Humans Don't Have Hairy Soles and Palms Because of One Protein

Life would be very different if we had hair on our palms. Handshakes would be cozier, eating hotdogs would be messier, and the glove industry would tank. Our hand-hairlessness sets us apart from other mammals, like polar bears and rabbits, who have hair alongside the pads of their feet. In a study that will be released Friday in Cell Reports, scientists explored why we lack hair on our palms and soles and discovered that those parts of the body contain a special something that’s missing from our arms and legs.

Scientists from the University of Pennsylvania announced that hairless skin contains a naturally-occurring, secreted inhibitor that blocks the WNT signaling pathway. This pathway controls hair growth and is critical for the development of hair follicles. The inhibitor — a protein called Dickkopf 2 (DKK2) — stops the pathway from doing its job and is why we have palms like guava skin.

“Based on some published data, we were initially expecting that DKK2 might play a role in generating the pattern of hair follicles in hairy skin, which proved not to be the case,” study co-author Sarah Millar, Ph.D. tells Inverse. “Instead, we found that it has an unexpected function in establishing areas of hairless skin.”

Millar, a professor of dermatology and the director of the Penn Skin Biology and Diseases Resource-based Center, and her team determined that hairless regions contain DKK2 by working with a subject analogous to the underside of a human wrist — the plantar skin from mice. This is the skin on the bottom of their feet. When they analyzed the skin tissue, they found that DKK2 was highly expressed. Furthermore, when they genetically removed the protein, hair began to grow in the previously hairless area.

This result was a good hint that some animals, like humans and mice, evolved in a way that now drives the production of DKK2 in specific skin regions. To test this further, they analyzed the plantar skin of rabbits — who do naturally develop hair on their soles. Millar and her team found that rabbits, unlike mice, do not have high levels of the protein — which explains why they can grow hair there.

While scientists don’t know why rabbits have plantar hair, they think that polar bears have hair in this region because it helps insulate their feet when they are walking on snow and ice. Meanwhile, scientists assume humans don’t have hairy palms and soles because smooth hands help us grip surfaces. We are less hairy than our mammal peers as a whole — an evolutionary difference that scientists theorize could have emerged as a means to help us stay cool while traveling, avoid parasites, and attract mates.

Now, Millar and her team aim to use their research to help people who are hairless because of diseases. According to the American Academy of Dermatology, more than 80 million people have female or male pattern baldness, technically known as androgenetic alopecia. DKK2 could be a potential therapeutic target associated with this condition.

“A nucleotide polymorphism in the human DKK2 locus is associated with increased risk for androgenetic alopecia,” says Millar. “It is possible that elevated DKK2 expression or function contributes to miniaturizing hair follicles, but whether this variant affects DKK2 expression or function, and whether it plays a functional role in this condition is not yet known. This is a fascinating area for future studies.”