- Body hair helps regulate heat.
- Head hair protects your scalp from the beating sun, but also retains heat when you are in cold weather.
- Eyelashes are like screen doors for the eyes that keep bugs and dust and small particles of dirt away when they are open.
- Eyebrows prevent sweat from getting into your eyes.
- Armpit hair, technically called "armpit hair", collects and spreads pheromones, acting like the WD-40 of body hair. This will reduce the friction between the skin on the bottom of the arm and the skin on the side of the chest when walking and swinging the arms.
- Pubic hair also helps reduce friction and provides a protective layer against bacteria and other pathogens.
But facial hair? You will find that it is not on this handy list of adaptive hair characteristics.
In the early days of studying this type of thing, evolutionary biologists thought it could serve thermoregulatory or prophylactic purposes, much like body hair and pubic hair. Beards and mustaches are after all around the mouth, and the mouth ingests food and other particles that can transmit disease. Beards and mustaches are also found on the face connected to the head, which loses a lot of heat from its top when it is not covered by hair. It all makes sense when you look at it that way.
Unless there is one problem with this theory: it leaves out 50 percent of the population; H. women. Natural selection is ruthless and has sent MANY species down the path of the dodo – the dodo, for example – but rarely, if ever, does it select any trait in such a species, and in particular, it leaves half the population hanging, half all babies makes (i.e. the most important half). If facial hair were to perform important functions, it would be present in both sexes. Instead, dense, mature facial hair is almost entirely on the male half of the species, and its only function is to sit there on the wearer's face as a signal to anyone who crosses his path.
Which signal makes Send facial hair? Well, this is where things get a little complicated when it comes to ornamental features. Geoffrey Miller, a professor at the University of New Mexico, one of the foremost evolutionary psychologists in the field, put it this: "The two main explanations for male facial hair are intersex attraction (attracting women) and intrasexual competition (intimidating rival men)." Facial hair gives potential partners one thing (namely, manhood and sexual maturity, Hubba-Hubba-type stuff) and potential rivals something else (formidability and wisdom or piety). Taken together, these signals give the men with the most majestic mustaches or the largest, stockiest beards their own status.
The signal that facial hair sends also tends to be stronger and more reliable in men and women who are more frequent rivals than men and women who are more frequent partners. In fact, evolutionary biologists will tell you (if you ask them) that some women really like facial hair and others don't and others care no less, but attraction mostly has as much to do with beard density as anything else. That said, if you're in a place that has a lot of beards – a lumberjack convention, for example – a clean-shaven face is more appealing. However, if you are surrounded by bare faces, a beard is best.
In evolutionary genetics, this is referred to as "negative frequency dependence" (NFD). This is scientifically speaking for the idea that a trait that is rare in a population tends to have an advantage. In guppies, for example, males with a unique combination of colored spots mate more often and are less hunted. This is a huge competitive advantage. It's like going to Vegas and expecting to lose $ 1,000 but hoping to break even to win $ 1,000 instead. That's a $ 2,000 swing! This also applies to a feature with NFD selection. The characteristic ranges from the struggle for his life to the life of the party. The downside is that competitive advantage can very quickly lead to overpopulation of others with the same trait because the very interesting looking guppy turns everything upside down – which means that it will lose its rarity and become common. Don't worry, nature has a solution for this: the more guppies that share the same trait, the less interest the partners and the greater the attention of the predators. In other words, what was once the hot new guppy thing is becoming old news.