NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE
The Evolution of Human Skin Color
by Annie Prud’homme-Généreux Life Sciences Quest University, Canada
Part I Skin Cancer
“Stop it!” called Tatiana, playfully. Her boyfriend, Zach, was inspecting her skin very carefully. “Look,” he answered her, his voice taking on a more serious tone. “Today a woman walked into the clinic for her annual physical. Everything about her seemed ne. She leads a balanced lifestyle, she eats well, she exercises: she’s healthy! But as she was about to leave, I noticed a mole on her arm. It had many of the warning signs of skin cancer. So, I removed the mole. is woman now has to wait for the lab results to ...view middle of the document...
Chimpanzees are therefore often used as a surrogate to make inferences about the physical and behavioral attributes of our common ancestors. e skin of chimps is light and covered with hair. From this observation, it has been inferred that our earliest ancestor was also probably light-skinned and covered with hair. Since humans and chimps diverged, humans left the protection of trees and adapted to a new environment (the open savannah). is change in habitat required several adaptations. Life on the savannah provided little shade and so little protection from the sun, and required a more active lifestyle (i.e., hunting as opposed to picking fruits). It is also hypothesized that the social interactions and strategizing required for successful hunting favored the development of a large brain, which consumed a lot of energy and generated heat. An increased number of sweat glands and loss of body hair evolved to dissipate heat. is created a new problem, as the light skin became exposed and vulnerable to the sun’s damaging ultraviolet (UV) radiation.
Melanin: Natural Sunscreen
UV light is harmful to living organisms because it causes changes (i.e., mutations) in the DNA sequence. Skin cells that produced a pigment called melanin were advantaged because melanin is a natural sunscreen; it absorbs the energy of UV light and shields cells from the radiation’s harmful e ects. Such cells were favored in evolution and now all human skin cells can produce this pigment. People vary in their skin tone due to di erences in the distribution, quantity, size, and type of melanin found in their skin cells. As you might suspect, people with dark skin tend to have larger and more numerous melanin-containing particles in their skin. is provides protection from the sun’s UV rays. Many genes are known to a ect the production of melanin and cause skin color variation in humans. While skin color is an inherited characteristic, the fact that many genes code for this trait explains why children do not always exactly match their parents’ skin tone. Tanning is the process of producing more melanin in the skin in response to ultraviolet exposure, and does not require a change in the genetic code (if a parent gets a tan, the o spring will not be more pigmented).
Distribution of UV Light across the Globe
e following image (Figure 1) represents a map of the world on which the UV-light Index has been superimposed. e latitudes are shown on the left (latitude helps de ne a location on Earth, speci cally how far north or south of the equator a site is).
4. Does the amount of UV light reaching the Earth vary in a predictable manner? If so, describe the pattern you observe. 5. What latitude receives the greatest amount of UV light? e least? 6. Based on these data, where might you expect to nd the most lightly pigmented and most darkly pigmented people on the planet? Be as speci c as you can. 7. Provide a rationale to your answer above (i.e., why did you think that...