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In the human story, the head louse has been a constant bystander.KONRAD WOTHE/NPL/MINDEN
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When it comes to investigating the human story, scientists tend to focus on clues in our ancestors’ bones and artifacts. The tiny, bloodsucking parasites that infest our scalps? Not so much. But a new study published today in PLOS ONE suggests the genetics of head lice can shed light on when and where groups of humans split and came together in the past.
The authors present data suggesting European and American lice share a genetic affinity dating back to the European colonization of the Americas. Lice may even offer clues to ancient relationships not captured by human DNA or archaeological evidence, says Mikkel Winther Pedersen, a molecular paleoecologist at the University of Copenhagen who wasn’t involved in the study. “This could be a potential new angle to look at human migration and interactions.”
Head lice (Pediculus humanus capitis) cling to hairs and feast on blood from the scalp. They are an old foe; people around the world have complained about lice for thousands of years. Because head lice can only spread between people—and not from, say, human to rat or rat to human—they’re a good proxy for tracking human migrations, says the new study’s first author, Marina Ascunce, a molecular biologist at the U.S. Department of Agriculture.
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In the past, scientists have shown that the global distribution of lice strains mirrors past and contemporary population movements, and they have done similar studies with other parasites such as bedbugs and pathogens such as tuberculosis and the bacterium responsible for the Black Death. As a postdoc in the lab of University of Florida geneticist David Reed investigating the genetic relationships between mammals and their parasites, Ascunce wondered whether lice could offer even more detail about our history. Working with colleagues in Argentina, Mexico, and other countries, she sought to puzzle out whether genetic markers in lice could trace historical contacts between their human hosts. The idea is that when groups of people live in close proximity, they will share their parasites.
Ascunce and her collaborators gathered 274 lice from around the world, including a few they collected themselves from schools in Mexico and Argentina. They sequenced the insects’ DNA and singled out short, repetitive segments known as microsatellites. Lice that share these segments inherited them from a common ancestor, giving the researchers a tool to sort the parasites into closely related families. One cluster of microsatellites pointed to a genetic link between lice in Asia and Central America, reflecting the initial migration of people from East Asia into the Americas, Ascunce says.
Another cluster linked lice from the Americas and Europe. Based on how quickly the lice reproduce and accumulate the genetic mutations that lead to microsatellites, the researchers could estimate when the lice of Indigenous Americans might have hybridized with European lice. The most likely answer, they found, was about 500 years ago—during the time of European colonization. “I see this as a proof of concept that human lice are good markers for human evolution and migration,” Ascunce says. “We can see the louse DNA reflected in our own history.”
Alejandra Perotti, an invertebrate biologist at the University of Reading who specializes in lice, says the work is intriguing, but she wants to see the researchers expand and diversify their sample. For instance, only a single louse in the study came from Africa and relatively few came from South America, she notes, limiting the authors’ ability to infer how lice from around world relate to one another. She adds that sequencing the insects’ whole genomes—a project Perotti herself is working on—will offer scientists even more reliable means for pinning down the relatedness between lice groups, as well as matching those to human populations, past and present.
Ascunce says her team plans to include more lice in future studies. “It’s hard to get head lice,” she says. “Hopefully next year there will be another article with even more lice.”
One potential advantage to lice as markers of human migrations is that the parasites can hybridize even if their human hosts do not, Pedersen notes. The lice’s shared genes therefore could illuminate instances in which groups of people came together, perhaps to trade goods, but did not produce any offspring. “This must have happened a lot of times—maybe even most of the time,” he says. In these cases, examining our lousy freeloaders “might bring some new interactions to light.”
