When Neanderthals mated with modern humans, it may have been a significant factor in their eventual extinction. New research has suggested that when these two species interbred, some of the babies were born with a potentially fatal blood disorder.
To be more specific, scientists analyzed blood from three Neanderthals and they discovered that they had a certain set of genetic variants that made them susceptible to the blood disorder called “hemolytic disease of the fetus and newborn” (or HDFN). This disorder happens when the mother and baby’s blood types are incompatible. For example, if a woman is not Rh-positive but her partner is, the fetus could have a different Rh factor than the mother and that could potentially cause the woman’s immune system to attack the child’s red blood cells and possibly cause a deadly form of anemia.
Because of their limited gene pool, Neanderthals were very susceptible to the disorder and HDFN would have probably been worse during and after their second pregnancies which could have affected their “reproductive success” and not allowing Neanderthals to have as many children. This could have been a significant factor in their eventual demise.
And it was much more common than one might think as explained by Stephane Mazieres from Aix-Marseille University, “The fact that these forms of genes were detected in individuals separated by 4,000km and 50,000 years suggest that this genetic peculiarity — and the risk of [an] anemic fetus — would have been quite common amongst Neanderthals.”
While there is the possibility that the blood disorder could have developed from Neanderthals mating with their own kind, experts are more convinced that it occurred when they had relations with ancient humans as well as Denisovans.
While the disorder was obviously very bad and possibly affected how many descendants they had, it probably wasn’t the sole factor in their extinction. In fact, there are many theories regarding their demise approximately 40,000 years ago, such as being exposed to tropical diseases that early humans may have brought over from Africa; or that they died from competing (and losing) against Homo sapiens for habitat and food.
Whatever the reason was for their demise, if the blood disorder caused a drastic decline in their population, it really is unfortunate news. Obviously the sex wasn’t worth it as they’re not around anymore.
The study was published in PLOS One where it can be read in full.