Head lice are considered a nuisance — a pest to be evicted from the hair on your head or the head of a loved one with a special comb or shampoo. But there's more to lice than their elimination. These parasites have been stowaways on our heads for so long that they've recorded our history as humans in their DNA.

"We can think of human lice as heirlooms of our past," says Marina Ascunce, an evolutionary geneticist at the U.S. Department of Agriculture in Gainesville, Florida.

Bret Boyd, an entomologist at Virginia Commonwealth University, agrees. "They're really like a little tape recorder that's been following us around throughout our time on this earth," he says.

And Ascunce says lice are particularly helpful in answering questions about human history that we can't resolve using our own DNA or the archaeological record.

In a new study in the journal PLOS One, she and her colleagues present evidence that our head lice seem to have recorded in their DNA the massive human migrations that led to the inhabitation and colonization of the Americas.

That is, where humans went, so did our head lice.

Looking at the DNA of lice

Head lice are the tiniest of hitchhikers, each one about the size of a sesame seed. They grab hold of our locks, glue their eggs to our hair and annoy us for a time by tickling our scalps and making our heads itch — before crawling into the next person's head of hair. We may not need these pesky little insects, but they sure need us.

"These are a parasite that live [on] our head," says Ascunce. "And to survive, they need to take our blood and suck our blood. So they cannot live outside of our head." In biology parlance, they are obligate parasites. To survive, they are obligated to live upon us.

Like gazillions of humans, Ascunce has had head lice. "When I was a kid in Argentina, I remember one time at least that I have for sure," she says. "It wasn't fun. My mom [was] freaking out."

Ascunce's mom's generation battled with lice, too. As did her grandmother's generation. In fact, head lice have been clinging to human hair for as long as there've been humans — and likely even before that to the hair of our hominid ancestors.

"Basically," says Ascunce, "both we humans, which are the host, and the lice, which is the parasite, have evolved through time together."

And so, while still a researcher at the Florida Museum of Natural History in the early 2010s, she set out to see what these parasites and their DNA could tell us about our past.

The first thing she needed was a bunch of lice. So she teamed up with collaborators who collected them from 25 places around the world and sent their corpses to her in Florida.

Ascunce then began her laboratory procedure, which, to anyone who's ever felt tortured by lice, may feel like a kind of karma.

"So first we put them under a microscope, and actually we cut them in half," she says. "And then we put them in another tube to do the DNA extractions."

After she and her colleagues analyzed all that lice DNA, they found further evidence that lice operate as mini recorders of human history. In this case, she says she detected two distinct genetic clusters, which suggest that human head lice arrived in the Americas twice.

"We humans, we migrate and we take the lice with us," she summarizes.

First, some 15,000 to 35,000 years ago, when humans crossed the Bering Land Bridge from Asia into North America, there were likely lice gripping their hair, along for the ride. So it confirms what we knew about humans crossing continents.

"The Native Americans," says Ascunce, "different populations, they went south through the Americas," as did their lice.

Then, 500-some years ago, the Europeans showed up with their own strain of hitchhiking head lice.

In other words, "these lice are mirroring the colonization of the Americas," says Ascunce, "the two migration waves."

Alejandra Perotti, an invertebrate biologist at the University of Reading who wasn't involved in the study, says the approach is solid. But she says the researchers didn't have enough lice from every part of the world to get a complete picture of their diversity — which could lead to a better understanding of broad human movement patterns over the centuries.

"If you look at the data they gather," she says, "some of the populations have only one louse, including Africa, for example. So there is an issue with the sampling size."

Future work will correct this data gap. And Ascunce and her colleagues plan on looking for signals in our head lice of ancient interactions between our human ancestors and Neanderthals who would have carried their own lice as well. These interactions would have included "any type of close contact from sharing sleeping sites to fights to interbreeding," she says.

You just can't keep a juicy secret from a head louse.

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