first phase of admixture is dated to about 45.000 years ago and is associated with most of the Neanderthal DNA carried by today's populations outside Africa. The second phase dates to more than 200.000 years ago, leaving traces that were retained in Neanderthal populations rather than passed on to modern humans. This evidence suggests that contacts were repeated over hundreds of thousands of years.
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Despite the new evidence, the scientists stressed that the genetic data describe statistical patterns at the population level and cannot explain the exact conditions under which these contacts took place. It remains unknown whether the relationships were based on alliances, whether they were consensual or whether they involved violence and conflict, as well as who took care of the offspring that were born.
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