One of the more embarrassing mysteries of human evolution is that people are host to no fewer than three kinds of louse while most species have just one.

Even bleaker for the human reputation, the pubic louse, which gets its dates and residence-swapping opportunities when its hosts are locked in intimate embrace, does not seem to be a true native of the human body. Its closest relative is the gorilla louse. (Don’t even think about it.)

Louse specialists now seem at last to have solved the question of how people came by their superabundance of fellow travelers. And in doing so they have shed light on the two major turning points in the history of fashion: when people lost their body hair, and when they first made clothing.

Three kinds of louse call Homo sapiens their home, but each occupies a different niche on the human body. The head louse, Pediculus humanus, lives in the forest of fine hairs on the scalp. Its cousin, the body louse, lives not on the skin but in clothes. And the exclusive territory of the pubic louse, Phthirus pubis, is the coarser hairs of the crotch.