Our lack of fur compared to other apes is a long-lasting mystery in biology. There are several hypotheses to explain the reasons behind our lack of hair, but so far we have little evidence to go on. While the why remains mysterious, the how is getting clearer.

In a new study, published in the journal Cell Biology, researchers have discovered that the abundance of a certain protein determines hair growth on paws. If the protein, called Dickkopf 2 or Dkk2, is in low abundance (like in rabbits and polar bears), the plantar region is full of hair. In mice, who have little hair on their paws, Dkk2 was present in higher amounts.

The team proposed that the protein might be blocking a specific signaling pathway, known as WNT, responsible for the growth of hair. To test this, the researchers engineered mice to not produce Dkk2. These animals developed fur on their plantar skin, but the fur there was thinner, shorter, and more randomly spread out than the rest of the animals' hair. The protein clearly plays an important role, but it is not the full picture.

“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,” co-senior author Professor Sarah Millar, a dermatology expert from Perelman School of Medicine at the University of Pennsylvania, told the Smithsonian. “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. No one understands really at all how these differences arise.”

Although these observations don't provide the full picture, they do reveal intriguing clues about hairiness and baldness. Researchers think that the WTN pathway is key, and the next step is to investigate what other proteins might be inhibiting this pathway. This could lead to a more complete picture of how we lost our fur and even provide potential medical applications. It will certainly help in conditions like baldness or alopecia. It might also help us understand better certain skin conditions such as psoriasis and vitiligo.

The mainstream theory for why we lost our fur suggests we needed to better thermoregulate when we moved from shady forests into the hotter savannah, allowing us to hunt during the day. Another suggests it reduced parasites. It could be the “aquatic ape theory” as well, which suggests that our ancestors waded and swam for food and fur was not ideal for that. It could even be multiple factors.

[H/T: Smithsonian]