When observing images of human populations around the globe, it is difficult not to notice our variance in skin color. This visible variety is not often seen in other species and even sets us apart from our primate relatives. Like many other traits, skin color is one that exists as a result of evolution through natural selection. How and why has this trait evolved with such variety? What was once a mystery has been solved thanks to anthropology and genetics.
In HHMI's newest video, Dr. Nina Jablonski, a Penn State biological anthropologist and author of Living Color: The Biological and Social Meaning of Skin Color, takes viewers along on her quest to solve the mystery of skin color. The journey begins with an explanation of melanin, which comes in two forms: reddish-yellow (i.e., pheomelanin) and black and brown (i.e., eumelanin). Eumelanin in particular is responsible for protecting our body from harmful ultraviolet (UV) radiation that penetrates cells, possibly causing mutations. The narrative further explains that melanin protects cells by forming supernuclear caps that absorb UV rays, protecting the nucleus.
With this in mind, the film explores the connection between UV intensity and skin color. Using data collected by NASA in the 1980s, a map overlay of UV radiation measurements across the globe is presented. The result illustrates a clear gradient from the equator, with exceptions by altitude and places with consistent cloud cover. This map is then compared to a map of skin color based on skin reflectometer measurements collected by anthropologists studying indigenous populations. A correlation does indeed exist. Where UV radiation is strong, skin is dark, in both equatorial and high-altitude regions. These data suggest that humans adapted to various solar conditions over time.
Additional evidence gathered through DNA sequencing comes to the forefront of the film. Geneticist Dr. Rick Kittles describes genetic marker MC1R, which codes for a protein used in the production of eumelanin. Around the globe, there is variation in this sequence, but not in African populations where the majority have the MC1R allele. Fossil evidence suggests this has been present for about 1.2 million years. While reduction of skin cancer is the assumed explanation for its presence, skin cancer fatalities occur after reproductive age, which would not have an impact on reproductive success. Rather, folate may be the influencing factor. Some birth defects are more common in pregnant women with a deficiency in folate, and strong sunlight breaks down this nutrient as it circulates in blood vessels of the skin. This would make melanin influential in reproductive success.
This leaves the question: Why aren't we all dark skinned if melanin provides an advantage? The answer lies in the migrations of populations. While UV is harmful, UVB – a type of UV – is critical for the synthesis of vitamin D. Close to the equator, humans get more than enough UV exposure for vitamin D synthesis to take place, even through dark skin. Yet, as populations move north, UV exposure decreases. It is more difficult to absorb UVB in these areas. Selective pressure for MC1R lessens and variation occurs as a result. Supporting evidence is seen in polar indigenous populations that still have dark skin but also have a diet rich in vitamin D. While we need to protect ourselves from UV, there is still a benefit from absorption, resulting in a balancing act driving skin pigment variation. Today, with the ability to live in a variety of environments, this may mean cultural adaptations such as vitamin supplements. As her quest comes to an end, Dr. Jablonski reflects that skin color is a flexible trait that changes over time – one that is inherited independently of any other trait.
This video would be a great addition to any unit on evolution, adaptation, or genetic variation. It is highly engaging and adds personal relevance that is often missing from these topics of instruction. HHMI provides additional resources for both the teacher and the student. The film guide includes background information, discussion questions, and a written assessment with key. Student resources included interactive and traditional animations that support the science content covered in the video, each about three minutes in length. The piece I found most valuable was the activity “Human Skin Color: Evidence for Selection.” Here you will find a series of case studies based on the video. Each case study uses real data in the form of charts, graphs, and other visuals that students can analyze and interpret. This allows for application of skills while still engaging your classroom. Visit the website to learn more about how this video may connect with your middle school, high school, or college course.
REMY DOU taught high school life science for eight years before becoming an Albert Einstein Distinguished Educator Fellow. He now works as a Graduate Assistant at Florida International University contributing to STEM education research. For column queries: email@example.com