Millions of modern humans ask themselves the same question every morning while looking in the mirror: Why am I so hairy? As a society, we spend millions of dollars per year on lip waxing, eyebrow threading, laser hair removal, and face and leg shaving, not to mention the cash we hand over to Supercuts or the neighborhood salon. But it turns out we are asking the wrong question—at least according to scientists who study human genetics and evolution. For them, the big mystery is why we are so hairless.
Evolutionary theorists have put forth numerous hypotheses for why humans became the naked mole rats of the primate world. Did we adapt to semi-aquatic environments? Does bare skin help us sweat to keep cool while hunting during the heat of the day? Did losing our fur allow us to read each other's emotional responses such as fuming or blushing? Scientists aren't exactly sure, but biologists are beginning to understand the physical mechanism that makes humans the naked apes. In particular, a recent study in the journal Cell Reports has begun to depilate the mystery at the molecular and genetic level.
Sarah Millar, co-senior author of the new study and a dermatology professor at the University of Pennsylvania’s Perelman School of Medicine, explains that scientists are largely at a loss to explain why different hair patterns appear across human bodies. “We have really long hair on our scalps and short hair in other regions, and we’re hairless on our palms and the underside of our wrists and the soles of our feet,” she says. “No one understands really at all how these differences arise.”
A reconstruction of the head of human ancestor Australopithecus afarensis, an extinct hominin that lived between about 3 and 4 million years ago. The famous Lucy skeleton belongs to the species Australopithecus afarensis. (Photo by Tim Evanson / Reconstruction by John Gurche / Flickr / CC BY-SA 2.0)




In many mammals, an area known as the plantar skin, which is akin to the underside of the wrist in humans, is hairless, along with the footpads. But in a few species, including polar bears and rabbits, the plantar area is covered in fur. A researcher studying the plantar region of rabbits noticed that an inhibitor protein, called Dickkopf 2 or Dkk2, was not present in high levels, giving the team the fist clue that Dkk2 may be fundamental to hair growth. When the team looked at the hairless plantar region of mice, they found that there were high levels of Dkk2, suggesting the protein might keep bits of skin hairless by blocking a signaling pathway called WNT, w.
A more widely accepted theory is that, when human ancestors moved from the cool shady forests into the savannah, they developed a new method of thermoregulation. Losing all that fur made it possible for hominins to hunt during the day in the hot grasslands without overheating. An increase in sweat glands, many more than other primates, also kept early humans on the cool side. The development of fire and clothing meant that humans could keep cool during the day and cozy up at night.
But these are not the only possibilities, and perhaps the loss of hair is due to a combination of factors. Evolutionary scientist Mark Pagel at the University of Reading has also proposed that going fur-less reduced the impact of lice and other parasites. Humans kept some patches of hair, like the stuff on our heads which protects from the sun and the stuff on our pubic regions which retains secreted pheromones. But the more hairless we got, Pagel says, the more attractive it became, and a stretch of hairless hide turned into a potent advertisement of a healthy, parasite-free mate.
One of the most intriguing theories is that the loss of hair on the face and some of the hair around the genitals may have helped with emotional communication. Mark Changizi, an evolutionary neurobiologist and director of human cognition at the research company 2AI, studies vision and color theory, and he says the reason for our hairless bodies may be in our eyes. While many animals have two types of cones, or the receptors in the eye that detect color, humans have three. Other animals that have three cones or more, like birds and reptiles, can see in a wide range of wavelengths in the visible light spectrum. But our third cone is unusual—it gives us a little extra power to detect hues right in the middle of the spectrum, allowing humans to pick out a vast range of shades that seem unnecessary for hunting or tracking.
Changizi proposes that the third cone allows us to communicate nonverbally by observing color changes in the face. “Having those two cones detecting wavelengths side by side is what you want if you want to be sensitive to oxygenation of hemoglobin under the skin to understand health or emotional changes,” he says. For instance, a baby whose skin looks a little green or blue can indicate illness, a pink blush might indicate sexual attraction, and a face flushing with red could indicate anger, even in people with darker skin tones. But the only way to see all of these emotional states is if humans lose their fur, especially on their faces.
In a 2006 paper in Biology Letters, Changizi found that primates with bare faces and sometimes bare rumps also tended to have three cones like humans, while fuzzy-faced monkeys lived their lives with just two cones. According to the paper, hairless faces and color vision seem to run together.