Mr. James Winters at A Replicated Typo pointed me to a short hypothesis paper, Neanderthal-human Hybrids. This paper argues that selective mating of Neandertal males with females of human populations which had left Africa more recently, combined with Haldane’s rule, explains three facts:
– The lack of Neandertal Y chromosomal lineages in modern humans.
– The lack of Neandertal mtDNA lineages in modern humans.
– The probable existence of Neandertal autosomal ancestry in modern humans.
If you don’t know, Haldane’s rule basically suggests that there’s going to be some sort of breakdown in the heterogametic sex. In humans females are homogametic, XX, and males are heterogametic, XY. The breakdown need not be death (or spontaneous abortion). It could be sterility (e.g., some mutation or genetic incompatibility which results in the malfunctioning of the flagella of sperm would do it).
So you have a scenario where only Neandertal males are interbreeding with the intrusive groups from the south. The hybrids from these pairings would then lack Neandertal mtDNA, since mtDNA is passed only from mothers. But the male offspring would have Neandertal Y chromosomes. This is where Haldane’s rule kicks in: these males in their turn would not reproduce. Therefore only the female hybrids …