Why do we have hair in such random places? - Nina G. Jablonski

We have lots in common with our closest primate relatives. But comparatively, humans seem a bit... underdressed. Instead of thick fur covering our bodies, many of us mainly have hair on top of our heads—and a few other places.

So, how did we get so naked? And why do we have hair where we do? Human hair and animal fur are made of the same stuff: filaments of the protein keratin that grow out of organs known as follicles, which go through cycles of growth and shedding. Across mammalian species, hairs have been modified for numerous purposes, ranging from the soft fluff covering rabbits to the rigid quills protecting porcupines.

But for many mammals, hair grows in two layers consisting of a shorter undercoat of ground hairs covered by longer guard hairs. Together, they help insulate the animal’s body and protect its skin. Human hairs, on the other hand, are kind of a combination of these hair types. Unfortunately, hair is rarely found in fossils, making it hard for researchers to pinpoint when and how our ancient ancestors lost their coats.

But scientists have developed some working hypotheses. It seems that, millions of years ago in Africa, early hominins first transitioned out of trees and adopted a more active lifestyle. Keeping cool became increasingly important. Eventually, they developed more sweat glands, which helped them lose heat by evaporating moisture through the skin. In fact, humans have 10 times more sweat glands than chimpanzees, for instance.

But efficiently losing heat by sweating is harder to do when you’re covered in fur. Scientists believe that early humans lost much of their coat around this time to help their sweat evaporate faster. However, if losing our hair was so advantageous, why do we have any left at all? It seems that there are unique uses for hair in different parts of our bodies.

When it comes to the tops of our heads, temperature regulation likely played a part again. Since early humans began venturing into the open, their heads would’ve been exposed to the scorching sun. Thicker, longer-growing hair protects our sensitive scalps and keeps our brains from overheating. Dark, tightly curled hair is most effective at keeping solar radiation off of skin. Other kinds of head hair evolved as humans moved to different places.

Meanwhile, researchers think eyebrows are especially useful for communication because they sit atop active facial muscles that convey our feelings. Eyelashes have been shown to minimize airflow over our eyeballs, preventing them from drying out and catching debris. And maybe facial hair proved helpful in distinguishing identity from a distance, but we really don’t know. Evidence is stubbly at best.

Why we have hair in other regions is... more pungent. Our armpits, nipples, and pubic areas are dotted with apocrine glands. They produce oily, smelly secretions which the thick, curly hair that often grows in these spots helps disperse. The secretions that waft off these hairy patches may be useful for identification. For example, several studies have shown that people are able to identify their own armpit odors as well as those of people they’re close with.

The final type of notable human hair is the vellus hair that covers our bodies. We don't know if these hairs serve any purpose themselves, but the follicles vellus hair grows from are essential banks of stem cells that repair damaged skin after injury. They’re also important sites of nerve endings that convey signals of gentle touch to the brain. In fact, although it’s much finer, humans have roughly the same density of body hair as apes of comparable sizes.

So despite all this talk of human nakedness, we're not actually as hairless as we look.