Researchers itching to understand how modern humans evolved are turning to lice for answers. A genetic analysis of head lice suggests that two species of early humans had close physical contact after millennia of isolation, information that may help anthropologists narrow the possibilities for the origin of humans.

According to one theory, Homo sapiens migrated out of Africa and quickly replaced other human species, such as H. erectus in Asia, without interbreeding. The competing theory of "multiregional evolution" contends that modern humans appeared when Homo sapiens from various geographical regions mated with each other as well as with archaic Homo populations, blurring regional and species boundaries. A middle-ground proposal suggests that as modern humans from Africa spread across the globe, they interbred with other archaic humans, but that only African genes persisted. After analyzing the genes of lice, Dale Clayton, an evolutionary biologist at the University of Utah, Salt Lake City, says that the history of these pests best fits the third hypothesis.

Clayton and postdoc David Reed, now an evolutionary biologist at the University of Florida, Gainesville, compared mitochondrial DNA from lice, primarily Pediculus humanus, to published data on human evolution. The data reveal that two genetically distinct lineages of P. humanus appeared about 1.18 million years ago, Reed, Clayton, and their colleagues report online 5 October in the Public Library of Science, Biology.

Clayton argues that the two subgroups must have diverged when two human lines--perhaps Asian H. erectus and the African ancestors of H. sapiens--went their separate ways, which anthropologist believe happened at about the same time. The fact that the lice evolved into subgroups suggests that they had little or no contact with each other, which implies that their human hosts were also isolated--contrary to the multi-regional hypothesis.

But the data do suggest that there must have been some contact among different kinds of early humans. Today, there is only one species of human--but two subgroups of lice. So the lice thought to have been living on H. erectus must have jumped to H. sapiens at some point before H. erectuswent extinct, perhaps as late as 30,000 years ago, possibly through fighting or sex, and that conclusion weakens the African replacement idea.

Milford Wolpoff of the University of Michigan, Ann Arbor, author of the multi-regional hypothesis, doesn't agree with Reed and Clayton's interpretation, calling the new study a "fringe explanation." But paleontologist Chris Stringer of the Natural History Museum in London is quite pleased with the work as it provides "an indirect but informative new window on ... modern human origins."