Humans are considered strange members of the mammalian class. Almost every other mammal, aside from hippopotamus and naked mole rats, has fur covering on its body. Save for the hair on our heads, humans are practically naked.
Importance of Mammalian Body Covering
For evolutionary biologist Mark Pagel from the University of Reading in the UK, it is crucial to understand why mammals need fur in the first place. It has something to do with the fact that mammals are warm-blooded animals and need to regulate their body temperature to survive. As a body covering, the fur keeps the mammals warm when it is cold at night and protects them from the Sun during the day.
Being exposed to harsh environments means that mammals should have protection from various elements of nature. This is achieved by covering their body with a layer of fur, which can also serve as sensory input.
Fur is also important in waterproofing, camouflaging, or enticing a potential partner during mating season. Some mammals have evolved their fur by producing a skin pigment that gives the hair its color.
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Long Head Hair in Humans
Experts believe that hairlessness must have granted humans some evolutionary advantage. There are three main theories as to what these advantages could have been.
First, a thick coat of body covering could have led prehistoric humans to overheat under the Sun. Ancient humans also evolved to have more sweat glands than primate relatives. It would have gotten soaked with sweat if they kept their long body hair. This would make it hard for the sweat to evaporate and do its important job of cooling the body.
However, this body-cooling hypothesis failed to explain other aspects of human body hair patterns, like the tendency of men to be hairier than women. It should be noted that humans are covered with tiny and colorless vellus hairs, except on the lips, nipples, palms, and soles of the feet. During puberty, some vellus hairs are transformed into longer, colored terminal hairs.
The second theory is known as the aquatic ape hypothesis. It proposes that ancient humans spent much of their time in water. Since fur weighed them down while swimming, they evolved to lose their hair gradually. However, Pagel claims this hypothesis is hard to believe since no evidence exists that humans spent a significant amount of time in water during their evolutionary past. It also failed to explain why humans did not evolve to regain their fur after leaving the waters.
A third theory was proposed by Pagel himself, which he called the ectoparasite hypothesis. Ectoparasites are parasites that live outside the host's body, like fleas, ticks, and lice. They are a major cause of diseases and mortality across different animal species. Pagel believes that ectoparasites are less attracted to hairless skin, and it may also be easier to get rid of them when they are not buried in fur. In short, having less hair and, thus, fewer parasites may have granted a survival advantage to ancient humans.
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