Slap me some skin, brother. 2/26/22

 Teachers, what is your go to when teaching evolution or natural selection? I think most of us at some point use the example of the peppered moths. That is a good example, but I have recently become aware of what I consider to be an even better one. Skin.

The news of ‘wear sunscreen to prevent skin cancer’, and ‘get outside to get your vitamin D’ is widespread, but did you know that it is rooted in the evolution of skin color? I didn’t, until I read a paper and watched a lecture by Nina Jablonski.  Her research was partially inspired by a photography project by Angelica Dass called Humanae ( https://angelicadass.com/public-speaking/ted-the-beauty-of-human-skin-in-every-color-vancouver-canada/ ). Skin color is not a set of colors with discrete boundaries between each race. In fact, I would say race doesn’t really exist.  Skin color is a continuum of shades and tones and so completely beautiful you might need a tissue.

                      

In a lecture supported by The Leakey Foundation, Dr. Jablonski took us on a trip through evolution and showed us the meaning of skin color. I hope I can give a synopsis of her research; I would encourage you to watch the lecture yourself (https://www.youtube.com/watch?v=sc4OFcT5m1Y) - to get all the nuances not possible in this format.

It begins with why we lost our fur about 2 million years ago. Fur was insulation, protection, it was how most animals and early hominins connected with the environment. Remember climbing trees when you were a kid? How many abrasions and bruises did you get? And that was wearing clothes. What would that have been like if you were naked and hairless? As the environment and their society changed, they climbed trees less and ran more. This necessitated a way to cool down, to release heat from their bodies and sweating with fur is not an efficient was to do this. A change to less fur did allow for more efficient heat loss, but that also opened the door for UV radiation damage. The polymer eumelanin created pigments in the skin to protect it against the damaging rays of UVR as the hair on Homo’s body became sparser. But the big idea for our ancestors was not so much skin cancer, but folate destruction. Folate is needed for reproductive success; it is important in preventing such birth defects as anencephaly and spina bifida. It is also needed for DNA production and repair.

Time marches on, the kids grow up and leave home. Homo migrated out of Africa about 100,000 years ago and to areas that had less direct sunlight, hence less UV radiation. The new problem was adequate vitamin D production. The darker the skin pigmentation the more time needed in the sun to make the needed amount of vitamin D. Since this vitamin is needed for calcium and phosphorus uptake, getting the needed amount is vital to life. It is also necessary for proper immune function. Without the ability to absorb calcium and phosphorus from your diet, the disease Nutritional rickets was a problem. This would cause the deformation of bones, and if it happened in females, chronic disease could result in a deformed pelvis preventing child birth.  “The Vitamin D Compromise” is all the different populations in low UVR areas experiencing parallel evolution of depigmented skin. With lighter colored skin less time was needed to get enough UVR to make the daily requirement of vitamin D.

One question that may come up is why hominins didn’t re-evolve fur in the higher latitudes? The answer is that there was no selective pressure to do so. In current temperate zones, it would be possible to go outside without clothing for much of the year, allowing for vitamin D production and heat dissipation.  During winter months when clothing would be needed to protect much of the body, the exposed depigmented skin would still allow for some vitamin D production.

As a lesson activity, create your own continuum. Take a picture of the back of your students’ hand or their forearm against the same background, with no way to identify the student. Then order them from dark to light. You will have a unique spectrum of skin tones. Dogs don’t care about ‘breed’, why should humans care about ‘race’?

 The peppered moth story shows us how natural selection can be a strong force for evolution, but the skin color story makes it personal.