Evolution of Skin Color in Humans SE PDF

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Cameron Follett Name________________________________________________________________________________ Period_______

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. 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. Figure 1: Ancient Popul Populations ations Analysis Qu Questions estions 1. Explain what a gene pool is, and what it has to do with skin color in humans. The Gene Pool is the set of all genes or genetic information for any population, It generally specifies what color hair you get, what color your eyes are, and skin. It has to do with the color of skin a human has.

2. What technological advancement has changed the size of the human gene pool, allowing humans to travel and exchange genes? The advancement is being able to travel and live anywhere.

3. Make a prediction as to why people in different regions developed different skin colors I think the reason is because life originated in Africa and as humans moved to colder regions their skin grew whiter and whiter.

UV Light and Ski Skin n 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. F igure 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 3: UV exposure Figure 2: Skin Color Di Distribution stribution

Analysis Qu Questions estions 4. What is meant by the quote “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,”? The quotes means that traits aren't bad, they are just features that enhances organisms chances of surviving and to pass on genes.

5. What general correlations can you see between skin color distribution and UV exposure? The general correlations are the more UV humans are exposed to the darker the skin.

6. Why do you think Dr. Jablonski rejected this theory based on the fact that skin cancer doesn’t show up until a person is in their 50’s? Hint: think about reproduction and fitness. Because more exposure to UV radiation leads to a higher risk of cancer, and pigmented skin protects against damage from UV. Jablonski argued that protection from skin cancer may not explain the evolution of darker skin color.

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). 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). Figure 4: Folate Levels vs. UV exposure

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?

Analysis Questi Questions ons 7. Based on the graph above, what is the correlation between UV exposure and blood folate levels? The greater amount of eumelanin in darker skin protects folate from being broken down by UV radiation and thus increases fitness among populations in high-intensity UV areas.

8. How does the theory that darker skin was related to blood folate levels serve as a more reasonable hypothesis than the theory that darker skin was related to lower risk of skin cancer? Because skin cancer does not usually arise until after an individual's peak.

9. Predict why people would have lighter skin in areas of lower UV radiation. People in lower areas of UV have less sun on them which means lighter 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. If you return to page 2 and look at Figur Figure e 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 Figure 5: Child with Rickets regardless of where they live. Moderately dark-skinned people can synthesize enough vitamin D if they live near the equator. Lightskinned 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. Analysis Qu Questions estions 10. Explain the relationship between UV light and Vitamin D. When your skin is exposed to sunlight, it manufactures vitamin D.

11. Based on the above graph, who is most at risk for vitamin D deficiency? How might it affect their fitness farther from the equator? People who are obese, who have dark skin and who are older than age 65 may have lower levels of vitamin D.

12. How would a light-skinned person’s fitness be affected if they lived close to the equator? Living near the equator increases skin reflectance.

13. Summarize in terms of UV light light, Folate and Vitamin D : Why, as you move farther from the equator, does skin color tend to get lighter? Skin tends to get lighter the farther you get because there is less UV light and colder climates....