The Conflicting Stresses of Solar Radiation

   

1.          Of the four stresses listed in the assignment, I would argue that solar radiation is likely the most dangerous, for a couple of reasons.  First, solar radiation is composed of UV (ultraviolet) radiation, which, unchecked, can cause direct genetic damage to the DNA of our skin cells.  If enough damage arises, this can lead to skin cancer. While this may not impact homeostasis immediately, untreated, this will eventually lead to severe health problems and even death, reducing the survival and reproductive success of the afflicted individual.  A second confounding factor of solar radiation is that we also receive a positive impact from the sun’s rays, namely Vitamin D (appropriately called the “Sunshine Vitamin”), which our body produces in our skin with the help of solar radiation and uses for many physiological processes, primarily for the development, maintenance and repair of our skeletal system.  So while we need to be careful about how much solar radiation we receive, our skin must have access to the sun.  A balance must be achieved between avoiding damage and absorbing sufficient Vitamin D.

2.          There are four types of adaptations that will be considered here in the human body’s response to solar radiation stress:  Short term, facultative, developmental and cultural.

Nothing to do but hide from the sun.

Short Term:  Of all the stresses listed in the assignment, solar radiation is the only one where humans have failed to evolve a short term physiological adaptive response.  This is one of the reasons why solar radiation is such a dangerous stress, as the damage occurs before we know it.  Sun burn is not an adaptive response.  It is a sign that damage has already happened to our skin’s DNA.  It reminds us to get out of the sun, but it is too late to avoid the negative impact.

 

Most of the feet are protected.

Facultative:  Once our skin is exposed to solar radiation, a regulated response occurs in our skin cells, turning on the genes that produce melanin, a protein that blocks UV radiation.  This describes the process of tanning.  The process takes time and damage may occur before sufficient melanin can be produced to provide protection.  Because this is a facultative response, melanin will only be produced as long as the stress of UV radiation is present.  Once the stress is reduced or eliminated, melanin production slows to its base level of production (most of us produce some levels of melanin all the time, the exception being those with albinism).  The tan will fade as skin cells with high levels of melanin are replaced with skin cells that have the standard amounts.

More evidence of the human continuum

Developmental:  In populations that have resided in a region with high levels of solar radiation annually, such as in equatorial populations where the sun is more intense throughout the year, natural selection will have created populations that have higher base levels of melanin to provide constant protection against UV radiation.  As you move to higher latitudes, where solar radiation is less intense, populations have evolved lighter skin tones with less melanin content.  It is worth asking the question of why these populations don’t just retain the high levels of melanin, just in case, to protect against possible radiation exposure.  The reason why they don’t is two-fold.  Melanin production is costly in terms of energy and resources.  Better to not use those resources if it isn't necessary.  But remember that solar radiation also gives us Vitamin D.  As you move to high latitudes, not only does the intensity of UV radiation weaken, so does the exposure for the production of Vitamin D.  Populations more distant from the equator must have lighter skin in order to absorb sufficient radiation for producing Vitamin D (which is also blocked by melanin).  Hence, you see that important balance between protection against UV radiation and absorption of Vitamin D shaping our skin tones globally.

Umbrellas aren't just for rain anymore.

Cultural:  Fortunately for humans, we have learned to use our cultural tools and practices to reduce the impact of solar radiation, as well as possible Vitamin D deficiencies if we don’t receive enough sun exposure.  Sun screen, UV-blocking clothing, and umbrellas all help us live in the sun without being damaged by it.  Dietary Vitamin D (in fatty fish, egg yolks, beef liver and Vitamin D supplemented milk and cheese) and Vitamin D supplements all help to provide us with this necessary building block for a healthy skeletal structure and other physiological processes.

3.           Benefits of the adaptive approach include the concrete by-products that can arise from the information provided by these types of studies.  Understanding how the human body uses, protects itself and is damaged by solar radiation can contribute to medical advances in cancer research and nutritional research.  The information can also lead to new inventions that help protect humans from the damage of solar radiation.  It would be helpful if clothing could be developed that blocked the harmful rays and still allowed for Vitamin D production, wouldn’t it?  There are social benefits as well.  The more humans understand that their differences are environmentally-based, the more likely we will be to reduce or even eliminate misconceptions that arise from racial biases that convince many that phenotypic differences reflect the value or the intelligence of humans.  Imagine what the world would be like if all people recognized that this was false.

4.          There is no way to use race in an unbiased way to accurately and objectively understand human variation.  This is because race does not have the causal relationship with variation that we see with the environment.  Race is a social construct, not a biological one, and varies subjectively across cultures to categorize humans based upon external appearances and is influenced by preconceived cultural biases.  Race can only describe and categorize humans, not cause the variation to occur.  No causal relationship means there is no explanatory power, so race cannot be used to explain why humans vary as they do.  Fortunately, we have the adaptive approach to enlighten and teach us about why we, as a species, differ as we do.