Microscopic mites living within hair follicles have been with modern humans since our earliest days. Their evolution mirrors divergences in ancient human populations. According to new findings published in the Proceedings of the National Academy of Sciences this week, they may provide a window into human migration and evolution. 

Our evolution didn’t happen in isolation. Rather, it occurred alongside many closely associated species, from bacteria and viruses to lice and rodents. Mites in the genus Demodex live within the hair follicles of mammals, and two species are known to inhabit human skin: Demodex folliculorum is often found near the skin surface, while Demodex brevis is typically found deep in the glands that secrete the oils in our skin. These mites are ubiquitous, and they’re implicated in human skin disorders like rosacea. 

To better understand the genetic diversity of mites, a team led by Bowdoin College’s Michael Palopoli examined DNA of mites sampled from the hair follicles of 70 people with diverse geographical ancestries. These include people of European, Asian, African, and Latin American descent, though the majority of the participants currently live in the U.S. For 31 of the volunteers, intact mites were directly collected by gently dragging the curved end of a bobby pin across their forehead.

People’s geographical ancestry, the team found, predicted the mite lineages they would harbor. Their genetic analysis revealed four deeply divergent lineages of mites: a globally distributed lineage that’s common among people of European descent and three that are common in people of Asian, African, and Latin American descent. Specifically, lineages (or clades) A, B, C, and D were recovered from African and Latin American recruits, while Asian participants hosted only clades A, B, and D. Europeans primarily hosted mites from clade D (see figure below).

Since Demodex folliculorum populations can stably reside on individuals for years, these associations stick around even after they had spent generations living in a new place. The team also found that genetic variations in the mites were much more likely to be shared within families and between spouses than between unrelated individuals, which means mite transmission likely requires close physical contact. Additionally, human populations vary in terms of skin hydration, hair follicle density and morphology, and lipid production and composition – attributes that might favor certain mite lineages over others.

Molecular variation in these mites could give people some information about where their ancestors lived, Palopoli explains to IFLScience, although we're not to the point where that's possible yet. Demodex folliculorum likely originated with (or even predated) our species in Africa, and then diverged among populations of their descendants as Homo sapiens left the continent and migrated across the planet. The researchers estimate that the most recent common ancestor of mitochondrial clades A, B, and C lived between 2.4 million and 3.8 million years ago – roughly corresponding with the origin of Homo. 

^{Clades A, B, C, and D were recovered from African and Latin American hosts; Asian participants hosted only clades A, B, and D; Europeans primarily hosted mites from clade D. M.F. Palopolia et al., PNAS 2015}

^{Image in the text: California Academy of Sciences}