In 2004, Dove, an American cosmetics company based in New Jersey, launched its Real Beauty campaign. The campaign was designed to show that the criteria used to determine whether or not a person is beautiful is subjective, and women often internalize a very exacting beauty standard. Though it never appears in any Dove ad, there is a physical feature that almost all human faces have in common.

Unfortunately, it’s not a feature that would make most people feel beautiful. Face mites are transparent, microscopic, eight-legged creatures. They measure roughly one-hundreth of an inch (three-hundreths of a millimeter). According to a 1992 article in Clinical and Experimental Dermatology, twelve face mites can live on one hair follicle, and each mite lives for about two weeks. Face mites┬ábelong to the phylum Arthropod and the Genus Demodex.

These mites are believed to be transmitted by mother-infant contact. Once they are transmitted, they live in the pores. Face mites aren’t usually harmful—unless a person is sickened merely by the idea of hosting them—but they are gluttonous. They burrow into the pores of humans’ hair follicles, searching for sustenance. Face mites feed on sebum, a yellow, oily substance secreted by the sebaceous glands, small, oil producing glands found on the skin of mammals.

Sebaceous glands are usually attached to fair follicles, and sebum, an oil they produce, naturally moisturizes humans’ skin and hair. Sebum contains triglycerides, free fatty acids, wax esters, squalene, cholesterol esters, and cholesterol, all of which are part of an appetizing diet for face mites.

During the day, face mites feast on the oils in a human’s pores. At night, they mate on the skin’s surface. Sometimes, they cause a skin disorder called, demodectic mange. Those who are experiencing a decline in immune system response due to immunodepressive drugs, chemotherapy, or an immunodeficiency disease. Kanade Shinkai, a dermatologist at the University of California, San Francisco, demodectic mange is easily identifiable, because it gives a person’s skin a red or white tinge. In most cases, face mites aren’t harmful, but, from a geneticist’s perspective, they are defining facial features.

Michelle Trautwein, an entomologist at the California Academy of Sciences in San Francisco, studies the relationship between humans and their face mites. Trautwein collects the face mites by scraping a spoon across the oily parts on the faces of the two thousand humans she has worked with in her studies.

All of the people Trautwein has examined have had face mites, and, she says, those face mites reveal a person’s genetic history. People whose ancestors came from different parts of the world have different face mites. Trautwein told National Public Radio (NPR), “Face mites are definitely the species of animal that we have the closest connection with as humans, even though most of us don’t know about them or even see one in our lifetime. We still have this very ancient and intimate relationship, and it seems clear that we’ve had these face mite species with us for all of our history.”

[H/T LiveScience]