While most parents worry about their schoolchildren contracting lice, U scientists are fascinated by precisely which lice they are contracting.

A recent study at the U found that humans have two genetically different species of head lice, which indicates that the ancestors of current humans (Homo sapiens) had contact with a species of ancient humans, possibly Homo erectus.

One type of louse is found worldwide and comes from current humans. The other type of louse occurs only in the Americas, which evolved on the early species of human.

Scientists don’t know exactly how the transfer took place.

“We don’t know the nature of the contact. It could have been fighting, interbreeding, cannibalism or sharing caves,” said study leader Dale Clayton, professor of biology.

By studying lice’s mitochondrial DNA, scientists were able to track when population bottlenecks and explosions occurred.

Populations of lice parasites boomed at the same time human host populations exploded, according to Alan Rogers, professor of anthropology and co-author.

The lice population grew between 150,000 and 50,000 years ago, which helps prove the theory that Homo sapiens left Africa at that time.

The research also shows when species might have branched off the human and primate family tree.

Researchers aren’t 100 percent sure of their findings, but they say it fits best with other data.

“We sort of stumbled into the rich sort of data set. We might not be correct, but it’s most consistent with other research of parasites,” Clayton said.

That uncertainty has raised many questions.

Rogers doesn’t know why head lice that can only survive on the head live in the Americas while lice that can switch between the body and head live all over the world.

One theory he proposed is that the New World lice can’t live on the body or clothing because of competition. When head lice from ancient humans jumped to modern humans, body lice had already adapted to living on humans’ bodies and in clothing, so head lice couldn’t survive there.

David Reed, the lead author of the study, thinks head lice were brought by Europeans centuries later, according to Rogers.

The questions raised will likely be answered by further research of pubic lice, according to Rogers, which will allow researchers to help determine what kind of contact the two species made.

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