It's a head-scratching development

Lice: Researchers have unexpectedly found that one variety of the head-dwelling parasite spread to modern humans by direct contact with one of our ancestors.

Medicine & Science

October 25, 2004|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

After riding silently on our scalps for thousands of years, human head lice have finally begun to speak.

Genetic evidence teased from the DNA of Pediculus humanus suggests that modern man caught one variety of the itchy pest during an encounter with one of our archaic cousins - possibly Homo erectus, who died out in Asia a short time later, at least 25,000 years ago.

Scientists say the findings are the first evidence of direct contact between our own Homo sapiens ancestors, and their smaller-brained relatives, according to a study published this month in the online journal PLoS Biology.

Whether it was wrestling, trading hats or something more intimate that brought the two close enough to catch each other's lice is open to speculation. But David L. Reed, leader of the study and curator of mammal sciences at the Florida Museum of Natural History on the campus of the University of Florida in Gainesville, took some delight in imagining the meeting.

"To see a human you haven't shared a common ancestor with for over a million years would be really quite amazing," Reed said.

"You would clearly know it was another species," he said.

Homo erectus had bigger brow ridges than our ancestors, and a receding chin. He lacked our high, domed forehead and big brain, and his branch of the family was generally stockier, heavier and more robust than we are. And the species probably had full body hair.

Insight from parasites

Uncertainties abound, but the study of parasite genetics is producing new insights into the evolutionary history of their host species, including humans, said Carter Kimsey, program director for postdoctoral fellowships in biology at the National Science Foundation.

"It's an evolving field, and it's becoming important," she said. "The conclusions here are unique, and this is the first example of these particular species coming into contact."

Lice are parasitic insects that live only on birds and mammals. Each kind lives its entire life cycle on its chosen host species, breeding, laying eggs and feeding on the host's blood.

Reed's study was limited to head lice. But humans also are host to body lice, which adapted to live in our clothes, as well as pubic, or "crab" lice, a separate species that adapted to the body's nether regions.

Louse bites cause itching, which invites scratching and risk of infection. Body lice also can transmit deadly typhus.

Because head lice can't survive more than a few hours or days on their own, moving from one host to another requires close contact, Reed said.

Children playing or sharing clothes is usually opportunity enough. That's why head lice - to the horror of parents everywhere - spread so readily among children and in schools.

"When we see it among adults, it's usually when they've been infected by their children," Reed said.

Reed and his team gathered 127 head lice from people in many parts of the world.

The bugs' mitochondrial DNA - inherited from their mothers - confirmed an earlier discovery that there are two distinct varieties of head louse. One is found worldwide, while the other seems confined to the Americas. You can't tell them apart by looking - the difference is only visible in their DNA.

An age-old puzzle

Mitochondrial DNA accumulates mutations at a known rate, so investigators were able to use the differences they found to estimate how long ago the two varieties split from a common ancestor.

That ancestor, to their surprise, lived about 1.2 million years ago.

"Which is way older than we are as a host," Reed said.

About 10 times older.

That was a puzzle. Hosts and parasites should mirror each other's evolution. For example, genetic evidence shows that the ancestors of humans and chimpanzees diverged about 5.5 million years ago.

Not surprisingly, it turned out that chimp lice and human lice diverged about the same time.

But the human head lice data indicated that the New World lice split from the other variety a million years before the ancestor of all living humans appeared. So, on whose head did they spend that million years?

Looking at the human family tree, Reed's team realized it was also about 1.2 million years ago that Homo erectus split off from the lineage that eventually led to us - Homo sapiens.

Hence the New World lice probably departed from our lineage on Homo erectus' archaic head.

Our own lineage, meanwhile, kept evolving with our own head lice, now found worldwide.

But since people living today host both types, Reed's team realized that somehow, somewhere, modern humans must have gotten close enough to their archaic cousins to catch their head lice.

And it must have happened fairly recently.

"It could not have tagged along with us for 1.2 million years," Reed said, "Interbreeding, genetic drift and selective pressure would have extinguished one of the two [louse varieties] in a million years."

But where, when and how?

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