p-ter points me to a new paper which documents interspecies hybridization in monkeys whose lineages putatively diverged about 3 million years ago. Note that the hybridization follows Haldane's rule: the heterogametic sex (in mammals the males) exhibits sterility while the other sex does not. Whatever genetic incompatibilities built over the period during which the two populations became distinct the less robust sex (males have only one copy of the X chromosome, ergo, sex-linked diseases) naturally exhibits greater breakdown in hybrids. In any case, the story is obviously relevant to Neandertal introgression, since the divergence between moderns and Neandertals is on the order of 1 million to 100,000 years. Related:Mammalian hybridization potentialities.