To explore how the sun’s intensity at different latitudes has contributed to variations in human skin color.
In this lesson, students learn that variation in skin color can be explained by adaptations to the environment through natural selection. It makes use of two activities at a website created for the traveling science exhibit, RACE: Are We So Different?, developed by the American Anthropological Association in collaboration with the Science Museum of Minnesota.
By middle school, students are familiar with the concept of evolution, but they tend to have a weak grasp of the process through which evolution occurs. Natural selection is a difficult notion to grasp, even for high-school students. (Benchmarks for Science Literacy, p. 343.) It is not unusual for students to hold misconceptions, such as to believe that the environment can produce biological changes in an individual during his or her lifetime or that change occurs through conscious effort. Such misperceptions can cloud the understanding that chance alone produces the new combinations of existing genes or mutations of genes that are transmitted to offspring; the combinations that confer an environmental advantage gradually build up in the gene pool of a population, through natural selection. To reinforce the meaning of the term natural selection, it may be helpful to precede this lesson with another Science NetLinks lesson, Nowhere to Hide.
A few other terms and concepts also will need to be introduced or reviewed so that student learning from this lesson is maximized. One of those terms is people, which can be used to refer to all human beings but also to a specific group of people who make up a community, tribe, nation, or other group because they have something in common such as culture, background, or geographic origin. The Human Spectrum activity in this lesson uses a related term, native people, to refer to a group of people that shares cultural, behavioral, and physical traits because for many generations group members have mostly lived in a particular geographic location and reproduced with other members in the same group. Related to this is the concept of race. The Teacher’s Guide can help you navigate a class definition for this vague but problematic term. Finally, an important concept for this lesson is latitude, in terms of how distance from the equator is related to the intensity of sunlight. A useful resource for instruction on this topic is National Geographic’s Introduction to Longitude and Latitude.
Before guiding your students through this lesson, you are advised to read Skin, an article from Scientific American co-authored by the scientist whose work is featured in the main activity of this lesson. The article adds useful details to the hypothesis that geography—specifically, the sun’s intensity at different latitudes—has contributed to variations in human skin color.
You also should preview the FACITY website page that presents faces of people living in Berlin, Germany, which is used in the Motivation section of this lesson. If you will not be able to project the website for the class, print out one or more copies of the page in color, plus several individual photos from the page.
A useful resource in preparing this and other lessons on human biological variation is RACE: A Teacher’s Guide for Middle School, developed by the American Anthropological Association with funding from the Ford Foundation and the National Science Foundation. The Teacher’s Guide presents race and human variation through the integrated lenses of biology, culture, and history. The section Exploring Human Biological Variation offers four exercises that can help students understand why humans vary as a single biologically diverse species and why “races” do not actually exist the way species do but instead are social constructs.
Display to the class the page of facial portraits taken in Berlin, Germany as part of FACITY, an online photo project. Scroll slowly down the page through the nearly 600 facial portraits, so that students can see all of them. Ask:
- What facial features vary among people?
- (There are many possible answers and include: eye color, shape of nose or lips, type of chin, thickness and shape of eyebrows, degree of wrinkles, amount and type of facial hair, shape and texture of lips, and skin color.)
Click on several individual photos to enlarge them. Ask students to focus on skin color as they answer these questions:
- For any one person, is skin color the same all over?
- (The correct answer is no; no one has evenly pigmented skin.)
- What are some of the ways that skin color varies on a person’s face?
- (Answers may include: some people have skin that is darker or lighter in sections; some have freckles, moles, scars, or other spots).
Now tell the students you will ask them a question that they should answer based on their own experience:
- Considering all the people in your family, all the people of your community, and all the people of the world, how does skin color vary?
- (Answers may include: skin colors range from pale to dark; skin colors range in tone from pink to olive; some people have relatively smooth skin tones and others have skin that is more unevenly colored. Keep probing until someone mentions that skin color also varies depending on time of year and exposure to the sun.)
Say to the class:
You’ve mentioned a lot of the ways that skin color varies within a person, within a family and community, and within all humanity. One of those ways that you mentioned is very important: skin color varies based on exposure to the sun. Sunlight not only affects skin color from one day to the next; it also has affected the evolution of skin color among the peoples of the world. In this lesson, we’ll learn something about how the process of natural selection over many generations has protected peoples of the world from too much or too little sun exposure.
Begin by having students use their The Human Spectrum student esheet to visit The Human Spectrum, which is a section of the website, Race: Are We So Different? This interactive page teaches about the continuum of human traits and the arbitrary ways in which people divide up such continuums to categorize each other.
Assign students in groups to review The Human Spectrum and to discuss the questions on the student esheet. They can record their answers on The Human Spectrum student sheet. Have each group assign a note taker and a reporter. After the small groups have completed this task, reconvene the class as a whole to discuss each question:
- Short, medium, and tall is one way we categorize people. There are many others. For example: young, middle-aged, and old. What are some other ways people commonly categorize each other? What are the advantages and disadvantages of grouping people in the ways you have named?
- “Skin tones overlap both within and between native peoples.” Give an example using the named peoples from the graph of native peoples. For example: Although Holy Island people as a whole are lighter-skinned than Sherpa people, some Sherpa individuals are lighter-skinned than some Holy Island individuals. What does it mean for the concept of race if the quoted sentence is true?
- Do you think race is a good way to group people?
- (Answers may vary. Encourage students to explain their answers.)
Use the last question to transition to the second part of the lesson. Ask students to define race. Answers will vary; an example is a group of humans that shares distinct physical characteristics. Explain that race is difficult to define because it is a “social construct”—it is based on arbitrary groupings such as by skin color, with arbitrary cut-off points between one group and another: “The idea of races supposes that there are natural breaks in the distribution of human biological variation that produce clear clusters (or races) of people who share certain traits. However, the opposite is generally true: human variation is a continuum.” (RACE: A Teacher’s Guide for Middle School, p. 9.)
Now students should use their esheet to go to and read Only Skin Deep, a second section from the website, Race: Are We So Different? When they have read the material, reinforce the concepts presented by discussing these questions:
- What was the challenge for the human body when our early ancestors moved into hot, open places such as grassy savannahs, in search of food and water?
- (They were exposed to the hot sun.)
- How did they evolve in response?
- (They lost hair and gained more sweat glands, which were adaptations that enabled their bodies to stay cool.)
- What was the challenge for the human body as it evolved away from body hair and more skin became exposed?
- (They were more vulnerable to ultraviolet radiation of the sun.)
- What is a benefit of ultraviolet radiation for the human body?
- (It helps the human body make vitamin D, which the body must have to absorb the calcium necessary for strong bones.)
- How does ultraviolet radiation challenge the human body?
- (Too much ultraviolet radiation can destroy another vitamin called folate [folic acid], which is essential to the development of healthy fetuses.)
- Where is ultraviolet radiation the strongest?
- (It is strongest nearest the equator.)
- How did natural selection work to bring about the trait of darker skin in people who live near the equator?
- (Pigmentation was selected for, because it protected the body from too much penetration by ultraviolet radiation. In this way, the body’s store of folate was protected. At the same time, year-round exposure to strong sun ensured that the body could produce enough vitamin D. Folate and vitamin D are important for creating healthy babies. People with more pigmented skin were more likely to successfully reproduce, passing on their traits [including the trait of pigmented skin] to the next generation.)
- How did natural selection work to bring about the trait of lighter skin in people who live far from the equator?
- (Pigmentation was selected against, so that the body could absorb enough ultraviolet radiation to obtain vitamin D, which is necessary for maternal and fetal bone strength. People with strong bones were more likely to live to adulthood so that they could reproduce, passing on their traits [including the trait of less pigmented skin] to the next generation.)
- Why is the threat of skin cancer from ultraviolet radiation not a plausible explanation for why dark skin evolved in peoples who lived near the equator?
- (Skin cancer usually affects people after they have had children, so it is not likely to have had an effect on which traits were passed on.)
As a follow-up project to this learning, assign students to each investigate one of the peoples named in the sixth slide of The Human Spectrum. Ask them to find some facts about the culture of their assigned people and to locate the latitude at which that people has traditionally lived. Also ask them to find and print out a photograph of one or more representative members of that people. Students can collect this information on the Native Peoples, Skin Tones, and Latitudes student sheet. These sheets can be displayed in the classroom, arranged according to latitude, to create a visual display of the correlation between latitude and skin color.
Ask students to write a short essay on one or more of these questions:
- Considering the effects of sun and natural selection, how do you explain the fact that skin color tends to be lighter on the undersides of arms, on the palms of the hands, and on the soles of the feet?
- What health concerns related to sun exposure might there be for a native person from Nairobi, Kenya (1.2833 degrees south of the equator) who emigrates to Berlin, Germany (52.5233 degrees north), and vice versa? What actions can such persons take to protect themselves from adverse health effects from too much or too little sun exposure?
- Create a hypothesis to explain how natural selection has worked to evolve differences in another human biological trait besides skin color.
An artistic response to the concept of race is presented at The Human Pantone. Artist Pierre David used as his inspiration the Pantone color wheel, which is a visual dictionary of color. The exhibit was commissioned by the Museum of Modern Art in Brazil, and David photographed 40 staff members of the museum to create his color wheel.
Students can further explore the social construct of race at the PBS website, Race–The Power of an Illusion. The first two activities, What Is Race? and Sorting People, tie most directly to this lesson’s focus on variation in human skin color.