The Pthirus and Pediculus lineage diverged via co-speciation with their hosts at the gorilla — chimpanzee/human split 13 million years ago. Pthirus then transfered to the human/chimpanzee ancestor, a host switch, and both lice were present at the human — chimpanzee split. In chimpanzees, Pthirus eventually died out, leaving only Pediculus, whereas in the human lineage both lice species survived, and with the loss of body hair within Australopithecus around 3–4Ma, the two lice genera separated to colonise the pubic region and hair region.

This model has two evolutionary events, a host switch and one extinction, making it more parsimonious than the ‘pair of lost lice’ hypothesis (Reed et al, 2007).

References

Pair of lice lost or parasites regained: the evolutionary history of anthropoid primate lice