Why Did Humans Lose Their Fur?

Why Did Humans Lose Their Fur?
July 12 10:03 2019 Print This Article

We are the naked apes of the world, having shed most of our body hair long ago

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.”

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, which is known to control hair growth.

To investigate, the team compared normally developing mice with a group that had a mutation which prevents Dkk2 from being produced. They found that the mutant mice had hair growing on their plantar skin, providing more evidence that the inhibitor plays a role in determining what’s furry and what’s not.

But Millar suspects that the Dkk2 protein is not the end of the story. The hair that developed on the plantar skin of the mice with the mutation was shorter, finer and less evenly spaced than the rest of the animals’ hair. “Dkk2 is enough to prevent hair from growing, but not to get rid of all control mechanisms. There’s a lot more to look at.”

Even without the full picture, the finding could be important in future research into conditions like baldness, since the WNT pathway is likely still present in chrome domes—it’s just being blocked by Dkk2 or similar inhibitors in humans. Millar says understanding the way the inhibitor system works could also help in research of other skin conditions like psoriasis and vitiligo, which causes a blotchy loss of coloration on the skin.

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.

With a greater understanding of how skin is rendered hairless, the big question remaining is why humans became almost entirely hairless apes. Millar says there are some obvious reasons—for instance, having hair on our palms and wrists would make knapping stone tools or operating machinery rather difficult, and so human ancestors who lost this hair may have had an advantage. The reason the rest of our body lost its fur, however, has been up for debate for decades.

One popular idea that has gone in and out of favor since it was proposed is called the aquatic ape theory. The hypothesis suggests that human ancestors lived on the savannahs of Africa, gathering and hunting prey. But during the dry season, they would move to oases and lakesides and wade into shallow waters to collect aquatic tubers, shellfish or other food sources. The hypothesis suggests that, since hair is not a very good insulator in water, our species lost our fur and developed a layer of fat. The hypothesis even suggests that we might have developed bipedalism due to its advantages when wading into shallow water. But this idea, which has been around for decades, hasn’t received much support from the fossil record and isn’t taken seriously by most researchers.

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