A few days ago I was asked by Elizabeth Pennisi of Science to comment on a new paper by David Reed and collaegues at the Florida Museum of Natural History. The paper looks at the origins of human pubic lice (Pthirus pubis) and builds on the study we published in PLoS Biology in 2004, looking at the origins of another group of human lice. Here is what I said to Elizabeth...

It has long been known that there are anomalous distributions of lice on primates. Orangutans for example have no lice, whilst chimps and gorillas seem to be missing a louse lineage (Pthirus and Pediculus respectively). We humans are unique amongst the great apes in that we are parasitized by both louse lineages. David's recent paper helps to answer why, suggesting that we acquired our Pthirus louse lineage from gorillas. Much of this was clarified in the Reed et al 2004 paper, but what is new is that we finally have molecular data for the gorilla louse Pthirus gorillae, and this allows us to date the split between Pthirus pubis of humans and P. gorillae of gorillas. The sister group relationship between these taxa has never been in doubt; after all they are the only two taxa in a genus that is morphologically very distinct. However, we would have expected them to share a common ancestor that predated the split between the humans and chimpanzees. They fact that they don't suggests that either gorillas or humans acquired this louse lineage from the other host, and the most parsimonious explanation is that the hominid lineage acquired Pthirus from gorillas. Of course there are other scenarios that are slightly less parsimonious, and I have some concerns about the specifics on the dating. Notably that if you look at their table 1, the upper bound of the 95% confidence intervals for the Pthirus split range between 5.61-7.49 million years for the different gene combinations, which exceed the age of the two Pediculus species. I would have liked to see these dates cross-validated by the authors performing the same analysis using the chimp/human louse split. Arguably this calibration would have made more sense than using the more distant 20-25 million year old calibration between old world monkey lice (Pedicinus) and the great ape louse lineage. However, overall this is a minor concern in what is an interesting paper.

The full reference to the paper is:

• Reed, D.L., Light, J.E., Allen, J.M., and Kirchman, J.J. 2007. Pair of lice lost or parasites regained: the evolutionary history of anthropoid primate lice. BMC Biology 5:7 doi:10.1186/1741-7007-5-7.

The image below is figure 4 from David Reed et al's paper. This is a cophylogenetic reconstruction showing the evolutionary history of primate lice (in black) and host lineages (in grey). The important point to note is that the split between human and gorillae lice (3-4 mya) is younger than the split between their hosts. To explain the current distribution of these lice, gorillas and our human ancestors must have come into contact with each other 3-4 million years ago.