Most people would probably prefer to forget that their eyebrows are also shaggy ecosystems, home to scores of microscopic hair mites. But a DNA analysis reveals that your mites are incredibly loyal to you—and that could help scientists trace ancient human migrations and perhaps find new ways to treat common skin ailments.

Demodex folliculorum is a species of mite that lives in and around the hair follicles of humans and other mammals. Bowdoin College evolutionary geneticist Michael Palopoli and his colleagues sampled the DNA of these mites living on a diverse group of 70 human hosts.

Sequencing the mites' mitochondrial DNA revealed different lineages that closely match the ancestral geography of their human hosts. One mite lineage is common among people of European ancestry, no matter where they live in the world now, and is persistent even after generations in new locations. Other mite lineages are more common among people of Asian, African or Latin American ancestry.

There are a few possible reasons for this unusual mite fidelity, says Palopoli. His group favors the co-called skin traits model: “There may be something about the skin of people from different geographic origins that may be selecting for mites from different mitochondrial lineages,” he explains. “But we don't know what it might be about the skin that may be selecting for one lineage of mites over another.”

Following this line of inquiry could help researchers solve mysteries of how and why otherwise benign mites have been previously linked to skin disorders such as rosacea and blepharitis, or eyelid inflammation.

“One logical question that these results raise is whether one of these different, diverging mitochondrial lineages of mites might tend to be more or less likely than another in causing skin disorders," says Palopoli. "Maybe a mite from one mitochondrial lineage is particularly likely to cause rosacea. That could be really important, but we just don't know at this point.”

Mining the DNA sequences of our faithful mite pals could also provide a new tool for scientists to trace ancient human migrations.

George Perry, who heads an anthropological genomics lab at Pennsylvania State University, notes that some interesting findings have emerged from research on the various species that live with us, whether we like them or not.

“Probably the most widely studied is the stomach bacteria Helicobacter pylori,” he notes. “It's nearly ubiquitous in developing countries, and it closely tracks a lot of human migration movements.” Interesting theories of human history have also emerged from studies of head lice, he adds.

“There's a hypothesis that one ancient lineage of Pediculus humanus is the result of an archaic hominin speciation event, and then was transmitted by direct physical contact between those hominins and modern humans," Perry says. "So this theory suggests that although those hominins are now extinct, we still have their lice.”

The study by Palopoli and his colleagues, published this week in the Proceedings of the National Academy of Sciences, may add hair mites to the mix of species that can help track our species' history.

“We've got these genetically diverse mite lineages existing on all of us, and that provides a wealth of information, potentially, for unraveling different human migration patterns,” Palopoli said.

So far, the early exploration of mite lineages appears to tell a story consistent with the favored "out of Africa" model for human migration, which says that all humans alive today come from a group that left Africa about two million years ago.

“All four of the diverging clades appear in the mites on people of African ancestry, while only subsets appear on Europeans or Asians," says Palopoli. "So our hypothesis is that all four clades were present on us when we lived in Africa, but since we've come out, different subsets have migrated along with Asians and Europeans.”

Sampling mites from a wider variety of human ancestries, including more people now living in Africa, could help reveal how the mites and humans co-evolved.

“It looks like the mites are fairly faithful to people from a particular region, at least at this broad scale we've looked at so far, and the signal still remains that mites vary substantially in people from across different geographic areas, so it provides promise as a system to test where people are from,” Palopoli adds.

Using the mites for evidence of our origins may also spur more interest in understanding the habits of our largely unknown life partners. But breeding more familiarity with our hair mites could take some getting used to.

Human test subjects typically had two reactions to seeing the minute beasties who've been living in their hair, Palopoli reports. “One reaction is that they were sort of fascinated with them. The other reaction is that they were pretty grossed out.”