Sunday, March 18, 2012


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
                                                        Source: http://www.flickr.com/photos/32864904@N02/4596772664/

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.

                                         Source: Windows 7
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.
                                                                         Source: http://www.drclay.com/wp-content/uploads/2010/04/female-back.jpg

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.

Sources
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.  

12 comments:

  1. I thought this post was really fascinating. I'd never thought about why humans aren't any hairier before, but it really is surprising considering some of our closest living relatives, like chimps, are covered in hair. All three hypotheses seem plausible, to an extent. I'm interested to see if eventually one emerges as the most likely answer.

    -Hayley Hemstreet

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  2. I like your choice of pictures.

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  3. Jk. For your second conjecture, I was wondering if heat loss / prevention might be the reason that some insects have hair, or if they are simply for sensory purposes.

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    1. That's an interesting question! While the hair on insects and mammals can look and feel similar, they tend to serve wildly different functions. Insect hair, or setae, are hollow, allowing different functions. On example would be extending nerves into the setae to aid in taste, smell, sensing temperature or even triggering reflexes. They can also release harmful chemicals or provide extra surface area for water walking. In short, the setae on insects serve as containers for a variety of functions.

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  4. Do you think hairlessness has a correlation with intelligence or social advancement? As in, are humans more intelligent/socially advanced than other animals due to our lack of fur?

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    1. I think that hairlessness may have had an indirect effect on intelligence through allowing humans to hunt unhindered, but as a direct effect, no. While creatures such as dolphins and whales have reputations for being intelligent, other animals such as dogs, crows, and most of the primate family also have reputations for being intelligent as well. I hesitate to say this because I'm sure there are exceptions, but I think that environmental pressures has had a much larger effect to do with human intelligence than hairlessness. Environmental pressures as in that something in the environment, be it harsh weather, or predators caused humans to rely more on creativity than brute force to overcome and survive. If you've ever seen a chimpanzee or orangutan without hair, you'll note that they are extremely well muscled; they can be many times stronger than the average human through they may be much smaller. Somewhere along the line, instead of selecting for larger muscles or brute strength, intelligence was selected for instead. From a general increase in intelligence, we get an increase in social intelligence (which is needed for our form of the mating ritual).

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  5. For mentioning naked mole rats, my offer of marriage still stands. If you don't want to go with that, I'm good with just making you sandwiches.

    But that aside...

    I can definitely see that sexual selection might play a role (see Nate's first post). I also read a pretty interesting article about why humans HAVEN'T completely lost their hair a while back, although I can't find the link right now - I believe one study showed that it did play a slight sensory role.

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    2. From past experience, I know that my head hurt a lot less after bumping into sharp corners if I had more hair to cushion the blow. So, if having hair meant that potentially damaging injuries might be reduced and survival rate in creased, I think that factor could also play a role in natural selection.

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  6. So I was recently talking to my friends about how much harrier men are than women, and I realized that the "intense hunting" hypothesis doesn't make sense when you consider the original roles of males and females in early human societies: Men were the hunters and women were the gatherers. If the loss of hair correlates with preventing overheating during hunting, why do most men still have thick chest and back hair while women have thinner hair?

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    1. You're right! That's an interesting point that I hadn't thought about. The only thing off the top of my head would be that when the human population moved to colder climates, hairier men saw more success since they lost less body heat and were able to stay out to hunt for longer periods of time. In milder climates, the hairiness of men were less noticeable because there wasn't any need for hair to grow. 

      Another factor might be that hairiness could be stimulated by testosterone and sexual selection selected men who had more hair in more unfavorable climates.

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    2. Ahhh that makes sense...I wonder if there's ever been any studies comparing the hairiness of men from different climates...I'll look that up tomorrow.

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