tumbledry

Hair Theories

The ideas for the following theory have been steeping in my head for a while. And since I’m doing something similar to kottke’s operation clear all browser tabs, you get to read my thoughts on the topic. See, the theory is about why we have a different density of hair on different parts of our body. To begin: evoultionary biology dictates that the different hair densities on our bodies must have conferred an advantage to individuals who had favorable distributions of said hair.

Now, that previous sentence needs some clarification. First, the “advantage” asserted above is an evolutionary term. It refers to a key tenet of evolution: survival alone is not grounds for evolutionary selection - an enhanced ability to survive and reproduce indicate advantageous adaptation. To that end, I offer an oversimplification: the fish who developed a light sensitive patch on its body, the precursor to eyes, could beat out its cousins at finding food… but the fact that this allowed it to reproduce at a higher rate (not simply its survival) was what provided the impetus for the slow change in the gene pool ultimately resulting in eyes. Another point in that original sentence: a “favorable distribution” of hair varies from region to region, an idea which I will elucidate shortly.

Common Sense
This brings us to generally accepted observations: we have hair coverage at a higher density on the top of our forearms than on the bottom, extremely large amounts of hair on our heads, hair surrounding our reproductive organs, hair sprouting from our underarms, and no hair on the bottoms of our feet. Generally, men have denser hair coverage than women. Furthermore, hair coverage varies very widely between different ethnicities. A typical black man has relatively little hair coverage when compared to a typical (sterotypical, even) Italian man. What unified theory could explain all of these observances?

I’ve mixed concepts from my animal behavior, microbiology, immunology, and biochemistry courses in an attempt to propose an outline of a unified theory of human hair coverage. First, we’ll cover that theory, and then I will attempt to explain all of the observations from above using the ideas set forth.

History
Let us begin with early Homo sapiens. We all know they toiled in a hunter-gatherer lifestyle, constantly exposed to the sun’s beating rays. Aside from being uncomfortable and causing visible signs of aging, sun exposure has a far more serious effect: DNA damage (specifically dimerization of adjacent thymines bases). Now, these dimers can be repaired by excision systems (clipping out the damaged region), but we unlucky humans don’t have photolyases, which are enzymes optimized for fixing sun damage. If we did, it is likely that we would never ever get skin cancer from UV rays. So it goes. That’s not to say we’re entirely powerless against fixing DNA damage - we repair tremendous amounts of damage to DNA on a daily basis. However, it is important to avoid overwhelming our repair systems, because the cumulative effects of DNA damage result in one thing we all know: cancer.

The Theory
So, we end up here: hair, a non-living collection of proteins just above our skin’s surface, filters out sunlight/UV rays, thereby reducing DNA damage and ultimately reducing the risk of cancer. A lower risk of cancer allows the ancient human to pass this desirable pattern of hair on.

Application
Applying this theory is the fun part: we’ll take the forearm hair example first. The top of your forearms are exposed to the sun, yet the bottoms of your forearms remain facing the ground, or held in at the body, dramatically reducing sun exposure. Less sun exposure, fewer mutations - thus, less hair required. Why less hair? Well, why waste precious energy on pushing hair out of the skin when there is no benefit? Oh, and I see where you are headed you wily tumbledry reader: how does this sun exposure thing explain underarm hair? Think about what is directly beneath your armpits… right below the skin there… yes, it is an interesection of many blood vessels, but more importantly - what’s there? Ok, I’ve given you plenty of time to wikipedia into the article on lymph nodes. Specifically, you may have noticed the axillary lymph nodes sit right beneath your armpits. This is one of the reasons it hurts so much to get pinched there. Anyhow, the lymph nodes play key roles in your immune system, and this specific group has been linked to breast cancer. Thus, one could say that hair growth in the underarms mitigated carcinogenic UV DNA damage in ancient humans.

The theory seems to continue working, too. Hair around the reproductive organs would prevent sun damage to critical gametes in males, and prevent tumors from blocking the birth canal in females. A dearth of hair on the bottoms of the feet would occur as sun exposure in that region is so low. Hair on our heads? Protects from brain cancer. Hair along the center of the chest? Well, this would shade the sternum, which is extremely close to the skin, and would be susceptible to DNA damage (bone marrow, etc.). For the ladies among you wondering where your chest hair went, we move to the next point.

Men vs. Women
So, why have women ended up with less hair than men? Well, less testosterone produces the secondary sex characteristics we associate with females, but we are looking for an evolutionary reason for hair coverage. To this end, I would guess that if the men were our hunting, running over the plains and such in open sun, the women could select sheltered areas in which to anchor the community. This shelter from the sun could result in the “men are hairier than women” result we observe today.

Ethnicities
Those with the darkest skin color are best adapted to a life under the sun. Thus, there’s no point in making excess hair all over the body. Those of Italian ancestry have paler skin, yet a lot of sun exposure, which necessitates a lot of hair. Scandinavians - I know you think you’ve got me there - they are quite pale, why aren’t they quite hairy to compensate and block the UV rays? The cold areas of the world far from the equator never have very intense sunlight, plus any culture surviving there would have to develop a comprehensive garment system if it was to survive. Clothes for thousands of years prior to those in the south = less hair.

Another Theory
I did run across an extensive and surprisingly interesting scientific discussion about hair, which was kicked off in a post about the complications of Brazilian waxing. Though mostly anecdotal in nature, the opinions and perspectives of the commenters were frequently backed up by real science. This situation is the polar opposite of one like, say, YouTube comment threads, which will forever go down in history as putrid cesspools of thoughtless idiocy. Don’t get me wrong, though, I love the videos there—just not the comments. Anyhow, a man who does research on commensal skin bacteria wrote in that thread:

The reason humans have hair in the places they do, is to provide a proper niche for commensal skin bacteria. These are the autotrophic ammonia oxidizing bacteria (the subject of my research and my blog).

Of course bathing does remove these bacteria, and because they are slow growing (doubling time ~10 hours), they take a long time to grow back (a month or so without sufficient innoculation).

Interesting stuff, but most interesting is the folllwing (emphasis mine):

They suppress heterotrophic bacteria probably by oxidizing quorum sensing compounds. If you have a sufficient biofilm of these bacteria, you don’t need to bathe to remove the heterotrophic bacteria that cause odor.

This is a fascinating direction of reasoning as to why hair is where it is. You can read his full comment here.

Closing Remarks
That is about all I have to say about hair. I am writing this portion because when I outlined this post, I wrote “closing remarks” as a section, like I would have something meaningful to say here.

6 comments left

Comments

Dan McKeown

Question:

Why is it that people of Asian heritage have less hair (traditionally) even though many of their cultures live in tropic regions that receive a lot of sunlight? Also, facial hair, why not just cover the entire face with hair (besides the necessary openings)? How is it that we have eyebrows (even women!), mustaches, and beards but not say…nose sprouts?

Just a few things I was wondering after reading this very interesting post.

Alexander Micek

I actually got to wondering that same thing with regards to the Asian folks - considering they make up about 20% of the world’s population, it’s a significant hole in my thoughts/theory, haha. Haven’t thought of anything good yet, frankly.

That said, I believe I read eyebrows serve the purpose of keeping sweat out of the eyes. The facial hair, especially the lack of it on the forehead, is a bit vexing to me. Ideas are welcome…

Adam Caulfield

I think that this is the most interesting post I have ever read on Tumbledry. Good work Alex!

Nils

Interesting question and theory, but I think you have it a little backwards. Humans didn’t develop hair though evolution, we lost it. Our nearest genetic ancestors, the primates, are almost completely covered in hair whereas humans are almost completely bald in a total, bodily sense of the word. And on this same train of thought, wouldn’t baldness (just on the head) then be seen as a further evolution from our primate ancestors?

But, the ideas you had concerning the differing densities of hair could explain why that hair never went away over the thousands of years. But why did we begin losing hair to begin with? Sexual selection is the most likely culprit I think.

Dan McKeown

I cannot lie, I have been thinking about this post a lot over this past weekend and another thought came to me in the middle of a catchphrase game (I am not sure why but it sure threw off my description of ‘flake’). It would seem that the loss of hair on humans has been a completely a process of humans wanting to become more aesthetically pleasing. This would seem to suggest that as homo sapiens we hate our hair besides on the top of our head and in strategic places around the face. Think about it, how is it that all “unnecessary” hair on people has been lost depending on their ethnic origins. Nowhere can there be found a culture that has prized hair so that those who have more of it procreate at the expense of those without. Is there any other creature that has evolved in such a shallow manner? Perhaps my historically based mind is not addressing the problem the right way but it would seem that we have evolved to this point simply because humans think hair is unattractive and those ethnicities that have kept it have done so because they needed to. I guess that is all I have to say about that for now.

Nils

Yeah Dan, your thoughts pretty much expound on what I meant when I said sexual selection accounts for much of human’s evolutionary hair loss. But I don’t think it’s shallow by any means. It’s part of what makes us human, but it isn’t shallow. Almost all living creatures have physical traits that have been selected and passed on through sexual selection. A great example would be almost any bird. Take the cardinal: the male eventually adopted brightly colored feathers to enhance his protective, male dominance position with female cardinals while the female remained lacking of color (mostly). Do you think that original homo sapiens had any idea of the concept of being shallow? And no human ethnicity is different in this regard: we all lost a lot of hair, so no race is any less shallow than the next.

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