Our results indicate that the two El Sidrón Neandertal individuals were most likely homozygous for the O01 allele. Nevertheless given a low rate of potential modern human contaminants in an unknown allelic state, we cannot discard the possibility that both Neandertals could have been heterozygous (e.g. OA or OB). The results however suggest the presence of the human O01 allele already in the common ancestor of Neandertals and modern humans and thereby confirming an emergence of the O01 allele more than 1 Mya predating the divergence of the modern human and Neandertal populations. An alternative possibility would be that Neandertals have acquired this allele by gene flow from modern humans, although at present, other genetic evidences suggest that this scenario is unlikely [17,24]. Furthermore a potentially homozygous state in at least two Neandertals, even though from the same group, at least indicates that a potential selective advantage of the O allele could have also been present in the Neandertal population. Future analyses regarding the frequency of the ABO alleles will be necessary to reveal if all three genotypes were conserved in the Neandertal population.