Tweeze, shave, chemical cream, wax or electro-zap: Humans have come up with a pharmacy’s worth of ways to deforest their bodies of natural hair.
Yet, to evolutionary biologists, humans are amazing not because they have so much hair, but because they have so little. Whereas our ancestors once sported full-fur suits, Homo sapiens today are almost embarrassingly naked.
The story of how our bodies morphed from being upholstered in head-to-toe carpeting into a mosaic of hairy and non-hairy bits is one of sweat, sex, scent, and (not to be left out) climate change.
From the Woods to the Savanna, Shedding Hair Along the Way
Early hominids (our über-great grandparents), the story goes, made their living foraging for fruits, nuts, tubers, and vegetation in the cool shade of forest trees. Then, about three million years ago, a global cooling period dried out the regions of Central Africa where those early family members were living.
The fur that had once kept them warm became a liability, as forests changed into grasslands and early hunters spent long hours striding or running across the savanna in pursuit of dinner. From natural selection’s point of view, the ability to sweat and quickly dissipate heat became the neatest trick on the block. So, by and large, humans lost their hair.
What that did, says Russell Tuttle, anthropologist and evolutionary biologist at the University of Chicago and author of Apes and Human Evolution, is give humans the ability to be out hunting in the heat of the day when other predators (i.e. LIONS!) “are resting and trying to keep themselves cool, because all they can do is pant.”
If that evolutionary argument doesn’t convince you, that’s fine. But just consider the alternatives, like the charming but ridiculous “Aquatic Ape Hypothesis” that’s occasionally bandied about the scientific literature. Advocates think early hominids went through a phase of near-total water living and, like dolphins and whales, lost the bulk of their hair in order to reduce drag. Unfortunately, there is no particular reason to believe this is true.
Why Keep Any Hair Then?
Happily though, the gods of evaporative cooling didn’t demand the sacrifice of all our pilatory plumage. We’ve still got some hair left, and it (mostly) seems to serve a purpose—starting with the most highly-groomed bits.
Like your eyebrows, for example. Aside from being good for raising, furrowing, and piercing, the hair there keeps the sweat out of your eyes. And the hair on your head shields your noggin from the direct force of the sun. It also leaves an area of air between your scalp and hair’s hot surface, so sweat can evaporate and cool things down. (Curly hair, Tuttle says, does the best job at performing this task.) Head hair grows longer than other hair because these follicles remain in an active growth (or anagen) phase longer than other other hair follicles—generally a couple of years instead of just a couple of months.
That hair’s useful. But what about all the soft, downy stuff that barely shows up on your body? It’s called vellus hair, and it’s a safe bet that it’s an evolutionary relic. Humans don’t need it anymore, but it’s not doing any harm either, so there it stays. Vellus hair is closely related to another type of hair that definitely serves a purpose, though. Add just a little androgenic hormone (which starts circulating in both boys and girls around puberty) and voila—it becomes thicker and darker.
Of Pits and Pubes
You knew we were going to get here, didn’t you? Hair follicles in certain favorite regions of our body are differentially sensitive to androgens—along with other places, like lower legs, arms, and chests. Put another way, our hair gets dark and thick only in some places. And that so-called androgenic hair’s placement probably developed over millennia in response to humans’ behavioral needs.
Don’t tell the multi-billion dollar deodorant and perfume industry, but humans actually like how other humans smell. Our hairiest regions carry two kinds of sweat glands. There are eccrine glands, which are found on most of the body and open directly to the surface of the skin (needed for cooling), and apocrine glands, exclusive to the hairiest bits, which empty body odor-carrying fluid into the hair follicles.
Thicker, denser hair helps hold on to that scent and then disperse it. So our moist hairy regions may help in attracting mates. Once that work is done, the hair could also be doing double duty, by preventing chafing when we’re on the move and even potentially cushioning and protecting our delicate bits from infection during sex.
Now that you’ve got the whole vellus/androgenic hair thing down, the differences between male and female hairiness are a little easier to explain. Physiologically, it’s simple: Men are hairier than women because they have more androgenic hormones in their body—more androgens, more noticeable androgenic hair.
But there still are plenty of hypotheses that try to delve deeper into behavioral drivers for mane maintenance. Possibly females prefer males with fuller beards because they read it as a sign of virility. Possibly they like stubble. Possibly males prefer women to be less-hairy because it’s a trait associated with juvenile features—possibly not. By and large, though, these are “just-so” stories, common to evolutionary biology, which haven’t been tested in modern humans.
What we do know for sure is that humans’ site-specific hairiness forced us into new forms of communication. Now that we can’t raise our hackles or use coat patterns to signal who we are, we’ve lost a powerful—and badass—way to send messages about who we are and what we’re feeling. Instead, we’ll have to be content with piercings, tattoos, makeup…and a little thing called language. Losing our hair didn’t just cool us down. It made us the people we are today.