Humans are the most advanced mammal on this Earth. The occurrence of body hair is a characteristic feature of mammals. In fact, presence of body hair is an evolutionary attribute in the human lineage. However, loss of body hair in humans, was still a mystery. It is really amazing that humans have long hairs on their scalp; while some regions have comparatively shorter hair and some have no hairs like palms, soles of feet and underside of wrist.
What makes this difference to appear in all humans? Though, ancestors of humans were hairy like gorilla and chimpanzee; which mechanism made humans a naked ape? Several theories have been put forward by many scientists regarding this.
Darwin suggested that sexual selection is responsible for this as ancestors of humans preferred less- hairy mates. Some scientists proposed that it was an adaptation to get rid of parasites dwelling in the hair like hair- lice. On the other hand, some scientists argued that it was a mechanism for thermoregulation. They explained that when ancestors of humans started bipedal movement, they started to get more exposure of drier environment in comparison to earlier. Walking or running for hunting in this environment posed a risk of overheating. In this situation, reduced body hair and increased sweat glands were favored. This allowed thermoregulation more effectively via perspiration.
The aquatic ape hypothesis states that human ancestors inhabited Savannahs of Africa. They depend on the hunting to meet their food requirements. During summers, they used to shift near riverbanks. They used to gather aquatic weeds, shellfish and other food resources to fulfil their daily needs. As hair does not work as good insulator in the water, they developed another adaptation for thermoregulation. They lost their body hair and fat got deposited beneath their skin to provide the insulation.
Another fascinating theory proposed for hair loss in humans was put forward by an evolutionary neurobiologist, Changizi. He stated that disappearance of hair from the face and some of the pubic hair was to promote emotional communication. He argued that emotions can be seen on the face only when it is hair less, viz., skin becomes red in anger, pink during sexual attraction, green or blue in illness etc. The genetic cause behind human body hair loss was also tried to find. It is observed that in many mammals including human beings, the plantar skin which is present on the underside of the wrist as well as footpads are hairless. However, in few mammals viz., polar bears and rabbits, the plantar region remain covered by fur. During a study of plantar area in rabbits, it was analyzed that an inhibitor protein, Dickkopf 2 or Dkk 2 were not present is high concentration. On the basis of these results, it was concluded that Dkk 2 is essential for the growth of hair. Later, in an another experiment on mice, it was observed that high level of Dkk 2 protein was present in the plantar region. It suggested that Dkk2 protein blocks hair growth only on some parts. It is achieved by inhibiting a signaling trail known as WNT, which is responsible for managing hair growth.
When mutant mice, in which Dkk 2 production was inhibited, were compared with normal mice, it was discovered that in mutant mice hair growth occurred in plantar region also. It confirmed the previous hypothesis that inhibitor plays an important role in determining the area of hair growth. However, a significant difference was observed in the hair of mutant mice. They were comparatively smaller, thinner and less evenly spaced.
This is important for investigating the reason of baldness. In human beings also, WNT pathway exists whose blocking by Dkk 2 or any other inhibitor may result in baldness. It will be helpful in recognizing the cause of some other skin conditions like psoriasis and vitiligo etc.
Dr. Anamika Tripathi