Tens of thousands of years ago modern humans got frisky with our cousins, the Neanderthals. We inherited several genes from this rendezvous, some of which proved beneficial and helped our ancestors adapt to the new environments they encountered as they spread through Europe. However, not everything we got from Homo neanderthalensis proved beneficial. The genetic gulf between our two species meant that any hybrids would probably have a few genetic problems. Like how when tigers and lions mate the resulting ligers are typically sterile.
Vernot and Akey – the two researchers behind last weeks discovery that Neanderthal genes helped humans adapt – noted that the genes we inherited from Neanderthals are pretty evenly spread throughout the genome. However, there are some parts where there are no Neanderthal genes. This strongly suggests that the Neanderthal genes present in those regions were harmful, so have been removed via natural selection. Last Wednesday a report was published in Nature, delving further into the negative side-effects of breeding with Neanderthals. The researchers honed in on the “deserts” of Neanderthal genes, hoping to infer what the harmful genes which were removed might have done.
They found 4 regions in the European genome, and 14 in the East Asian genome, that were unusually devoid of Neanderthal genes. This could be explained by the fact that there were likely a small number of hybrids, so many genes were just lost by chance. However, many of these absent regions code for important things, which raises the possibility that there were genes there, but they were removed because they were harmful.
The largest of these useful deserts was located on the X chromosome, where many of the genes relating to male fertility are found. Given the fact that many modern hybrids are sterile, it’s obvious that genes relating to fertility are especially susceptible to being mucked up by hybridisation. So maybe the first Neanderthal/human hybrids had some fertility issues, which led to the Neanderthal genes relating to fertility being stripped from the genome by natural selection. They also found such deserts in regions of the genome which are conserved in modern humans. These are regions which have not changed much over the course of evolution, likely because they are so finely adapted that any change would be detrimental. In these regions apparently mixing with Neanderthals was also enough to tip them over the edge.
However, not all of the harmful genes we inherited from Neanderthals have been removed. They cross-reference the Neanderthal genes still in our genome with those associated with several diseases, including “lupus, biliary cirrhosis, Crohn’s disease…smoking behaviour…and type 2 diabetes“.
Turns out House was wrong, this time it was lupus.
Sankararaman, S., Mallick, S., Dannemann, M., Prüfer, K., Kelso, J., Pääbo, S., … & Reich, D. (2014). The genomic landscape of Neanderthal ancestry in present-day humans. Nature.