Among primates, humans have the dubious distinction of having not one, but two types of sucking lice--one that nests on the head and the other that calls pubic hair home. Where did these buggers come from? A new study suggests that while we've been sharing head lice with chimps since before we were even separate species, pubic lice came from gorillas--and not very long ago.

Each year, 3 million people in the United States catch a case of pubic lice, otherwise known as "crabs." It's an important public health problem, but evolutionary biologist David Reed of the Florida Museum of Natural History in Gainesville prefers to look at it as an evolutionary conundrum. Typically, lice share the same evolutionary history as their hosts. For example, if a population of hosts splits into two and each of the isolated populations begins to evolve into separate species, then the parasites evolve too. Human head lice, for example, share a common ancestor with a chimp-based louse, and the two lice are closely related--just as chimps and humans are. But the closest relative to human pubic lice is not found in chimps, but rather gorillas, which are more distant kin from humans. Reed wanted to know why.

He and his colleagues compared the DNA sequences from two genes in various lice and calculated when the different species appeared. As expected, human and chimp Pediculus (head lice) began to diverge about the same time as hominids and chimps started down independent evolutionary paths, about 6 million years ago. The gorilla and pubic lice (both Pthirus species) split just 3.5 million years ago, however, about 4 million years after gorillas and the line that led to chimps and hominids diverged. Thus rather than co-evolving with our ancestors from the very beginning, these lice were acquired at a later point in time, Reed and his colleagues report today in BMC Biology.

Because lice rarely survive more than a few hours off their hosts, this jump required close contact between the two species. "They must have been in the same place at the same time," Reed points out. Vincent Smith, a cybertaxonomist at the Natural History Museum in London, says the study "raises some interesting questions about what gorillas and our hominid ancestors were doing in such close proximity to share each other's lice.