That diversity was crucial. For the first time, Palopoli and Trautwein discovered that Demodex folliculorum comes in four distinct lineages, named A to D. Their proportions vary among people with different ancestries in ways that reflect humanity’s own history.

For example, our species originated in Africa, and people of African descent still have the greatest diversity of face mites, with representation from all four lineages. Other regions only have a fraction of this former diversity—people from Asian backgrounds mostly host mites from Groups B and D, while those of European ancestry are almost exclusively swarming in D. People of Latin American ancestry are exceptional in hosting mites from all four groups, but this might reflect the region’s history of colonialism, including the heavy historical influx of slaves from Africa.

These results make sense, but they’re also counter-intuitive. Remember that the volunteers aren’t people aren’t from all over the world; they’re all Americans with different ancestries. “The common sense idea would be that an African-American who had been here for generations would have picked up mites from people of European ancestry,” says Palopoli.

That wasn’t the case. Instead, “some of these people are maintaining mites for generations outside of their region of ancestry,” says Trautwein. Her team even sampled one volunteer who was born in Asia and had moved to the U.S. eight years before—and his face was full of the Group B mites that are common in Asia.

It might be that the mites simply don’t move very much. Indeed, one of the researchers—known in the paper solely as “host 206”—confirmed that mite populations are incredibly stable by sampling his or her own body for three years. Alternatively, it could be that different types of skin select for different lineages of mites, because of the qualities of their hair or oils. The environment might also be important. George Perry from Pennsylvania State University wonders if there’s only one mite lineage in Europe because others are sensitive to persistent outdoor cold.

It does seems that mites aren’t jumping from neighbor to neighbor, but can certainly jump from parent to child: When the team analyzed three families, they found that children share their parents’ mites. “We’re not sharing them in the subway but only among our close family,” says Trautwein. “That allows them to be a marker for human evolution.”

David Reed from the Florida Museum of Natural History, who has done similar studies on head lice, agrees. “This particular parasite has a few real strengths that makes it particularly good for studying human evolution,” he says. “It’s ubiquitous, it stays within individuals and family groups, and it’s been with us for a long time.”

For, perhaps, all the time. Trautwein and Palopoli estimated that the four mite lineages diverged from each other at least 200,000 years ago, in parallel with the rise of our own species or perhaps even earlier. “They’ve likely been with us through all of our history,” says Trautwein. But the exact timing of our partnership is hard to pin down, especially since we don’t know the closest living relative of our face mites. There are Demodex species that specialize on dogs, cats, goats, cows, but no one has sequenced the mites on chimp faces yet.

“Really, it seems like there’s an unending supply of good parasites to study but too few people doing the hard work of sampling them worldwide,” says Reed. Trautwein is on the case, teaming up with Rob Dunn from North Carolina State University to collect mites from people on seven continents. She also wants to compare mites in mixed-race couples and their children. “What’s shocking about the mites is that no one has done anything,” she says. “Anything we can think of, we can do.”

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