Complementing the myth of Junk DNA is that of what you might call Junk Body Parts. This myth points to supposedly useless or defective anatomical features that give evidence of “unintelligent design.” An example is the fine hair on our arms and legs.

Isn’t that nothing more than an evolutionary holdover from our apelike ancestors? A list of “God’s Greatest Mistakes” that Jerry Coyne likes to promote asks wonderingly, “If our skin is meant to be mostly bare, why do we have the tiny ineffectual hairs (and separate muscles and nerves for them) at all?” Discover Magazine has a similar list of “Useless Body Parts” that observes, “Brows help keep sweat from the eyes, and male facial hair may play a role in sexual selection, but apparently most of the hair left on the human body serves no function.”

If you ever entertained the intuition that that can’t be right, that we’d be missing something without our arm and leg hair, well it looks like you were…right. Researchers at the University of Sheffield in England explain that it helps fend off unwanted parasites on our body. New Scientist has the story:

We’re often reminded that we’re genetically almost identical to apes, so how did we become the baldest primate? Charles Darwin thought sexual selection explained it — as he put it, women with less hair were more attractive and men became less hairy as a corollary. Alfred Russel Wallace, who didn’t like the idea of sexual selection, attributed our naked form to God. In fact, we have the same density of hair follicles as chimps, it’s just our hair is much finer.
So the question becomes, why does fine hair persist in humans?

Now Michael Siva-Jothy and Isabelle Dean of the University of Sheffield, UK, have a suggestion: fine hair helps us detect parasites as they crawl over our bodies. It also makes it harder for the bugs to bite.
The results of their experiments — carried out by putting bedbugs like the one pictured on the arms of student volunteers — suggest that hair lengthens the insect’s search for a feeding site, and increases its chances of detection.

Which makes sense at the intuitive level and is, in any event, more that just a “suggestion.” The study, “Human fine body hair enhances ectoparasite detection,” is out now in the journal Biology Letters published by the Royal Society.
Photo credit: Robert France Photography