It's well-documented that as modern humans migrated out of Africa, they encountered and interbred with Neanderthals. This led to a significant integration of Neanderthal DNA into the human genome. Interestingly, it's less commonly known that Neanderthal genomes also contain segments of modern human DNA.
Inherited Neanderthal DNA varies among individuals, with different people possessing various fragments by chance. However, there are specific areas known as 'Neanderthal deserts,' where Neanderthal DNA is notably absent. The most extensive of these deserts is the X chromosome, sparking debates about whether this reflects the evolutionary success of the genes or is due to mating behaviors.
In a recent study from the University of Pennsylvania, researchers Alexander Platt, Daniel N. Harris, and Sarah Tishkoff conducted an analysis of Neanderthal genomes, specifically focusing on their X chromosomes. Their findings reveal a significant presence of modern human DNA, which they interpret as evidence of selective mating. This suggests Neanderthal males might have preferred modern human females for mating, influencing their descendants.
What type of selection are we looking at?
Given the prolonged separation of Neanderthals and modern humans, some genetic incompatibility could have developed. Proteins, which interact within complex networks, could have evolved together, meaning that reintroducing original gene variations might disrupt these networks and reduce fitness.
This implies that certain Neanderthal genes introduced into modern humans—or vice versa—could disrupt genetic networks and diminish the fitness of carriers, leading to their eventual elimination through natural selection. Of course, some genetic segments might disappear purely by chance, as the sheer size of the human genome and the expanding modern human population diluted external genetic influences. Determining which factor prevailed is a complex scientific challenge.