Lice are intimately adapted to their hosts and cannot long survive away from the body’s blood and warmth. If their host evolves into two species, the lice will do likewise. So biologists have long been puzzled over the fact that the human head louse is a sister species to the chimpanzee louse, but the pubic louse is closely related to the gorilla louse.

BITE ME!

Some seven million years ago, this ancient ape species split into gorillas and the ancestors of humans and chimps, with both lineages infected by both species of lice. But Pediculus then fell extinct in its gorilla hosts, according to Dr. Reed’s reconstruction, and Phthirus vanished from the chimp-human ancestor. Next, chimps and humans diverged, and their joint louse diverged with them into Pediculus humanus and Pediculus schaeffi.

The last event in this history of human-louse cohabitation was the transfer of the gorilla’s Phthirus louse to people.

Dr. Stoneking said Dr. Reed’s reconstruction was “pretty reasonable” and said he agreed that acquisition of the gorilla’s louse indicated people had lost their body hair by then. “The transfer doesn’t have to be sexual,” he said, “but presumably it does require reasonably close contact.”

Just another Just-so story.