The evolutionary tree shows how pubic lice – Pthirus pubis and Pthirus gorillae – afflict humans and gorillas. Humans and chimps, meanwhile, can host Pediculus head lice (Illustration: J W Demastes/T Choe/V S Smith)

A genetic analysis of pubic lice suggests the parasites were transferred between early humans and gorillas about 3.3 million years ago.

Researchers say the findings suggest close contact between our ancestors and gorillas. But they claim it is far more likely that early humans caught the lice from sleeping in abandoned gorilla nests than from having sex with gorillas.

Pubic lice – also known as crabs – can leave irritating spots on the skin when they feed on the blood of their hosts. The parasites lack wings and cannot jump like fleas, so they remain stuck on a host until they come into close contact with another individual. Each species of pubic lice thrives on only one host species.

In the first part of the new study, scientists collected Pthirus gorillae lice from primates in Ugandan wildlife sanctuaries. David Reed of the University of Florida in Gainesville, Florida, US and collaborators then analysed the DNA of these parasites and compared the gene mutations with those seen in the Pthirus pubis lice, which can live in the pubic hairs of modern-day humans.


Close encounters

Humans and gorillas are thought to have last shared a common ancestor 7 to 9 million years ago. Reed says that because gene mutations accumulate over time in both lice species, he expected substantial differences between them. But the DNA analysis revealed the genes of Pthirus gorillae and Pthirus pubis were surprisingly similar.

The small number of genetic differences suggest the lice transferred between gorilla and early humans as recently as 3.3 million years ago. According to Reed, this suggests that early humans and gorillas made close contact, though he “seriously doubts” that pubic lice transferred between the two during sex.

Instead, he suspects that early humans might have picked up pubic lice from scavenging on gorilla corpses or sleeping in the abandoned sleeping nests of gorillas. Pubic lice can live for up to 24 hours once removed from their host. But he acknowledges: “I don’t know if we’ll ever be able to know for certain which hypothesis is correct.”

Journal reference: BMC Biology (DOI: 10.1186/1741-7007-5-7)