The Evolution of Skin Color

Watch

1. The Evolution of Human Skin Coloryoutube.com/watch?v=-KkydZWlF3Y

Word of the day

Adaptation (ad-ap-TAY-shun). A heritable change in an organism, resulting from natural selection, that makes the organism better suited to survive and reproduce in a particular environment.

• Apt and aptitude come from Latin aptus, “fastened, fitted.”
• Adjacent and adrenal share the prefix ad-, “toward.”

Bonus words. Folate (from Latin folium, “leaf”) names a B vitamin whose main blood form, 5-methyltetrahydrofolate, breaks down under UV. Cline (from Greek klī́nein, “to lean”) describes a gradual change in a trait across a geographic range.

Predict

1. Jablonski and Chaplin mapped the world into zones by how well lightly pigmented skin can make vitamin D from casual sun exposure. In the equatorial zone, casual sun gives enough UVB all year. Before you read the paper, take a guess. What percent of the world’s people live in the equatorial zone? Give your reason in one sentence.

   Hint. The equatorial zone covers about 41% of the world’s land.

Read the paper

Jablonski and Chaplin, “Human skin pigmentation, migration and disease susceptibility,” Philosophical Transactions of the Royal Society B, 2012. The paper is free to read.

Jablonski and Chaplin sorted the world into vitamin D zones for lightly pigmented skin. In the equatorial zone, casual sun exposure gives enough UVB to make vitamin D all year. In the subtropical and warm temperate zone, at least one month has too little UVB, and people rely on stored vitamin D. In the cold temperate and polar zone, UVB averaged over the year falls short, and people need vitamin D-rich foods such as oily fish. Table 1 of the paper counts the people living in each zone, using population estimates from 2000.

1. Quantitative reasoning Start by adding the People column to get the world total. Then fill in the percent column. What percent of the world’s people live in zones where lightly pigmented skin has at least one month without enough UVB? And which hemisphere holds most of those people?

   Hint. All four zones outside the equatorial zone have at least one month with too little UVB.

2. Process of science Table 1 uses zones drawn for lightly pigmented skin. The authors write that for darkly pigmented skin, the vitamin D safe zones are smaller and shifted toward the equator. How would the numbers in Table 1 change for darkly pigmented skin? And what data would you need to make that version of the table?

   Hint. Think about which rows would gain people and which rows would lose people.

3. Evolution and systems People lived year-round in the cold temperate and polar zone long before vitamin supplements. The authors write that early people in Scotland lived mainly on the coast, gathering and hunting vitamin D-rich foods. How did skin color and diet work together to let people live in the zones with the least UVB?

   Hint. Think about the two ways a body gets vitamin D.

4. Science and society The authors write that over 500 years of voluntary and forced migration moved many people into UV conditions unlike their ancestors’. Many people now living in the Northern Hemisphere’s low-UVB zones have moderately or darkly pigmented skin. What vitamin D and sun-safety advice would you give a public health department? Answer in one paragraph that uses at least two numbers from Table 1.

   Hint. Think about who faces low vitamin D and who faces too much UV in each zone.

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