Researchers surveying mites living on the faces of a diverse set of human hosts have found that these ancient symbionts evolved alongside us since our migration out of Africa and have adapted to live on people from specific geographic regions.
Your face is covered in microscopic arachnids. These little guys aren’t spiders, however, they’re mites, specifically Demodex folliculorum.
They are so named because they live in the hair follicles of all human beings, munching on secreted oils and dead skin cells. These creatures are tiny and symbiotic, so we rarely think about them – but they have an evolutionary history as interesting and complex as our own.
An international team of researchers recently set out to understand some basic information about the mites that make our face home, since very little about these ubiquitous organisms has actually been studied. There were three main questions.
First, have mites evolved alongside human hosts over long periods of time, or has their colonization occurred more recently (e.g. as a result of animal domestication, or the origin of cities)? Second, are there predictable genomic differences between mites living on people of diverse ancestry? Third, do any patterns emerge that may influence human health?
The study, which was published in the Proceedings of the National Academy of Sciences, sequenced DNA from the mitochondria (an organelle within the mites’ cells) of D. folliculorum living on 70 humans of diverse geographic ancestries.
The results revealed that there are four distinct lineages of D. folliculorum corresponding to different regions of the world: Europe, Latin America, Asia, and Africa. Interestingly, most of the study participants were living in the United States, which suggests that mites are passed through close association with family members – generations maintain the mites of their forebears even when they move to a different geographic location.
The results also showed a pattern of mite diversity consistent with the human out-of-Africa migration, with splits in mite genomic history estimated to around 3 million years ago, and a majority of diversity occurring in the African-ancestral mites while only a subset of this diversity was found in mites associated with other regions.
One striking result showed that European-like mites were found on people from all over the world, while Europeans themselves had only their own regional symbionts. The authors suggest that this may reflect the unique era of imperialism in human history.
The study was not able to elucidate any information about the role of face mite diversity in human health (mites have been implicated in medically important skin disorders), but it does pave the way for further study.
The associated evolution of face mites and humans is probably quite similar to our shared evolutionary history with other symbionts such as gut or mouth bacteria. Understanding these patterns gives us clues about the origin of humans, as well as the likely course of symbiont evolution in other important organisms such as crop plants or livestock.