Philosophical Society of Washington

Minutes of the 2345th Meeting

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.


Attendance: 103
The weather: Overcast
The temperature: 18°C

Respectfully submitted,

Zeynep Dilli
External Communications Director & Recording Secretary


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