A scanning electron micrograph of Demodex folliculorum. Photo: Eye of Science/Science Source.
Right now, deep in your hair follicles and sweat glands, tiny mites are feeding on dead skin cells, mating, and laying eggs. The microscopic arthropods, known as Demodex folliculorum, live on virtually all mammals—especially their faces—and cause no harm under most circumstances. Now, a new study shows that people of different ancestry carry different subgroups of the bugs, and that the mites’ distribution throughout the global population may even reflect how our species has migrated and evolved over the course of history.
“When you think of all the parasites that humans play host to, and that each has something to say about our history, you realize that there is so much more to learn about who we are and where we came from,” says University of Florida in Gainesville evolutionary biologist David Reed, who was not involved in the study.
A breakdown of how the four mite clades (A, B, C, D) were distributed around the world.
The researchers estimate that the last common ancestor of the four mite clades lived more than 3 million years ago, meaning all four groups predate modern humans and our two species have evolved in tandem. A wealth of fossil and human genetic evidence suggests that modern humans first evolved in Africa and the distribution of mite species supports that hypothesis. Though it was the least sampled geographic area, people of African descent had the most diverse mites, possessing all four clades. From there, the authors theorize that people (carrying their mites) spread out to the other geographic regions and that, along the way, certain groups of the mites didn’t make it.
“As they diverged into Asia and Europe, some individual lineages were lost,” explains the study’s first author Michael Palopoli, an evolutionary biologist at Bowdoin College in Brunswick, Maine. Thus, all European mites are mitochondrial type D. Human genes show a pattern similar, in some respects, to Africans possessing the most diverse genomes. Exactly when these die-offs occurred is impossible to know from the experiment, but he suggests that future studies could give us a clearer idea of the timeline.
The researchers also found that each individual’s mite population was stable over time periods as long as 3 years—even in people who had moved to new regions of the world with different dominant clades. Furthermore, the mite profiles appear to be passed across generations—a second generation person of African descent living in Europe will most likely retain the mites of her ancestors, rather than acquire a European profile.
The stability of the mite populations, especially between generations, suggests to the authors that differences between the hosts’ skin may explain why people from different regions have different subsets of the bugs. Human populations differ in skin hydration, hair follicle density, and lipid production. These differences have likely arisen over evolutionary time, and the authors theorize that the changes may have given certain mite clades a competitive advantage in certain skin types. Ancient Europeans, for example, may have acquired mutations in their skin that heavily favored the D clade over A, B, or C.
The team suggests that the D. folliculorum mites offer a new way to study the history and relationships of human populations. Other research—especially on lice—has attempted to glean similar insights about human history, but the mites appear to have greater genetic diversity: Even people with the same ancestry had subtle differences in their mite profiles, and the most similar profiles came from individuals within the same family. These traits could provide researchers with a map of human history that is both more precise and more consistent than lice or other species.
This is the first experiment using the mites to study human history and behaviors. Some of the sample sizes are small and more work remains, but the results are promising so far, Reed says. “The authors have found a very promising new way to investigate human evolution.”