There's no other way to say it: human hair is strange. Compared to most other mammals, human beings are decidedly naked, and the little fuzz that we do have is oddly patchy and quite diverse.
The most obvious of our bizarre bald regions are perhaps the soles of our feet and the palms of our hands. Here, our sleek and hairless skin sets us apart from other mammals, such as rabbits and polar bears, who both sport fur along their paws.
It's an enigma that continues to puzzle, but researchers may now have an explanation for why some parts of the human body have hair and others don't.
A new study on mice has revealed an important molecular pathway that keeps the undersides of our feet and hands as smooth as a baby's bottom.
The explanation is centred on a small molecular messenger, called a Wnt protein, which carries information between cells about the initiation, spacing, and growth of body hair.
"We know that Wnt signalling is critical for the development of hair follicles; blocking it causes hairless skin, and switching it on causes formation of more hair," senior author Sarah Millar, a dermatologist at the University of Pennsylvania, told Cosmos.
"In this study, we've shown the skin in hairless regions naturally produces an inhibitor that stops Wnt from doing its job."
The inhibitor is a protein called Dickkopf 2 (DKK2), and when it is removed from mice completely, the researchers noticed something curious. While the mutant mice continued to be hairy in all the right spots, the normally bare regions of their paws were now also sprouting little hairs.
In normal mice, the plantar region of the foot is free of fur, but in all 40 mutant mice, the researchers found fully-formed, mature hair follicles embedded in that very spot. And even when these hairs were plucked, these unusually hirsute bits grew back, just like normal fur.
When the researchers turned their attention to rabbits, they noticed something similar. Like polar bears, rabbits also grow hair on the bottoms of their feet, probably to keep their toes warm while they hope across snow and ice.
In this case, DKK2 also appears to play a role – or, rather, its absence does. In rabbit plantar skin, the study found that DKK2 is not expressed at high levels, and this is probably why fur is able to develop there.
The results were unexpected. When the study was first started, the researchers thought that DKK2 might be responsible for the pattern of hair follicles that develops on the body. But the new findings suggest that its role is kind of the opposite.
The researchers now think that the presence of DKK2 is responsible for keeping certain parts of the body free from hair. So, in certain cases where this inhibitor is not present, the Wnt signalling pathway is left to its own devices, activating the appropriate stem cells, called β-catenin, and causing hair follicles to develop in places where they shouldn't.
As for why this happens, Millar and her team think the presence or absence of DKK2 is probably based on evolutionary benefits.
For instance, while rabbits and polar bears might need hairy paws to survive, if human hair extended all the way to our hands and feet, it would probably make our lives a hell of a lot more difficult, not to mention messy.
At this point, it's still not clear exactly why humans would have developed bare hands and feet, but there are a few hypotheses.
Some suggest that it could be about sexual selection; others say it has more to do with thermoregulation, as we moved from the forests to the Sun-beaten savannah; while still others think our nakedness protects us from external parasites, like lice and other pesky bugs.
There are even those who think there is no evolutionary benefit whatsoever, and this was just a sneaky little trait that hitched a ride into the future.
Regardless of which explanation is correct, if the new findings extend to humans, it means that with just one slight genetic change, we could get hair to grow on the otherwise bare surface of our hands and feet.
As such, the authors of the study hope that one day, their research might help us figure out a way to block or promote the growth of body hair.
This sort of treatment could be extremely useful for a whole variety of health issues, including male pattern baldness, and helping to treat burns patients and people with skin issues, such as psoriasis.
"While more research is needed to improve our understanding of this pathway, our results suggest that therapeutics capable of decreasing levels of Wnt/β-catenin signaling in the skin could potentially be used to block growth of unwanted hair, and/or to treat certain skin tumours," says Millar.
"Conversely, if delivered in a limited, safe, and controlled way, agents that activate Wnt signaling might be used to promote hair growth in dormant hair follicles in conditions such as male pattern baldness."
This study has been published in Cell Reports.