The Zoo We Bear

By Drs. David Niesel and Norbert Herzog, Medical Discovery News

Bacteria and viruses live on and in us as part of our normal flora or microbiome. In fact, microbes outnumber our own cells by 10 to 100 to 1. But there are even more creatures that we share our lives with, parasites and vermin.

That sounds disgusting, vermin? Really?

Studies of these creatures are actually quite revealing about human evolution and migration. For example, all of us harbor a mite that lives in the hair follicles on our faces called Demodex folliculorum. They are tubular in shape with iddy biddy legs and live deep in our follicles. Somewhere between 23 - 100% of people are colonized with them and this infestation is only rarely associated with a disease.

Scientists collected mites from the faces 70 people of different origins, most from the US. They sequenced mitochondrial DNA from these mites. Since these mites are only shared between sexual partners or close family members, they can help determine where people came from. People from different continents harbor different varieties of mites on their faces. The type of mites that people carry remain the same even long after they have traveled to another geographical region and even generations later. As ancient humans left Africa and moved to other areas of the earth and became isolated populations, mites co-evolved along with them. So the type of mite you carry can help to identify your origins.

Another group of scientists are studying the tapeworms that infest mammals as a way to determine when ancient humans began to eat meat. It has been assumed that humans acquired tapeworms when they domesticated cows and pigs some 12,000 years ago and consumed under-cooked meat which allowed the tapeworms to survive. However, the three types of tapeworm that currently infect humans are most like those that live in lions and hyenas. So humans acquired these parasites long before the domestication of mammals when they ate the same animals consumed by lions and hyenas. That moves ancient humans carnivorous origins back to the Pliocene and Pleistocene epochs between 5.3 and .01 million years ago.

Getting evidence about when our human ancestors lost their fur and began wearing clothes made from animal furs would have been impossible except for the lowly louse. Some 7 million years ago, our human ancestors acquired head lice from chimpanzees. Then about 4 million years ago ancient humans were colonized with pubic lice that came from gorillas suggesting that by that time the hair of the head and of groin were no longer connected by fur making them separate environments. Head lice genetics reveals evidence of a population expansion out of Africa about 100,000 years ago which mirrors human migration. Though modern humans have a relatively limited mitochondrial DNA variability going back to a common ancestor within 200,000 years, lice have 3 very different lineages that converge on a common ancestor 2 million years ago. So they have been around a great deal longer than their human hosts. Analysis of the DNA from pubic lice revealed that they diverged from head lice between 30,000 and 114,000 years ago and clothing is likely to have helped to isolate the two populations. We are unlikely to ever know for sure if this estimate is accurate, but it is interesting to speculate when these features of our lives arose. So we have been making fashion statements at least that long.

The multitude of organisms that live on us and in us have been shown to be important in keeping us healthy and defending us against the many pathogens to which we are exposed. They also are helping us learn more about ourselves, our origins and our travels.

Medical Discovery News is hosted by professors Norbert Herzog at Quinnipiac University, and David Niesel of the University of Texas Medical Branch. Learn more at