The hairlessness of humans compared to related species may be due to loss of functionality in the pseudogene KRTHAP1 (which helps produce keratin) in the human lineage about 240,000 years ago. Mutations in the gene HR can lead to complete hair loss, though this is not typical in humans.
In order to comprehend why humans are essentially hairless, it is essential to understand that mammalian body hair is not merely an aesthetic characteristic; it protects the skin from wounds, bites, heat, cold, and UV radiation. Additionally, it can be used as a communication tool and as a camouflage. To this end, it can be concluded that benefits stemming from the loss of human body hair must be great enough to outweigh the loss of these protective functions by nakedness.
Humans are the only primate species that have undergone significant hair loss and of the approximately 5000 extant species of mammal, only a handful are effectively hairless. This list includes elephants, rhinoceroses, hippopotamuses, walruses, pigs, whales and other cetaceans, and naked mole rats. Most mammals have light skin that is covered by fur, and biologists believe that early human ancestors started out this way also. Dark skin probably evolved after humans lost their body fur, because the naked skin was vulnerable to the strong UV radiation as would be experienced in Africa. Therefore, evidence of when human skin darkened has been used to date the loss of human body hair, assuming that the dark skin was needed after the fur was gone.
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