Microscopic mites live on our bodies and faces. We, as hairy mammals offer an interesting habitat to these eight legged creatures related to spiders, scorpions and ticks (all have eight legs). They have been found on all mammals except the platypus and the echidna, hairy monotremes that lay eggs and live in Australia.
There are two distinct species of mites on our faces, inside the pores of our skin . One is short and chubby and lives inside sebaceous glands associated with vellus hairs, it is the Demodex brevis. The other, longer and slender lives above the sebaceous gland, it is the Demodex folliculorum.
We have quite a few per pore, so this means a lot of mites living on our faces. We pick them up after birth, due to contact with our mothers, and relatives. Towels, pillows, sheets... are all vehicles of transfer for the mites.
But it seems that the D. brevis and the D. folliculorum are not closely related to each other. D. brevis is closer to the dog mite, but (see further down, both may even be closer to the goat mite). We may have picked them up from these animals when we domesticated them, or maybe even earlier, from primates.
A paper published in 2013,  by Zhao et al. compared some mtDNA from samples of D. folliculorum from China and Spain, and found that: "The average sequence divergence was 1.24% among the five Chinese isolates, 0.94% between the two geographical isolate groups (China and Spain), and 2.15% between the two facial tissue sources (facial skin and eyelids)" their conclusion: "it appears that subspecies differentiation might not have occurred and that D. folliculorum isolates from the two geographical sources are of the same population. However, population differentiation might be occurring between isolates from facial skin and eyelids."
In other words the D. folliculorum found in Spain and China are virtually identical, there was more diversity between Chinese specimens than between Chinese and Spanish ones... but those found on the face skin differ from those found on the follicles of the eye lashesf!
Another more recent study  (Thoemmes et al.) took anothe look at this issue and took samples which allowed them to build the tree shown below:
As can be seen the same individuals also carries different populations of D. brevis!
The paper gives a good reason for a greater diversity between geographic locations: D. brevis is buried deeper inside their hosts and this makes it more difficult for them to "jump" from one host to another. D. folliculorum is more superficial and therefore easier to pass on to another person: "D. brevis can be found more deeply embedded in sebaceous glands below the skin surface, in comparison to D. folliculorum that lives more superficially in the hair follicles. These contrasting habitat preferences may lead to more frequent transmission of D. folliculorum than of D. brevis, thus resulting in greater reproductive isolation and geographic structure in populations." 
It is very likely that we have carrid Demodex mites with us since we first appeared in Africa, and we carried them around the world with us. So if the American and Chinese mites are different this means that they have been separated for some time. But if European and Chinese mites are more similar , does this mean a later split?
When African mites are sampled (Demodex from African populations), will they appear closer to the tree's root? as the Out of Africa theory proposes? or will they be less diverse?
Thoemmes et al found that D. brevis "exhibited higher genetic diversity, not only between mites from the Americas and those from China [...] but also among mites collected from the same individual human [...]. Sequences of 18S rDNA from different D. brevis samples taken from the same face [...] exhibited more genetic variation [...] than those of D. folliculorum taken from Chinese and North and South Americans ..." .
They found that the "The Chinese D. brevis samples [and] samples from the Americas each form monophyletic clades with a relatively deep divergence between them [...] The distance between the two D. brevis clades suggests strong geographic isolation among populations of D. brevis. Based on sequence divergence, these two populations are as different as are many congeneric species and subspecies.".
Interestingly the paper  points out that: "Phylogenetic estimates based on 16S rDNA also find that dog-hosted Demodex mites share a recent common ancestor with a human-associated species, though in this case D. folliculorum and D. brevis are both more closely related to goat-associated mites, D. caprae. The known habitat of D. canis is deep within the pores and is most similar to that of D. brevis. It is tempting to posit that D. brevis may have colonized humans from wolves during their domestication but any such assertion would be premature. Until other primate species are sampled, the mystery of whether humans acquired Demodex mites from our ape/hominid ancestors or through other means such as our interactions with domesticated mammal species will remain.".
As there are 5,000 species of mammals and each may host 2 species of Demodex... this means 10,000 possible species of which only 13 have been sampled. Clearly we have a long way to go to be able to use them to define ancient human migrations. But the possibilities are enormous and intriguing.
 Ya-e-Zhao et al., (2013) Discrimination between Demodex folliculorum (Acari: Demodicidae) isolates from China and Spain based on mitochondrial cox1 sequences, Journal
Journal of Zhejiang University SCIENCE B Volume 14, Issue 9 , pp 829-836
 Thoemmes MS, Fergus DJ, Urban J, Trautwein M, Dunn RR (2014) Ubiquity and Diversity of Human-Associated Demodex Mites. PLoS ONE 9(8): e106265. doi:10.1371/journal.pone.0106265
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