Just as humans and gorillas share a common evolutionary ancestry, the pubic lice that infuriate some members of the two species are also related. Pubic lice–known to scientists as Pthirus pubis and to most other people as “crabs”–are thought to have evolved from Pthirus gorillae, the structurally similar species that infests gorillas. Genetic analysis by David Reed at the University of Florida indicates that the lice lineages split about 3.3 million years ago, whereas it is believed that humans diverged from gorillas at least 7 million years ago. This suggests that “early humans somehow caught pubic lice from their gorilla cousins.”

But apparently the lousy parasite didn’t make the jump because humans and gorillas tried to reunite their bloodlines; no, University College London biologist Robin Weiss suspects that humans picked up crabs by hunting gorillas. Because a predator can easily pick up parasites from its prey, the lice could have jumped to early humans while they butchered gorillas for bushmeat. Some researchers say that HIV made its more recent jump from chimpanzees to humans the same way.

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