Lice

Gorillas have one kind of lice, and chimpanzees have another kind, and humans have descendants of both kinds. Nevertheless, we had to have picked up the gorilla lice at a pretty late date, maybe from sleeping in gorilla nests:

"[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."