Being too lazy to make his own bed condemned mankind to millions of years of the embarrassment and discomfort of lice, research suggests.

Ancestors of modern Man would doubtless have thought they were on to a good thing when bedding down in an abandoned gorilla nest 3.3 million years ago.

But while early Man took a kip in the jungle, lice left behind by gorillas crawled on to the recumbent hominid and made themselves at home, suggests a study.

When they lived on gorillas the lice would infest fur, but on humans they evolved to live in the pubic area and would be transmitted during sex.

Genetic and fossil analysis of the pubic louse, Pthirus pubis, found on humans and the gorilla’s louse, Pthirus gorillae, shows that the lice became separate species about 3.3 million years ago.

This was long after Man and gorillas shared a common ancestor and scientists believe that the most likely explanation for the lice evolving into separate species is as a result of crossing from one host to another.

David Reed, of the Florida Museum of Natural History at the University of Florida, said it was unlikely that the louse spread from gorillas to Man’s ancestors through cross-species sexual transmission. Much more likely would have been that early man lived alongside the gorillas and took to sleeping in the same beds of vegetation or preyed on the primates.

He and his colleagues, reporting their findings in the journal BMC Biology, said: “Evidence suggests that the Phirus pubis has been associated with humans for several million years and likely arrived on humans via a host switch from gorillas.

“Despite the fact that human pubic lice are primarily transmitted via sexual contact, such contact is not required to explain the host switch.

“Parasites often switch from a given species to a predator of that species, and are sometimes found to switch to unrelated hosts in communally used areas, such as roosting or nesting sites.”

Dr Reed added: “It certainly wouldn’t have to be what many people are going to immediately assume it might have been, and that is sexual intercourse occurring between humans and gorillas.

“Instead of something sordid, it could easily have stemmed from an activity that was considerably more tame.”

Humans suffer from the blood-sucking habits of two families of lice whereas gorillas and chimpanzees each only have one, although they are closely related to the human types.

The second human species, divided into two subspecies, accounts for head lice (Pediculus humanus capitis) and body lice (Pediculus humanus humanus). They are believed to have evolved from a species that split into two when chimpanzees and humans became separated on the primate family tree six million years ago. Dr Reed said that studying the evolution of lice has the potential to provide clues to human development where the fossil record is thin.

It can also help in the understanding of how parasites move from one species to another, spreading diseases.

“If you look at emerging infectious diseases that affect humans all over the world, most have their origins on some other host before threatening humans,” he said.

“Studying what it takes for a parasite to be successful in switching hosts adds to our knowledge about what makes a good host for the spread of disease.”

Professor Dale Clayton, of the University of Utah, said: “Human pubic or ‘crab’ lice get transmitted between people on bath towels all the time. So it is easy to imagine that gorilla lice could have transmitted to humans via shared sleeping quarters, or predation.”

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