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The Evolution of Skin Color

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Predict

Human skin comes in many shades, from light to dark. Sunlight carries ultraviolet (UV) light, the part of sunlight that causes sunburn. Could the sun be part of the reason human skin comes in different colors?

Circle one. Yes / No

Why?

Watch

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

Word of the day

Adaptation (ad-ap-TAY-shun). A heritable trait that evolved through natural selection and makes a living thing better suited to survive and reproduce in its environment.

Word partComes fromMeaning
ad-Latin ad-to, toward
apt-Latin aptāreto fit
-ationLatin -ātiōmakes the word a noun
  • An adapter lets a plug fit an outlet with a different shape.
  • Apt means suited to the purpose, and aptitude means a natural ability. Both come from Latin aptus, “fitted.”

Bonus words. Ultraviolet (uhl-truh-VY-uh-lit) names light beyond violet, the shortest-wavelength color people can see. The word joins Latin ultrā, “beyond,” with violet. Cline (KLINE) means a gradual change in a trait across a geographic area, from Greek klī́nein, “to lean.”

Questions

  1. Eumelanin is the brown-black form of melanin. Eumelanin absorbs UV light, and skin needs UVB light to make vitamin D. What trade-off does eumelanin create between UV protection and vitamin D? And how does the trade-off change with distance from the equator?

    Hint. Name one thing eumelanin protects and one thing eumelanin limits.

  2. Charles Darwin noticed that skin was darker near the equator and lighter toward the poles. Much later, NASA’s Total Ozone Mapping Spectrometer (TOMS) satellites measured UV across Earth for years. In the video, Raven sets the UV map beside a map of human skin color. What pattern do the UV map and the skin color map share? And why does a match between two maps need more evidence before it shows that UV caused the skin color pattern?

    Hint. A match shows that two things change together. Think about where the first humans lived and what happened to skin color as people moved.

  3. In the video, Raven explains that sunlight helps the body make vitamin D. Skin uses UVB light to make vitamin D, and melanin blocks some UVB. Far from the equator, UVB is weak for much of the year. What does vitamin D do for the body? And why did less melanin help people far from the equator make enough vitamin D?

    Hint. Name two jobs of vitamin D from the video. Then think about how much of the weak UVB gets into skin with less melanin.

  4. In the video, Raven says people moved faster in the past 400 years than ever before in human history. Voluntary and forced migrations took many people to places with UV far different from their ancestors’ homelands. What health risk could a person with dark skin face far from the equator? And what risk could a person with light skin face where UV is strong?

    Hint. One risk involves vitamin D. The other risk involves sunburn.

Claim, evidence, reasoning

Read this claim. Human skin color varies with distance from the equator because natural selection balanced two needs, protecting DNA from UV and making vitamin D. In December, the midday UV index is about 11.6 in Kuala Lumpur, Malaysia, near the equator, and 0.0 in Tromsø, Norway, far to the north. What evidence from the video and the UV numbers backs this claim? And what reasoning links the evidence to the claim? List at least three pieces of evidence, then write your reasoning.

Hint. Use Darwin’s observation, the UV map and the skin color map, where the first humans lived and the December UV numbers.

Claim

Evidence

Reasoning

Going further: research beyond the video

  1. This question goes beyond the video. Scientists Nina Jablonski and George Chaplin study how human skin color evolved. They argue that strong UV also breaks down folate, a B vitamin the body needs to make new cells and to have healthy babies. Melanin blocks UV, which means darker skin protects folate as well as DNA. How does the folate idea from Jablonski and Chaplin add a second reason that dark skin helped people near the equator?

    Hint. The video gives one reason, protecting DNA. Think about what strong UV does to folate and why a growing baby needs folate.

Exit ticket

In the video, Raven describes skin color as a balance. Melanin blocks UV that can damage DNA, and skin needs some UV to make vitamin D. What two needs did natural selection balance to produce the range of human skin colors? And where was each need most important?

Hint. One need protects DNA from strong UV. The other need lets skin make vitamin D from weak UV.

Sources