Head lice hitched a ride on humans to the Americas at least twice

Lice have been bugging humans for as long as our species has been around, and the insects’ genes record the story of their hosts’ global voyages, a study finds. 

Lice DNA suggests that the scalp stowaways rode humans to the New World at least twice — once from Asia many millennia ago, and again much more recently via European colonists, researchers report November 8 in PLOS ONE.

Head lice (Pediculus humanus capitis) can’t jump or fly. They can disperse only by crawling, leaving them closely tied to the movements of their human hosts, says Marina Ascunce, an evolutionary geneticist at the U.S. Department of Agriculture’s Agricultural Research Service in Gainesville, Fla. 

Ascunce and her colleagues analyzed DNA extracted from 274 head lice found on people from 25 locations around the world, comparing differences between 15 small sections of the louse genetic instruction book. The lice clustered genetically into two groups — one found in Europe and North America, and the other in Asia and Central America, the team found.

Central America’s link to Asian populations, the researchers suggest, is the result of humans crossing a land bridge into the Americas many thousands of years ago (SN: 7/22/20). The other lice group prevalent in the Americas reflects the more recent colonization of the region by Europeans.

By acting as storytellers, lice may not be “all bad,” Ascunce says. “They can help us to see our history also.”

The strong genetic split between European and Asian lice was a surprise to Ascunce and her colleagues. Studying slower-evolving lice genes than what the team studied this time in future research might help researchers solve how that genetic rift formed and possibly yield insights into older events in human prehistory and the evolutionary changes spurred by them, she says. 

In general, studying lice evolution could provide a “fast-forward” perspective on the evolution of humans and other hosts because the bloodsucking parasites have faster generation times and mutation rates, says evolutionary biologist Andrew Sweet who was not involved in the new research.  

“You might be able to see [evolutionary] patterns that emerge from the parasites before you would see them in the hosts,” says Sweet, of Arkansas State University in Jonesboro.

Using the team’s approach with other parasites, Ascunce and her colleagues say, could also tell researchers more about the evolutionary history of other host species that are currently poorly understood.