A study of Neanderthal and Denisovan blood group sequences suggests that Neanderthals began to show evidence of low reproductive success at the point they began to share specific genetic information with Homo sapiens. This suggests interbreeding resulted in Neanderthals birthing children with disorders, specifically hemolytic disease.
Analyses of blood group systems of Neanderthals and Denisovans contributed to a better understanding of their origin, expansion and encounters with Homo sapiens. Blood group profiles revealed polymorphism at the ABO locus, ancestral and African-origin alleles, and a RH haplotype presently secluded in Oceania, plausible relic of introgression events into modern humans prior their expansion towards Southeast Asia. An additional contribution is the reduced variability of many alleles and the possible presence of haemolytic disease of the foetus and new-born, which reinforces the notion of high inbreeding, weak demography and endangered reproductive success of the late Neanderthals, giving to our species the great opportunity to spread throughout the world.