A new paper in The American Naturalist should interest some in these parts, Placental Invasiveness Mediates the Evolution of Hybrid Inviability in Mammals:
...Here, we show that the maximum genetic distance at which interspecific mammalian pregnancies yield viable neonates is significantly greater in clades with invasive (hemochorial) placentation than in clades with noninvasive (epitheliochorial or endotheliochorial) placentation. Moreover, sister species with invasive placentation exhibit higher allopatry in their geographic ranges, suggesting that formerly separated populations in mammals with this placental type fuse more readily on recontact.
Here is a high resolution phylogram of mammalian lineages showing those with invasive placentation as solid. Hominidae are invasive.
