New research suggests humans got pubic lice by close contact with gorillas.
DNA studies reveal that pubic lice on humans, and their relatives on gorillas, split from a common ancestor between 3 and 4 million years ago. This is several million years after their human and gorilla hosts are known to have diverged.
By reconstructing the evolutionary history of these parasites, scientists have been able to show that the ancestors of gorilla lice switched hosts to start parasitizing our ancient human relatives.
'This research raises some interesting questions about what our hominid ancestors were doing in such close proximity to share each others lice,' says Vincent Smith, cybertaxonomist at the Natural History Museum.
Lice only survive for a few days away from their host and the most likely scenarios are that our human ancestors lived alongside gorillas and used their abandoned bedding.
There are two families of old world primate lice, and like their hosts, these louse families are closely related. Gorillas are parasitized by Pthirus , and chimpanzees by Pediculus . Interestingly orangutans do not have any lice.
'We humans are unique amongst the great apes,' says Vincent, 'in that we are parasitized by both louse lineages.'
We share the human head and body louse Pediculus with our closest living relative the chimpanzee. Both the lice and their hosts split from each other about 6 million years ago.
Human head louse,
Because gorillas are the more distant relatives of chimpanzees and humans, we would expect their lice to be at least 6 million years old. The fact that they are not suggests Pthirus has not been acquired by the same pattern of shared descent.
'The simplest explanation for this pattern is that our human ancestors acquired pubic lice from gorillas, and not the other way around,' says Vincent.
Pthirus gorilla lives on the fur of gorillas but its ancestor became specialized to live in the pubic areas of humans, evolving into what we recognize today as the human pubic louse Pthirus pubis .
Vincent concludes, 'We finally have molecular data for the gorilla louse Pthirus gorillae (and getting hold of gorilla louse specimens isn't easy) and this allows us to date the split between Pthirus pubis of humans and P.gorillae of gorillas.'
Studying louse evolution gives us clues to how humans evolved. It also helps us understand the development of diseases and how parasites spread from one host species to another.
The research was carried out by David Reed of the Florida Museum of Natural History and colleagues and is published in the journal BMC Biology.