Human Hair Loss

I’ve read a few watershed books over the years that seriously changed the way I thought about things. One of these – which I read way back in my sophomore year of high school – was Desmond Morris' The Naked Ape.

The book made a splash in its day mostly because it argued that, despite our high-falutin' opinion of ourselves, much of human behavior can be explained away as just updated versions of fairly undignified primate habits.

One of the issues addressed by the book is the question of why Homo sapiens became hairless. "There are 193 living species of monkeys and apes," wrote Morris. "192 of them are covered with hair. The exception is a naked ape self-named Homo sapiens." Beyond that, there are very few mammals of any stripe without hair and, when that rare occurrence happens, it’s due to an adaptation to a unique environment, like naked mole rats who live entirely underground or the totally aquatic whales and dolphins.

Morris theorizes that we are hairless to facilitate an increased sensation and intimacy from mutual touching, which strengthens pair-bonding. According to Morris, we humans are a type of hyper-sexualized ape. Men and women are so attracted to each other that they end up staying together for long periods of time.

Pair bonding was necessary for several reasons. For one, it permitted a division of labor between parents so that one could care for a long-term dependent juvenile and rely on the other to regularly bring home the bacon for both of them. It also reduced competition for mates and promoted cooperation within the tribe, which was required for a successful hunt.

I must say, I found this all pretty intriguing as a 16-year old guy: we lost our hair as a kind of sex aid to help bring men and women closer together. Somehow, that made the world seem like a better place. And, I’ve accepted Morris' explanation over the years – perhaps it was the romantic in me – even while recognizing that he was hardly the foremost authority on the issue.

Well, the cover story in the February issue of Scientific American is an article by Penn State’s own Nina G. Jablonski, "The Naked Truth." She proposes another explanation for the loss of our fur: to keep cool.

We lost our hair while simultaneously developing an extraordinary number of eccrine sweat glands, which produce a particularly watery and easily evaporated sweat. Other mammals have a more oily sweat which coats the hair and cools the animal but does not dissipate heat.

As climate change turned the jungles into savannahs, our ancestors had to travel ever longer distances in search of food and water. At the same time, probably also because of food shortages, hominids began to incorporate meat into their diets. Unlike plants, animals are moving targets and require the expenditure of even more energy in order to eat. Projectile weapons like spears had not yet appeared on the scene and early hominids were not fast enough to catch their prey. The only reliable way to kill it would have been to run it down to exhaustion over a long distance, usually in the heat of the day.

This persistence hunting, however, created a serious risk of overheating. To reduce this risk, hominids developed both an increased sweating ability and the loss of hair to promote evaporation. As Jablonski points out, according to a recent paper in Sports Medicine, "our cooling system is so superior that in a marathon on a hot day, a human could outcompete a horse."

This is one more piece of evidence that humans were designed for endurance running. This is who we are. Hey, if you’re not out on the roads regularly, you are denying your own humanity.

So, between Morris and Jablonski, who’s right? Did we lose our hair so that we could become better lovers or better runners? (There's a third possiblility rejected by Jablonski in her article that says that our hair loss was to make us better swimmers.) I honestly like both Morris' and Jablonski's explanations. Stevie Wonder sings “I was made to love” but Springsteen reminds us, “Baby, we were born to run.”