Why Aren’t Humans as Hairy as Other Mammals?

Humans are members of a weirdly exclusive club of mostly naked mammals, sharing that honorable distinction with the likes of elephants, rhinos, and whales. Mole rats, too, but they’re gross. Most mammals are hairy, and our ancient ancestors were too, but we ditched most of it at some point along our evolutionary journey. Why?

According to University of Pittsburgh assistant professor of computational and systems biology, Maria Chikina, it all comes down to heat management.

In a recent article in The Conversation, Chikina explained that around seven million years ago, we split from our chimp cousins and evolved to become the sweatiest mammals on Earth to give us a competitive edge.

Early humans living in Africa developed more sweat glands, slowly ditching the fur in the process. In doing so, we gained the ability to endure long-distance runs that helped us become persistent hunters, creatures that could chase animals across the savanna until they passed out while we were just out of breath.

Hair still has plenty of practical uses. It’s our natural sunscreen, armor, and an early-warning system that, for instance, can tip you off to the spider on your arm. Humans didn’t lose hair so much as we toned it down. We have all the genes to go full Chewbacca, they’re just turned off.

Sometimes, those genes accidentally flip back on. You might have heard of or have seen pictures of people with “werewolf syndrome,” like the teenager who set the Guinness World Record for the hairiest face.

Since “werewolf syndrome” is a bit of a pejorative, the condition’s scientific name is hypertrichosis, and it’s what humans would look like if the genes that pull back on our body hair were suddenly flipped back on, revealing that deep down, we’re just as hairy as our primate ancestors.