Evolutionary Origins of the Hairiness of Humans
In biology, hair serves as one of the many defining traits of the mammalian family (others include: mammary glands and the middle ear). When thinking of appendages, you would probably think of protrusions from the midline of the human body, such as arms, hands, fingers, legs; i.e. something large and thick. However, hair is also considered an appendage of the human, with quite a few useful functions. While some humans prefer to shave off hair in many areas of their body, they should keep in mind that hair can serve as protection from solar radiation as well as function as a social tool that aids in expression of emotion and social intercourse. However, no matter how much we compare our hairiness with each other, we will rarely if ever surpass the furriness of our primate family. Why are humans the only primate show such a lack of fur throughout the body that they are often described as naked? All of our closest ancestors have fur, so why don't we?
This critter is smiling because it has fur, and can still swim faster than humans
Conjecture 1: Aquatic Ape
In 1960, Alister Hardy, an English marine biologist, voiced his thoughts that the loss of hair for homo sapiens may be related to how blubber evolved in aquatic mammals. He hypothesized that the ancestors of humans may have undergone a phase where they adapted to a wet environment and then returned to terrestrial living before they undergone more changes that would render them completely aquatic. However, the fact that many aquatic mammals such as otters and seals do have dense fur sheds doubt on this hypothesis. While dolphins and whales have smooth skin similar to that of humans, they also have much greater mass than humans. Hair in this situation might cause these large mammals to overheat in the tropical climates that they migrate to. The hypothesis has also been criticized for not being parsimonious, because this sort of evolution would need to explain why fur had been lost as humans were forced to aquatic environments and then need to explain why the lack of hirsutism remained as a positive trait on land.
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Conjecture 2: Intense hunting leads to rapid release of heat
While Charles Darwin introduced the idea of hair loss due to thermoregulation in humans, Dr. Peter Wheeler came up with the underlying theory behind the thinning of body hair. He argued that with the period of global cooling, the forests became plains and savannahs, leading to much hotter and less humid weather. With the lack of abundant shade, and the need to travel farther distances to hunt prey, a dense heavy coat would cause humans to overheat and prevent perspiration. The prevalence of hairless humans also include an overlap with the Endurance Running hypothesis that explains the reason for certain human traits are due to adaptations to long distance running. However, how come humans are still the only hairless species on the savannah? Many animals such as cheetahs and baboons live just fine on the plains. Advocates for the hypothesis point to the study that states bipedal hominids actually show less water loss when the naked skin is exposed to the African savannah.
This female can be seen as a suitable mate. She has no ticks.
Conjecture 3: Ectoparasitism
The last hypothesis introduces that the loss of hair in humans as a counterattack to the prevalence of parasites (and the diseases they bring). The thinning of hair would allow for faster and easier removal rate of harmful parasites on the human body. The hypothesis becomes more fleshed out when we learn that at this point in time, humans had already established a primitive culture, found fire, and had started creating clothes. These three factors would allow for humans to retain body heat while allowing the selection for thinner hair to continue. Further selection for thinner hair in humans may also be due to sexual selection; potential mates with thinner hair and smooth, unmarred skin could openly exhibit their lack of parasitic infection to potential mates. In an interesting side note, naked mole rats live in huge, underground colonies where the parasites could be easily transmitted. However, due to the hairlessness of these critters, they do not suffer from parasitic problems due to the easy detection of foreign objects on their bodies. Researchers have hypothesized that a head full of hair allows for insulation of the part of body that loses heat the fastest, and the abundance of hair in the nether and axilla regions allow for pheromones to linger further., but no concrete studies have been conducted.
Each hypothesis discussed have their strengths and weaknesses with no single hypothesis completely dominating the discussion due to lack of conclusive findings. The above hypotheses discussed above only serve to act as a very general overview of the reasons for the nakedness of humans.
Bhattacharya, Shaoni (2003), “Early Humans Lost Hair to Beat Bugs,” New Scientist, [On-line],URL: http://www.newscientist.com/news/news.jsp?id=ns99993807.
Morgan, Elaine (1997). The Aquatic Ape Hypothesis. Penguin.
Wheeler, P (1984). "The evolution of bipedality and loss of functional body hair in hominids". Journal of Human Evolution 13: 91.