Human Skin Color
Its Evolution and Relevance to Health and Human Society
Evan Pugh Professor of Anthropology
Pennsylvania State University
Videography by Nerine and Robert Clemenzi, Edited by Nerine Clemenzi Copyright © Philosophical Society of Washington. All rights reserved.
About the Lecture
Variation in human skin color has fascinated and perplexed people for centuries. As the most visible aspect of human variation, skin pigmentation has been used in the past as a basis for classifying people into races. Studies conducted in the past 25 years have shown that skin pigmentation is a biological adaptation that regulates the penetration of ultraviolet radiation (UVR) into the skin, and represents an evolutionary compromise between the conflicting demands of protection of the skin against UVR and of production of vitamin D by UVR. This compromise represents one of the best examples of evolution by natural selection acting on the human body. In the history of our species, Homo sapiens, skin pigmentation has been a highly changeable trait. Genetic evidence indicates that similar skin colors have evolved independently numerous times in response to similar environmental conditions and, because of this, skin color is an inappropriate trait for grouping people according to shared ancestry. This lecture will discuss the evolution of the “human rainbow”, how skin pigmentation influences our health, and how skin color has influenced societies and social well-being through color-based race concepts.
About the Speaker
NINA G. JABLONSKI is Evan Pugh Professor of Anthropology at Pennsylvania State University. For the last 25 years, she has pursued questions in human evolution not directly answered by the fossil record, foremost among these being the evolution of human skin and skin pigmentation. Her research has extended beyond her primary interest in the evolution of skin pigmentation phenotypes to the study of issues surrounding the health and social implications of skin pigmentation. She currently divides her time between basic research and educational projects. She is the lead investigator on a pilot project examining the factors that affect vitamin D status in healthy youth in the Western Cape of South Africa. She is the convener of a five-year research and education initiative, “The Effects of Race,” based at the Stellenbosch Institute for Advanced Study (STIAS) in South Africa. And she is working with Henry Louis Gates, Jr. to develop a new curriculum on “genetics and genealogy” for US middle and high school students and university undergraduates. Nina earned an AB in Biology from Bryn Mawr College and a PhD in Anthropology from the University of Washington in 1981. She is an author on over 110 research publications as well as numerous chapters in scholarly books. She has written two books for more general audiences: Skin: A Natural History and Living Color: The Biological and Social Meaning of Skin Color. And she has appeared frequently on radio, television and other media to discuss her research. Nina is a member of the American Philosophical Society, a Fellow of the AAAS, and a member of the Board on Behavioral, Cognitive, and Sensory Sciences of the National Research Council. She has received a Fletcher Fellowship and a Guggenheim Fellowship. She is the recipient of an Honorary Doctorate from the University of Stellenbosch for her contribution to the worldwide fight against racism.
President Larry Millstein called the 2345th meeting of the Society to order on April 10, 2015 at 8:06 p.m. He announced the order of business and welcomed new members to the Society. The minutes of the previous meeting were read and approved. He presented a summary of the 20th meeting of the Society held on February 5, 1872. President Millstein then introduced the speaker for the evening, Nina Jablonski of the Pennsylvania State University. Her lecture was entitled “Human Skin Color: Its Evolution and Relevance to Health and Human Society.”
Dr. Jablonski began her lecture by showing the project entitled “Humanae,” by Angélica Dass. This project shows the wide variety of skin tones among people through a collection of images arranged by the Pantone number of the skin tone. Dr. Jablonski said she feels a link to this artist, but her interest is in how this variety of human color evolved in the first place.
Throughout human history, people have noticed that skin color seemed to be linked to sunlight intensity. For example, Ibn Sina (Avicenna) noted that dark skinned people experienced the sun at its zenith for a long time, and attributed a “fiery humor” to such people. He also thought that people who lived far from the sun had pale skin and lacked intelligence. In the 19th century Samuel Smith noticed that color gradation had a regular progression with latitude. In the 20th century scientists developed skin color maps and correlated pigmentation to UV light intensity and temperature, aided by the ability to directly measure UV. Today, NASA provides worldwide maps of UV averages. Using these, Dr. Jablonski’s husband, George Chaplin, has constructed color-UV correlation maps. Dr. Jablonski showed in such a map that the latitude correlation holds except where regions are too humid (low UV) or too high up (high UV).
Dr. Jablonski then presented a statistical analysis showing a very strong correlation between skin reflectance and average UV intensity. UV radiation is an important evolutionary force. In primates, UV radiation is mostly known for damaging DNA, which was the first explanation for skin pigmentation as a protection. However, Howard Bloom noted that skin cancer shortens lifespan mostly after reproduction; so it cannot be a evolutionary pressure factor. In 1978, a paper reported that UV radiation causes a breakdown of folic acid (folate) in surface capillaries. Folate is important in DNA production and genetic expression. Its deficiency can cause serious birth defects and miscarriages. Protecting against folate deficiency due to UVB exposure is an evolutionary fact.
Dr. Jablonski pointed out that near the equator, UVB is seasonally high and UVA is high all year round. Our closest living relatives, chimpanzees, are fairly light skinned underneath their dark hair. It has been theorized that human ancestors shared this phenotype. But bipedalism, and walking and running under the sun for long distances, led to the evolution of a more efficient mechanism for heat dissipation: sweating and evaporation. This, in turn, led to hair loss exposing the fair skin. We adapted to protect against folate loss by the evolution of permanent dark pigmentation. Melanin, the pigment used for coloration throughout the animal kingdom, also provides UV protection. Eumelanin can absorb through the UV and visible spectrums.
The evolution of dark pigmentation in humans around one million years ago was a response that conferred a great reproductive advantage, which caused a “selective sweep.” The characteristic was elevated to 100% incidence in a short time. Scientists find that in current indigenous populations of Africa, all variation in a key pigmentation gene has been eliminated. The waves of human migration out of Africa led humans to higher latitudes, where almost all UVB is also absorbed in the atmosphere. But vertebrates need a small amount of UVB to stay healthy, since UVB is necessary for them to make vitamin D. Different waves of migration led them to areas which do not have strong year-round UV. Light skin can produce vitamin D faster than dark skin, an advantage in relatively limited UVB exposure. A “Vitamin D compromise” evolved. Reduced pigmentation to enhance vitamin D production in low UVB evolved independently in multiple lineages in different geographical areas. Another adaptation was a diet richer in vitamin D. Cultures with such diets, for instance the Inuit, could evolve a good tanning ability and moderate pigmentation to regain some protection against folate loss.
We have a coherent picture of evolution by natural selection of a stable pattern of skin pigmentation. But this pattern has been disrupted by human behavior. We have had huge involuntary migrations due to slavery from high UVB to low UVB areas. Now there is voluntary rapid migration in every direction. We take vacations to places with very high UV levels and live in cities with low sun exposure. We have created a pattern of sun exposure that is very unusual for our species. Vitamin D deficiency is more prevalent, due to all these factors and other recent developments in the history of the species, such as particulate pollution and wearing very concealing religious clothing. This has caused a variety of ills to people.
Skin pigmentation is a great example of evolution by natural selection, and Dr. Jablonski pointed out that it is a teaching opportunity, right there on our own bodies. But instead of appreciating this rainbow, we see segmentation along skin colors in many places. What happened, she asked.
Carl Linnaeus initially had a simple four-color classification. By 1758 he added more features to his descriptions of humans, including a characteristic of temperament. This characterization was avidly read by philosophers of the time, including Immanuel Kant, an erudite but very narrow-minded philosopher with strong opinions about human classification. He was the first person to use the concept of "race" and the first person to attribute an order to races. He felt that these groups had a distinct biological nature, and mixing was very unlikely, despite evidence to the contrary. Then he attached moral characterizations to racial physical differences, considering some races inferior. His views were influential and widely read. The 17th and 18th centuries saw the creation of racial stereotypes, and eventually the psychosocial template for racism which was strengthened in the 19th century. In the 20th century we have taken legal steps to protect against the effects of this template, but it is there. When people believe that a hierarchy exists, their behavior is affected.
After the question and answer period, President Millstein thanked the speaker, made the usual housekeeping announcements, and invited guests to join the Society. At 10:10 p.m., President Millstein adjourned the 2345th meeting of the Society to the social hour.
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