AILSA CHANG, HOST:
Next up, something most of us don't even want to think about - head lice. Yep, it's a nuisance - a pest to be evicted from you or your loved one's hair by some special comb or shampoo. But as NPR's Ari Daniel explains, lice have been stowaways on our heads for so long that they have recorded some of our human history in their own DNA.
ARI DANIEL, BYLINE: Head lice are the tiniest of hitchhikers, grabbing hold of our locks, gluing their eggs to our hair, and annoying us for a time before moving onto the next person's head of hair. You may not need these annoying little insects, but they sure need you.
MARINA ASCUNCE: These are a parasite that live in our head. To survive, they need to take our blood and suck our blood, so they cannot live outside of our head.
DANIEL: Marina Ascunce is an evolutionary geneticist at the U.S. Department of Agriculture in Gainesville, Fla. And like many, she's had head lice.
ASCUNCE: When I was a kid in Argentina, I remember one time at least that I have for sure. It wasn't fun, my mom freaking out.
DANIEL: Ascunce's mom's generation battled with lice, too. In fact, head lice have been clinging to human hair for as long as there have been humans, and likely even before that, to our hominid ancestors' hair.
ASCUNCE: Basically, both we humans, which are the host, and the lice, which is the parasite, have evolved through time together.
DANIEL: And so Ascunce set out to see what these parasites and their DNA could tell us about our past. 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 them to her in Florida. And for any of you who've ever felt tortured by lice, what Ascunce did next may feel like a kind of karma.
ASCUNCE: So first we put them under a microscope. And actually we cut them in half, and then we put them in another tube to do the DNA extractions.
DANIEL: After all that lice DNA was analyzed, here's what Ascunce and her colleagues found - further evidence that lice are like little tape recorders of human history. In this case, she says she detected two distinct genetic clusters, which suggests that human head lice arrived in the Americas twice.
ASCUNCE: We humans, we migrate, and we take the lice with us.
DANIEL: First, some 15 to 35,000 years ago, when humans crossed the Bering Land Bridge from Asia into North America, there were lice gripping their hair along for the ride.
ASCUNCE: The Native Americans, different populations, they went south through the Americas.
DANIEL: As did their lice. And then, 500 some years ago, the Europeans showed up with their own hitchhiking head lice.
ASCUNCE: These lice are mirroring the colonization of the Americas, the two migration waves.
DANIEL: These findings are published in today's edition of the journal PLOS One. Alejandra Perotti is an entomologist at the University of Reading who wasn't involved in the study. She says it's a good approach, but the researchers didn't have enough lice from every part of the world to get a complete picture of their diversity.
ALEJANDRA PEROTTI: If you look at the data they gather, some of the populations have only one louse, including Africa, for example. So there is an issue with the sampling size.
DANIEL: Future work will correct this and look for signals in our head lice of ancient interactions between our human ancestors and Neanderthals, who would have likely carried their own lice as well.
Ari Daniel, NPR News.
(SOUNDBITE OF MOODS, YASPER AND PHILANTHROPE'S "BUCKET LIST")
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