By now, it is firmly established that modern humans and their Neanderthal relatives met and mated during the spread of our ancestors out of Africa, resulting in a large amount of Neanderthal DNA dispersed throughout our genome. What is less widely recognized is that some of the Neanderthal genomes we have seen also contain fragments of modern human DNA.
However, not every modern human has the same set of Neanderthal DNA; Different people may, by chance, have inherited different pieces. But there are some areas, called “Neanderthal deserts”, where no Neanderthal DNA is present. Notably, the largest Neanderthal deserter has a complete X chromosome, raising the question of whether this reflects evolutionary fitness or mating preferences of the genes there.
Now, three researchers from the University of Pennsylvania, Alexander Platt, Daniel N. Harris and Sarah Tishkoff have done the opposite analysis: examining the X chromosomes of the handful of genomes we have. It turns out that there is also a strong bias towards modern human sequences, and the authors interpret this as selective mating, with Neanderthal males showing a strong preference for modern human females and their descendants.
What kind of selection are we looking at?
Given how long modern humans and Neanderthals have been evolving as separate populations, some degree of genetic incompatibility is certainly possible. Many proteins interact in different ways, and the genes behind these interaction networks will evolve together – changes in one gene will often lead to compensatory changes in other genes in the network. Over time, those changes could mean that reintroducing the original genes would actually disrupt the network, which would have a negative effect on fitness.
This means that the introduction of some Neanderthal genes into the modern human genome (or vice versa) would be disruptive and their carriers would be less fit. So they will be selected against and lost to future generations. Of course, some sections will be lost at random – the genome is quite large, and the modern human population was probably large and growing, allowing its DNA to minimize the influence of other human populations. Figuring out which effect is stronger can be challenging.
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