Tiny mites living on your face hold the key to your ancestry
When people talk about wildlife, they're generally thinking about wolves running through forests, or maybe squirrels skittering about neighborhood trees. But let's look a bit closer to home: Blink. Wildlife just took a ride on your eyelashes. If that makes you lift your eyebrows in puzzlement (or dismay), well, there's animals in your eyebrows, too.
In fact, your face is an ecosystem of long standing for two species, which have been passed on for generations in your family. They probably climbed aboard while you were nursing at your mother's breast. Demodex folliculorum has evolved to live in human hair follicles. Its cousin Demodex brevis ensconces itself slightly deeper in the microhabitat of your sebaceous glands.
Follicle mites, as they are commonly known, are distant relatives of spiders. They eat our dead skin cells, or maybe the oils, bacteria, and fungi on our skin, and they are, I should quickly add, utterly harmless. It's even possible they perform some sort of housekeeping service, making us mutually beneficial: We give them habitat, they minimize zits. But no one really knows for sure. In any case, mites have co-evolved with humans and pre-humans for millions of years.
A study released Monday in Proceedings of the National Academy of Sciences suggests that mites may also provide a sort of skin-deep view of our genealogy and shed new light on the history of human migration. Bowdoin College evolutionary biologist Michael Palopoli and his co-authors sampled the follicle mites from 70 individuals, using the standard technology in the field: They scraped a bobby pin across the forehead of their test subjects. Then they analyzed the scruffy material trapped in the crook of the bobby pin.
I wasn't part of the study but, as it happens, I once went through at least the first part of this process in the course of reporting a story about life on the human body. The researcher I was working with placed my sample onto a glass slide and examined it for a moment under his microscope. "Oh wow," he said, causing my eyebrows to rise nervously. Then, as if congratulating me, he said, "You've got the best population I've ever seen!" And this was a guy who had spent his entire career looking at follicle mites. He showed me the evidence under the microscope. The adult mites looked like sticks of wood lying crisscross (lots of them), with stumpy little legs that wriggled and twitched. They had tiny claws and needlelike mouthparts for consuming skin cells. "Look at 'em all," he cried, unable now to suppress his delight. "Holy moley!" He admired my forehead as if there were gold in them thar hills.
I'm just putting this out there to establish my credentials as a biodiversity expert.
The researchers in the new study took a more sophisticated approach, analyzing a short segment of DNA from each sample. (As is common in such studies, they used DNA from the mitochondria, the energy factory of the cells. In most species, it's inherited only from the mother, and that makes it a useful tool for tracing evolutionary lineages.) The analysis revealed that different mite lineages are associated with different continents, and that these ancestral lineages tend to stay with families even generations after they move to a different continent.
The greatest diversity of mites occurred in people of African origin. That's not surprising because most of human evolutionary history took place in Africa, and previous studies have shown that the genetic diversity of humans is also greatest there. Europeans and Asians are subsets of that diversity. But the researchers were surprised to find that African Americans with a long history in North America "have retained mites originally inherited from Africa rather than exchanging mite populations regularly with individuals of European descent."
That suggests, first, that mites are not highly mobile, and are passed on only through intimate contact within the family. Because the ancestry of African Americans is on average 24 percent European, it also suggests that other factors may minimize the spread of mite lineages. The "skin traits" theory argues, for instance, that mites may be keyed in to differences in skin types, meaning hair follicle density and distribution, skin hydration, oiliness, or other factors.
The current study was too small to say anything definitive. But for co-author Michelle Trautwein at the California Academy of Sciences in San Francisco, it suggests bigger studies to come. "The exciting thing, first, is the idea that we have animals living in our face. It's just shocking for people, and it's a reminder that we are intimately part of nature, even though we think we are distant from it. And then that we are part of this evolutionary process and that this interesting arachnid can be a storyteller, and help us recover some of our evolutionary history in ways that were unexpected." Further studies, she suggested, might help sort out the migratory history of the people in Madagascar, just 250 miles off the east coast of Africa, but with strong genetic links to Borneo. Or they might help decipher the early history of humans in the Americas. The study of mites could also provide medical insights into the skin condition called rosacea, a reddening and swelling that has afflicted Bill Clinton and W.C. Fields, among others. It appears to be associated with certain European mite lineages.
Some people worry that even talking about the existence of mites on the face is a bad idea. They're afraid it will cause people to suffer delusory parasitosis, the disabling paranoia that invisible insects are crawling on our bodies. But here is a better way to respond to the latest news from the world of follicle mites: You can start your day now in front of the bathroom mirror and set aside all that nonsense about fat or thin, young or old. Instead stare into the mirror and in a clear, proud voice announce, "This is an ecosystem!" Given the abundance of humans on Earth, it's also one form of biodiversity that's unlikely to go away any time soon.
Mites aren't just genetically telling, they're also fast:
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Why We Have to Save Wildlife to Save Ourselves
The False Promise of Ecological Restoration Projects