A truth universally known is that tiny mites are present on the human body: staying in hair follicles, mating and laying eggs on dead skin cells, generally causing no harm. But a new study has made another fascinating discovery: the mites' distribution in the global population could hold clues on human migration throughout history.
Scientists writing in the journal PNAS used genetic testing to link the evolution of the mite species Demodex folliculorum to human evolution, zeroing in on the mitochondrial DNA of mite samples worldwide.
The team found that different human lineages have a corresponding variety of mites, which followed them throughout generations and were hardly casually passed on between individuals.
"They aren't just bugs on our faces, they are storytellers,” said senior author and entomology curator Dr. Michelle Trautwein, citing that mites give a glimpse into complex human ancient history.
The team collected samples from 70 human hosts, with intact mites obtained from the forehead, cheek or outer nose. The specimens were then sequenced to see the mitochondrial DNA of each.
Trautwein said that the continent of someone’s ancestral origin had the tendency to predict the kind of mite on their face, with bug lineages persisting in hosts for generations. Even if one moves to a far-flung region, the mites stick with that person, she added.
African-Americans who lived in the United States for generations, for instance, still sheltered African mites. The authors suggested that the differences found in mite lineages aligned with the “Out of Africa” hypothesis, which states that every human today descended from a group that had evolved in Africa and dispersed far and wide.
Mites were also suggested to predate modern humans: researchers estimated that the last common ancestor of the studied mites existed over three million years past. Certain mite groups, however, might not have made it during human migration.
“As they diverged into Asia and Europe, some individual lineages were lost,” explained study first author Michael Palopoli, an evolutionary biologist at Bowdoin College in Brunswick. Case in point, all European mites are mitochondrial type D.
But individual mite populations were stable over a maximum period of three years, even in humans who transferred to new regions with different dominant mite clades. Their profiles appeared to be passed on as well, with a second-generation male of African descent now in Europe likely retaining the mites of his ancestors rather than obtaining a European profile.
Finally, mites are not shared easily, such as casual transfer to passersby. “We seem to share mites primarily with our family, so it likely takes very close physical contact,” explained Trautwein.
The team aims to continue their mite research, with Trautwein in the middle of a project that collects arthropods, including mites, around the seven continents.
