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.