While human beings have many living ancestors — bonobos, chimps, gorillas — our absence of fur immediately marks us as something different.
And though our big brains and bipedal posture have taken us to outer space, the reason our species transitioned to a mostly hairless body remains somewhat of a mystery.
Only a few other mammals share our genetic preference for sleek bodies, including rhinos, whales, elephants, and — everyone’s favorite — the naked mole rat.
In fact, humans do technically have lots of hairy potential. The average person has about five million hair follicles across the body, but the vast majority produce only what’s known as vellus hair. These fuzzy little hairs often grow all over us but are too small to really help with insulation.
So why, other than our heads and a few other places, don’t our bodies grow thick hair anymore? Scientists don’t know when or why this evolutionary shift happened.
But they definitely have some theories.
One theory suggests that hairless bodies helped early humans stay cool during prolonged hunts. Photo: Shutterstock
Dominant hair-loss theories
Currently, the prevailing view is the body-cooling hypothesis, also known as the savannah hypothesis. It suggests that early humans ditched the fur coat to help regulate their body temperature.
When Homo erectus and other ancestors of modern humans hunted during the Pleistocene, they pursued their prey for many hours to drive it to exhaustion. That allowed them to take down big animals without advanced hunting tools, which the fossil record suggests they didn’t have yet.
A 2011 study in the Journal of Human Evolution supports the theory of shedding fur to keep the body cool while running for long distances under the African sun.
“Our model suggests that for endurance running to be possible, a hominin would need locomotive efficiency, sweating rates, and areas of hairless skin similar to modern humans,” the authors wrote.
Many human populations haven’t lived in extremely cold parts of the planet for thousands of years while endurance running. Our ancestors did mainly hot-weather marathons.
Two other biologists have a supportive theory based on microbiology. Called the ectoparasite hypothesis, it was first posited in 2003 by Mark Pagal of the University of Reading and Walter Bodmer of the University of Oxford. They argued that a furless hominin would have fewer parasites, resulting in less death from disease.
“If you look around the world, ectoparasites are [still] an enormous problem in the form of biting flies that carry disease,” Pagal said in a BBC story. “And those flies are all specialized to land on and live in fur and deposit their eggs in fur…Parasites are probably one of the strongest selective forces in our evolutionary history, and still are.”
Australopithecus afarensis, one of our most ancient ancestors, had quite a bit more hair than modern humans. Photo: Shutterstock
What’s up with our still-hairy parts?
There are other theories, of course.
Some scientists believe humans lost thick hair just 200,000 years ago when we started using clothing made from the fur of other animals. And Charles Darwin even took a swing at the problem: He said it was simply a result of the sexual selection of humans looking for sleeker mates. (Unsurprisingly, modern researchers reject this idea.)
As for the thick hair that remains on our bodies (heads, armpits…you get the idea), biologists have a few ideas, though nothing backed by iron-clad evidence.
Our hairy heads likely continue to protect us from solar radiation. And the hair in our armpits and pubic areas might be what scientists call a spandrel — leftovers from primate ancestors that used pheromones to communicate. (There’s no evidence humans still do that.)
The mystery of our hairless bodies might remain an evolutionary secret, but it almost certainly coincided with humans gaining darker skin to guard against UV radiation.
“It’s the logical inference that we can make,” Tina Lasisi, a biological anthropologist at the University of Southern California, told the BBC.
It’s clear from recent scientific discoveries, however, that humans still have the latent genes for a luxurious, full-body fur coat. And whether it’s Robin Williams or Mexico’s Wolf Man, it’s clear that some human bodies still want a furry exterior.
