Figure 1. Neanderthal woman (left) and modern human (right)
It is not widely known, but modern humans carry up to 4% of Neanderthal DNA. This occurred because of interbreeding between the two species about 50,000 years ago when Homo sapiens were expanding to Eurasia. Of this 4% of DNA, there is none coming from the Neanderthal Y chromosome. Some scientists believe this merely due to chance that it was lost in the human gene pool, but new research presents a more likely option. Several of the Y chromosome genes of Neanderthals are a part of the immune system, so a woman's immune system would attack these genes because they see it as a threat because of the incompatibility. This would have also led to miscarriages between the two species. So far this is only a hypothesis, but the mother's immune system has been known to perform similar acts.
Before reading this article, I had no knowledge that a part of our DNA was that of the Neanderthal's. Though it is insignificant it is still fascinating to see that it is even present. It makes one wonder if there is other Homo DNA in our genomes. I also believe that the 4% will surely disappear completely within a few hundred years. If their hypothesis is correct, then mother immune systems will continue to eradicate any remaining Neanderthal Y chromosomes due to its incompatibilities.