Scott Norris
for National Geographic News
What exactly went on between gorillas and early humans? No one knows for sure, but scientists say one thing, at least, seems certain: The big apes gave us pubic lice.
Researchers made the uncomfortable discovery during a DNA study reconstructing the evolutionary history of lice in humans and our primate relatives.
The transfer occurred about 3.3 million years ago, said study leader David Reed, of the University of Florida in Gainesville. That's when the gorilla louse and the human pubic louse separated into distinct species, the research revealed.
Modern humans (Homo sapiens) weren't around at the time. So the first to be infested by the new lice species were probably Australopithecus, a group of human ancestors that include the famous "Lucy" fossil.
Prior to the transfer our ancestors were troubled by only one species of body louse, as chimpanzees and gorillas are today. Why humans can harbor two species—head lice and pubic lice—has been a mystery until now.
The discovery raises the same vexing question faced by anyone who has contracted pubic lice: How exactly did this happen?
Pubic lice are spread most commonly through sexual contact, but that's not necessarily how our ancestors acquired the parasite from gorillas.
"Unfortunately, we'll never know for sure," Reed said. "Given that the [gorilla louse] species occurs primarily in the pubic region, it is quite possible that the lice were transmitted sexually."
A more likely scenario, though, is that early humans picked up the parasites simply by living in close proximity to gorillas, perhaps using the animals' sleeping sites or scavenging gorilla remains, he said.
The study appears in a recent edition of the journal BMC Biology.
Good Night, Gorilla
Reed's team studied changes in primate and lice genes to determine when different louse species originated.
Human head lice share a common ancestor with chimpanzee lice. The new data showed that the two emerged as separate species at the same time that humans and chimps parted evolutionary ways, about six million years ago.
Such co-speciation is typical of lice and other parasites, which can often evolve in tandem with their animal hosts.
Our pubic lice, by contrast, are most closely related to the gorilla louse. But gorilla and human lineages diverged seven million years ago, so co-speciation couldn't explain the origin of the pubic louse.
Instead, the parasite must have spread from one primate to the other long after they had evolved into separate species.
Even discounting the possibility of sexual transmission, the discovery gives anthropologists intriguing new information about the lifestyle and behavior of early human ancestors.
"These results may suggest that [our ancestors] lived or partially dwelt in forests and perhaps even slept in nests of foliage built by gorillas," said Mark Pagel, of the University of Reading in England.
Our Bodies, Our Habitats
Understanding where human lice came from still doesn't fully explain our unusual capacity to harbor two distinct varieties of the bloodsucking parasite.
For lice, each host species is, in effect, a unique "island" of habitat, study leader Reed noted. The parasites become adapted to local conditions such as hair size and blood type.
"[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."
