Around 3.3 million years ago, gorillas gave our early hominid ancestors pubic lice, and humans have had them ever since, according to new research.

The pesky parasites, which on humans live in pubic hair and other hairy areas except the scalp, suck on blood and can only survive for a short time without a host.

"We know that humans have had close interactions with gorillas in recent times; unfortunately there is still a bushmeat trade to attest to this," says lead author Dr David Reed, assistant curator of mammals at the Florida Museum of Natural History.

"However, we now can conclude that archaic humans and gorillas may have lived in close proximity 3.3 million years ago, which was not known previously."

Reed and his colleagues speculate the contact could have occurred in at least three different ways: sexual contact between gorillas and early hominids, ancient humans consuming or handling gorillas, or by sharing sleeping areas.

"We cannot rule out the first, or any of the other possibilities," Reed says.

He and his team studied lice collected from primates in Ugandan wildlife sanctuaries. Gorilla lice today represent a different species than modern human lice, as parasites evolve along with their hosts.

Reed and his colleagues extracted DNA from the lice. They then used fossil data from humans and gorillas to estimate how long ago the two parasite species shared a common ancestor.

Their findings are published in the current issue of the journal BMC Biology.

"The time at which these two louse species shared a common ancestor, 3.3 million years ago, is much younger than the time that gorillas and humans last shared a common ancestor, 7-9 million years ago," Reed says. "That means a host switch occurred 3.3 million years ago."

The study also suggests human head lice originated from contact with chimpanzees. Today gorillas suffer from pubic lice, but not head lice, while chimps only have the latter.

Lance Durden, associate professor of biology at Georgia Southern University, says the findings make sense.

"It has long puzzled parasitologists as to why humans are unique ... in having two [kinds] of sucking lice," says Durden, referring to the head louse and the body louse.

"Through some elaborate molecular phylogenetic detective work, David Reed and his research team have come up with a compelling and parsimonious answer," he says.

"[It is] entirely feasible that humans acquired lice from gorillas by means other than sexual contact, such as nest sharing - not at the same time, hopefully - or handling gorillas while hunting or feeding on them."

Dale Clayton, a biology professor from the University of Utah, says the possibilities are a real, and almost literal, head-scratcher.