Human head lice share a common ancestor with chimpanzee lice. The new data showed that the two emerged as separate species at the same time that humans and chimps parted evolutionary ways, about six million years ago.
Such co-speciation is typical of lice and other parasites, which can often evolve in tandem with their animal hosts.
Our pubic lice, by contrast, are most closely related to the gorilla louse. But gorilla and human lineages diverged seven million years ago, so co-speciation couldn't explain the origin of the pubic louse.
Instead, the parasite must have spread from one primate to the other long after they had evolved into separate species.
Even discounting the possibility of sexual transmission, the discovery gives anthropologists intriguing new information about the lifestyle and behavior of early human ancestors.
"These results may suggest that [our ancestors] lived or partially dwelt in forests and perhaps even slept in nests of foliage built by gorillas," said Mark Pagel, of the University of Reading in England.
Our Bodies, Our Habitats
Understanding where human lice came from still doesn't fully explain our unusual capacity to harbor two distinct varieties of the bloodsucking parasite.
For lice, each host species is, in effect, a unique "island" of habitat, study leader Reed noted. The parasites become adapted to local conditions such as hair size and blood type.
"[Lice] are simply stranded on their hosts with no means of escape," he explained. "They can't fly, they can't jump, and they can't live apart from the host for any period of time."
The loss of hair over most of our bodies may have created two distinct habitat islands in humans. The scalp and pubic regions differ significantly—and they are separated by largely inhospitable terrain.
"Pubic lice could not have established on humans without suitable habitat," Reed said. "Loss of body hair would have left the pubic region an open island of habitat that [the gorilla louse] could have colonized."
Dale Clayton, of the University of Utah in Salt Lake City, called it "a fascinating example of ecological opportunism."
"Different hair diameters [in the scalp and pubic regions] probably represent different habitat templates," he said, "just as different-size tree branches are used by different species of birds."
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