From an article in Genetics:
Well-documented cases of natural hybridization among primates are not common. In New World primates, natural hybridization has been reported only for small-bodied species, but no genotypic data have ever been gathered that confirm these reports. Here we present genetic evidence of hybridization of two large-bodied species of neotropical primates that diverged ~3 MYA. We used species-diagnostic mitochondrial and microsatellite loci and the Y chromosome Sry gene to determine the hybrid status of 36 individuals collected from an area of sympatry in Tabasco, Mexico. Thirteen individuals were hybrids. We show that hybridization and subsequent backcrosses are directionally biased and that the only likely cross between parental species produces fertile hybrid females, but fails to produce viable or fertile males. This system can be used as a model to study gene interchange between primate species that have not achieved complete reproductive isolation.
The fact that the species (two species of howler monkey) diverged about 3 million years ago is pretty striking– the divergence time of humans and chimps is “only” one or two million years more than that, but creating a humanzee doesn’t seem to be possible. Humans and Neandertals, though, only split a few hundred thousand years ago.