Skin Color Lab

This is a DBQ (Data Based Questions Exercise).  It is required by the school and your results will be analyzed and compared with others from the school.  You will be graded on your ability to come up with conclusions from the information given.  Here is the grading criteria.  

Evolution of Skin Color in Humans

We all have the same bone structure, the same color blood, the same sized brains… in fact, if you looked at just the insides of a human, you would have no idea what race the person belonged to.  So why, then, did we evolve to have different skin colors?  This simple outwardly difference has been exploited and perverted by humans to enslave one another, go to war, and discriminate against one another.  Let’s take a look, from a biological perspective, as to how different skin colors evolved in different indigenous groups of humans. 

A gene pool is the sum total of genes and their alleles within a population.  As humans migrated out of Africa, certain groups stayed and made their homes in different places, creating their own gene pools.  Due to the vast geographic obstacles (mountains, deserts, oceans, etc.) in between these groups, populations mainly stayed within the same area for thousands of years. 

Figure 1: Ancient Populations

The isolation of these different populations for such a long period of time helped to isolate different skin tones within these different gene pools.  This also explains why people from different parts of the world have their own particular features, heights, genetic disorders, etc.  Nowadays, people have the ability to travel across the world in just a few hours, allowing genes to be exchanged. The isolation of different gene pools for such a long period of time helps to explain why different skin colors stayed within different regions, but it doesn’t explain how each of these regions developed these different skin tones in the first place.  

UV Light and Skin Cancer

Biological traits aren’t good or bad. They’re features that have evolved because they enhance an organism’s odds of surviving and passing on its genes.  When scientists first began to investigate the idea that skin colors were simply due to natural selection, they wondered why it was advantageous for different skin colors to develop in different regions.

Scientists first looked to the skin cells that produce the pigment melanin. These skin cells were what caused differences in skin color - dark-skinned people tend to have more melanin-producing cells than lighter-skinned people.  Melanin provides protection from the radiation in UV light (like a natural sunscreen), which can damage DNA and cause skin cancer. 

Natural selection favors traits that provide an advantage to a species.  If melanin developed as a protectant from skin cancer, then there should be more dark skin where there was more UV light. Take a look at the figures below.  Figure 2 shows distribution of skin color over the face of the Earth.  Figure 3 shows the UV exposure over the face of the Earth. 

One thing that became obvious right away, was that as you moved farther from the equator, skin color tended to be lighter.  The other, was that as you move farther away from the equator, UV radiation tended to be weaker.  This supported the theory that darker skin was selected for to protect those who were exposed to a high amount of UV radiation!  But then again, damage from UV light (skin cancer) doesn’t usually show up until the person is in their 50’s.  This was a “red flag” for Dr. Nina Jablonski, who rejected this theory that melanin was selected to protect people from skin cancer.

Figure 2: skin color Distribution                                                                           Figure 3: UV exposure

UV Light and Folate

 Dr. Nina Jablonski had been investigating this theory when she came across a 1978 paper by Branda and Eaton that showed a statistically significant correlation between the amount of folate in a person’s blood and the amount of UV exposure.  Folate is an essential vitamin for DNA synthesis.  Pregnant women need a high amount of folate, because cells divide at a rapid pace during fetal development (meaning there is a lot of DNA to be synthesized). 

Figure 4: Folate Levels vs. UV exposure

Pregnant women who have low levels of folate can have anemia, while the fetus can have severe defects with their nervous system, aorta, digestive tract, and bones. Low levels of folate in pregnant women has also been linked to higher rates of miscarriages.  Men who have low levels of folate tend to have a lower sperm count.  All of these factors would have direct impact on an organism’s fitness (it’s ability to survive and reproduce). 

Dr. Jablonski saw a connection between skin color, UV exposure, and fitness. This provided a different explanation as to why dark skin was selected for in areas of high UV levels.  One problem still left to solve, was that if dark skin protected folate levels, why wouldn’t everyone have dark skin?  What was the biological advantage in having light skin?  

UV Light and Vitamin D

 Vitamin D3 is essential for normal growth, calcium absorption, and skeletal development. It is particularly important in maintaining and repairing healthy bones and teeth. People who have vitamin D deficiencies can have rickets (softening of the bones), osteoporosis, deformities and even death.  Pregnant woman especially need vitamin D, as their bodies are busy making another human skeleton.  We can obtain it by consuming foods high in D3 like fish liver oil, or our skin cells also have the ability to synthesize it with the help of UV radiation. 

Figure 5: Child with Rickets

If you return to page 2 and look at Figure 3, you can see that UV levels decrease as you move farther from the equator.  This would mean you would need to have less UV protection to receive an adequate amount of Vitamin D. Because of this, dark-skinned people are least likely to have sufficient vitamin D levels - they cannot produce enough vitamin D regardless of where they live. Moderately dark-skinned people can synthesize enough vitamin D if they live near the equator.   Light-skinned individuals are better able to synthesize sufficient vitamin D, especially at higher latitudes. That means that light skin increases fitness away from the equator.

Too much UV exposure can lead to lower folate levels and skin cancer, but too little UV exposure can lead to Vitamin D deficiencies.  Over thousands of years, in many climate zones, genetic selection has helped indigenous human populations adapt toward skin pigmentation levels that provide a healthy level of UV exposure.