Lice aren’t nice, at least not when they’re attached to people. In a bid for scientific respectability, however, these pestering parasites may have yielded provocative new genetic insights into human evolution.
Head lice found on people today consist of two lineages that diverged about 1.18 million years ago, say biologist David L. Reed of the Florida Museum of Natural History in Gainesville and his colleagues. Analyses of mitochondrial DNA, inherited solely from the mother, indicate that the group that includes body lice evolved with the ancient Homo species that became modern Homo sapiens, the researchers contend. This genetic category of lice now inhabits all regions of the world.
The other evolutionary lineage, consisting of only head lice, latched on to a now-extinct species, probably Homo erectus, Reed’s team proposes. Physical contact occurred between H. erectus and H. sapiens, probably in eastern Asia between 50,000 and 25,000 years ago, giving these head lice access to our species, the researchers theorize.
This lineage of lice now lives only in the Americas, apparently after having been transported by Asian migrants during the late Stone Age, the researchers suggest in the November Public Library of Science (PloS) Biology.
Anthropologist Alan R. Rogers of the University of Utah in Salt Lake City, a study coauthor, says the findings fit best with a theory that modern humans evolved by spreading out of Africa in a series of migrations between 95,000 and 20,000 years ago and interbreeding occasionally with other Homo species.
For their study, Reed and his colleagues extracted mitochondrial DNA from 114 head and body lice found on people around the world.
Computerized analyses of the DNA yielded an estimated date for the evolutionary split of the lice lineages. The data also indicated that the louse lineage with the global distribution experienced a population decline around 100,000 years ago. That roughly coincided with a population decline of modern H. sapiens, according to prior mitochondrial-DNA studies.
The two lice lineages diverged at about the time that a genetic split occurred between H. erectus and ancestors of modern H. sapiens, upping the odds that the worldwide lice group evolved in concert with people, Rogers argues.
John H. Relethford of the State University of New York at Oneonta notes that the data can’t rule out the possibility that the early Homo groups could have had enough physical contact for the transfer of lice, yet didn’t interbreed.
Milford Wolpoff of the University of Michigan at Ann Arbor has another concern. Those who use mitochondrial DNA to reconstruct animals’ evolutionary histories assume that its chemical sequence changes only at random, but mounting evidence indicates that natural selection molds the makeup of mitochondrial DNA, making analyses such as Reed’s useless, he argues.
What’s more, a prior, independent analysis of head lice mitochondrial DNA didn’t yield the same two geographical lineages reported by Reed’s group, raising questions about whether the new data can help answer questions about human evolution, says Mark Stoneking of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.