New research indicates that "crabs", or pubic lice, began to evolutionarily diverge after human-leading lineages and gorilla-leading lineages split. As there are very few ways to spread public lice, it suggests that there was some hominid-on-gorilloid action after speciation.
Eww, you might think. But this is to be expected in evolutionary terms. If it wasn't a case involving us or our near relatives, we'd say, "sigh... yet another case of divergent species occasionally doing the nasty". For instance, ducks, which have a deep evolutionary history, will often hybridise. Even if the two species didn't actually hybridise, as they are supposed to have done with chimp-leading lineage species, there may have been attempts at sex, and why would we expect otherwise?
Cross-species sex is a widespread phenomenon in vertebrate biology. We have been misled by our "intuitions", based on the one hand on an over-strict application of reproductive isolation concepts of sex, and on the other of projection of moral standards.
Alan Templeton's classic paper on species concepts has a section entitled "Too much sex". It's worth reading just for that.
Templeton, Alan R. 1989. The meaning of species and speciation: A genetic perspective. In Speciation and its consequences, edited by D. Otte and J. Endler. Sunderland, MA: Sinauer.
Late note: John Hawks notes that a better explanation is that gorillas were eaten by hominids, as they are today, which would allow transfer easily. But I still like the horizontal hominoid hypothesis.
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This will be confirmed or refuted over time - we should see examples of sexual diseases and other external parasites crossing evolutionary lines in other mammals/vertebrates. I'm not sure how conclusive we can be about this single example, although it is fascinating.
For some reason I am reminded of a statement regarding species hybridization by Steve Martin's Dr Hfuhruhurr in 'The man with two brains'.
Fascinating stuff! Carl Zimmer reports the authors suggesting that early hominids might occasionally have slept in gorilla's nests (the nests being the supposes vector of the lice).. reminded me of the toilet seat hypothesis of transmitting VD ;-)