The first humans to arrive in America left scattered evidence behind, from ancient stone tools to fossilized footprints. And among descendants on either side of the Bering Strait, genetic studies have helped to map out the whens and wheres of these migrations. But another clue to when the first humans arrived in the Americas lives on our bodies—lice. The blood-sucking parasites persistently bedded down in human hair and moved with hunter-gatherers from Asia into and throughout the Americas. The descendants of those critters cling to us today. And since lice have their own distinct genetic lineages, evolved while living on humans, we can study them to yield clues about the history of our species.
Now, a study of louse genetics suggests that two distinct clusters of the pests migrated to the Americas with different human hosts. One group arrived tucked into the hair of the East Asians who first populated the Americas. Then, thousands of years later, a second louse lineage came over nestled on the heads of European colonists. While the former louse cluster has populated much of the world, the latter is concentrated in Europe and the New World. The only place that the authors of the new research, published Wednesday in PLOS One, found the two lice clades commonly hybridize is in the Americas. That revelation shows the insects’ migration patterns mirror the New World mixing of their human hosts from East Asia and Europe.
“It is amazing to think that the lowly louse can shed a bit of light on our human journey around the world,” says David Reed, a biologist at the Florida Museum of Natural History and a co-author of the study. “Written in their DNA is a script of our story. It reminds us that even the less-than-charismatic creatures can be very important to our understanding of the world around us.”
Lice are an itchy annoyance, and we are stuck with them. The parasites can’t live more than a day or two away from their human host. Their eggs, or nits, won’t hatch unless they stay in a special “Goldilocks zone” near the human scalp. Some nits are attached with cement so strong they have been found still stuck to ancient hair after 10,000 years.
Lice and their predecessors have been intimately living with primates and co-evolving together for at least 25 million years. Studies show that human and chimpanzee lice diverged sometime around 5.5 million years ago, roughly the same era as the lineages of their hosts.
Marina Ascunce, an evolutionary geneticist with the U.S. Department of Agriculture and a co-author of the new study, is interested in tracking the relationship of the infamous parasite and its human hosts to learn much about both. “I think many people might be aware of a co-evolutionary relationship between us and the microbiome we carry,” she says. “Lice can be similar, except in this case, instead of an internal microbe it’s an external insect.”
“I think lice deserve a more positive view,” she says. “Yes, they can be annoying, but they also have real value for us.”
For example, one milestone in louse evolution likely reveals the timing of a major development in human history and culture. DNA studies show that two types of common lice, head and body lice, diverged from each other around 190,000 years ago. Since body lice are evolutionarily adapted to living in fabrics, the divergence likely indicates that humans began to wear clothing in that era, and the pests adapted to make themselves at home. Because no clothing of that era survives, the lice are the closest thing to smoking-gun evidence.
For the new study, Ascunce and colleagues sampled 274 head lice from 25 sites around the world and found that they exist in two distinct clusters that, from what scientists have discovered so far, have rarely interbred. Of the lice analyzed, only 33—12 percent—were hybrids. Of those hybrids, 25 individuals—76 percent—were found in the New World.
To determine when and where that mixing of lineages might have occurred, the team created models by using highly variable genetic markers and the timing of known human movements. Those models tested a wide range of possible mixing events—from the interbreeding of modern humans and Neanderthals, to the population movements of the world wars, to the dawn of economic globalization in the 1980s. The results suggested hybridization occurred in the relatively recent past. “By comparing the distribution of the current-day European lice and the current-day Asian lice,” Ascunce says, “we suggest that these hybrid lice are the result of the admixture of lice descendent from Native Americans and lice descendent from European colonizers.”
Invertebrate biologist Alejandra Perotti, of the University of Reading in England, says the study’s sample size needs to be expanded and more lice included from more populations around the globe. Still, Perotti, who is not affiliated with the research, says the paper is a good step forward.
“They found two groups of lice, genetically differentiated,” she says. “One group shows a genetic affinity between American or Amerindian lice and East Asia, confirming once again the colonization of the New World from East Asia.” She previously recovered ancient human DNA from louse cement in 2,000-year-old mummies preserved in Argentina’s Andes Mountains.
To reach these new conclusions, the scientists looked at markers of nuclear genetic diversity recently identified by geneticists, which carry far more information on evolutionary history than commonly-used short sequences of female-only mitochondrial DNA. Numerous scientists are now working on the sequencing of an entire louse genome. That would enable some intriguing future research on louse evolution—with accompanying insights on our own history.
Ascunce looks forward to investigating long-debated theories about the louse and human speciation. “Our hypothesis is that one of these two louse clusters evolved in anatomically modern humans, and the other evolved in Neanderthals,” she says. Using a genome to date the clusters could shed light on whether they did evolve in different human species.
And lice like those found in American mummies can also be a source of ancient DNA. “Would the ancient lice be like some louse populations from the present-day Americas and different from the European lice?” Ascunce asks, reinforcing one of this study’s key finds. “That would be a cool question to answer in future studies.”
Whatever the results of such research, one thing seems certain: Our ancient, intimate relationship with lice will continue in the future—for nitpicking parents and inquiring scientists alike.