Body hair helps with thermoregulation.
Head hair protects your scalp from the beating sun, but also traps heat in if you’re in a cold weather climate.
Eyelashes are like screen doors for the eyes, keeping bugs and dust and little debris particles out whenever they’re open.
Eyebrows impede sweat from getting in your eyes.
Armpit hair, technically called “axillary” hair, collects and disseminates pheromones while acting like the WD-40 of body hair, reducing friction between skin on the underside of the arm and skin on the side of the chest as we walk and swing our arms.
Pubic hair also helps reduce friction, as well as provides a layer of protection from bacteria and other pathogens.
But facial hair? You will notice it doesn’t appear on that handy list of adaptive hairy traits.
In the early days of studying this kind of stuff, evolutionary biologists thought it might serve thermoregulatory or prophylactic purposes similar to body hair and pubic hair. Beards and mustaches are around the mouth, after all, and the mouth takes in food and other particles that might carry disease. Beards and mustaches are also on the face, which is connected to the head, which loses a lot of heat out of its top if it isn’t covered by hair. It all makes sense when you look at it that way.
Except there’s a problem with this theory: It leaves out 50 percent of the population, i.e., females. Natural selection is ruthless, and it has sent A LOT of species the way of the dodo—for instance, the dodo—but rarely, if ever, does it select for a trait in a species like that and leave half the population hanging, especially the half that makes all the babies (i.e., the most important half). If facial hair were meant to perform important functions, it would be present across both sexes. Instead, thick, mature facial hair is present almost exclusively on the male half of the species, and its only job is to sit there on the face of its wearer as a signal to everyone who crosses his path.
What signal does facial hair send? Well, here’s where it gets a little complicated, as ornamental traits go. University of New Mexico professor Geoffrey Miller, one of the preeminent evolutionary psychologists in the field, put it this way: “The two main explanations for male facial hair are intersexual attraction (attracting females) and intrasexual competition (intimidating rival males).” Basically, facial hair signals one thing to potential partners (namely virility and sexual maturity, hubba-hubba-type stuff) and something else to potential rivals (formidability and wisdom or godliness). Taken together, these signals confer their own brand of elevated status to the men with the most majestic mustaches or the biggest, burliest beards.
The signal that facial hair sends also tends to be stronger and more reliable between males, who are more commonly rivals, than it is between males and females, who are more commonly partners. In fact, evolutionary biologists will tell you (if you ask them) that while some females really like facial hair, and some don’t, and some couldn’t care less, more often than not attraction has as much to do with beard density as anything else. That is, if you’re in a place where there are a lot of beards—say, a lumberjack convention—then a clean-shaven face is more appealing, but if you’re surrounded by bare faces, then a beard is best.
In evolutionary genetics, this is called “negative frequency dependence” (NFD), which is science-speak for the idea that when a trait is rare within a population it tends to have an advantage. In guppies, for example, males with a unique combination of colored spots mate more often and are preyed upon less. This is a huge competitive advantage. It’s like going to Vegas expecting to lose $1,000 but hoping to break even, only to end up winning $1,000 instead. That’s a $2,000 swing! It’s the same thing for a trait with NFD selection. The trait goes from fighting for its life to being the life of the party. The downside is that the competitive advantage can result in overpopulation of others with the same trait very quickly, because of all the getting-it-on the very interesting-looking guppy does—which means it loses its rarity and becomes common. Not to worry, nature has a solution for that: As more guppies bear that same trait, it leads to a decrease in interest from mates and an increase in attention from predators. What was once the hot new guppy thing becomes old news, in other words.