Face mites have evolved with humans — and can tell us about our ancestry

Scientists have known since the 1840s that masses of tiny mites live on our faces, but they’ve only recently begun to study these creepy-looking arachnids in earnest.

Last year, researchers reported that all of us (not just 10 to 20 percent of us, as previously believed) are hosts to the two species of face mites that “belong” specifically to humans. One species, Demodex folliculorum, lives in our hair follicles. The other, Demodex brevis, resides deeper, in our skin’s sebaceous glands.

Both species feed on oil in our skin. Indeed, Demodex means “the worm that bores into fat.”

The mites appear to be harmless, although some evidence suggests that the tiny creatures may be associated — in some cases, at least — with skin and eye disorders, such as rosacea and blepharitis.

Fascinating new findings

But it’s not only the bugs’ implications for human health that have led scientists to study them. Face mites also provide an intriguing system for exploring human evolution. 

Indeed, in a study published this week in the Proceedings of the National Academy of Sciences (PNAS), researchers report some fascinating new evolutionary-related information about human face mites: Different populations of people have Demodex folliculorum mites with different genetic profiles. Furthermore, these genetically different mites appear to follow families from generation to generation. They do not, however, jump casually to non-related humans.

“It’s shocking that we’re only just discovering how deeply our histories are shared with the mites on our bodies,” said study co-author Michelle Trautwein, a scientist at the California Academy of Sciences who studies the evolution and diversification of insects, in a released statement. “They aren’t just bugs on our faces, they are storytellers. Mites tell us about our own ancient history — it’s a complex story, and we’ve only just scratched the surface.”

Study details

For the study, Trautwein and her colleagues took mite samples from the faces of 70 Americans from diverse backgrounds, and then sequenced the DNA in those samples. The diversity among the study’s participants was critical. Here’s why, as explained by British science writer Ed Jong for The Atlantic:

For the first time, [biologist Michael] Palopoli [of Bowdoin College] 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  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.

What might explain these results? Writes Yong:

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.

A family matter

The researchers also found that mites are not shared easily among strangers — or even among non-family friends.

“Mites are not casually transferred to passersby on the street,” said Trautwein in the released statement. “We seem to share mites primarily with our family, so it likely takes very close physical contact to transmit mites.”

You can read the study in full on the PNAS website. You’ll find Ed Yong’s summary of the research on The Atlantic’s website. For even more information on this topic, I recommend Yong’s earlier article on “Everything you never wanted to know about the mites that eat, crawl and have sex on your face,” which appeared on his Not Exactly Rocket Science blog for National Geographic.