Head lice may shed light on early man

A study of head lice has led researchers and scientists from the University of Utah and elsewhere to find that modern humans fought with, ate with, shared clothing and mated with a more primitive human species between 25,000 and 30,000 years ago.

Genetic differences in two species of head lice found on people are the basis for the conclusion, which the study's authors admit is not chiseled on stone plates.

The study is being published online by the Public Library of Science journal called "PLoS Biology."

Lead researcher was David Reed, former U. postdoctoral fellow who works at the Florida Museum of Natural History, University of Florida, Gainesville. Co-authors include Dale Clayton, professor of biology at the U.; Alan R. Rogers, U. professor of anthropology; Vincent Smith, University of Glasgow, Scotland; and Shaless Hammond, who as a high school student worked in Clayton's laboratory.

Two different species of head lice parasitize today's humans, according to the report. Analyzing the rate at which genes change, they calculate that the two species diverged about 1.18 million years ago.

That's close to the time paleontologists postulate that archaic humans left Africa. Modern humans evolved in Africa around 200,000 years ago, the researchers say.

The lice diverged when the groups separated.

Although the earlier humans died out, the lice they carried continue to thrive on human heads, having infected modern people that the earlier species came into contact with.

The lice from the earlier humans only live on people of the New World, who also have the same lice that infect other modern humans.

This has led the scientists to speculate that ancestors of today's Indians came into contact with the earlier type of humans. Head lice jumped from the earlier to modern people 25,000 to 30,000 years ago, Rogers believes.

That date is not based on genetic studies but on when scientists think modern humans reached east Asia and ran into pockets of the earlier types, he said.

Asked how he knows one species of head lice originally was carried by a now-extinct form of human, he replied, "The short answer is that we don't." The argument for believing that may have happened "is more involved."

Lice are found on chimps, humans and other primates. Genetics can give some idea of when the common ancestors of these lice split from each other, he said. Knowing how fast the lice evolve, that gives the figure that they split apart roughly 1 million years ago.