Why Different Skin Colors? – Part 1
The evolution of diverse complexions around the world.
The concept of “race,” especially in the context of “people of color,” and what that does or doesn’t mean, is a controversial subject today. But that controversy might be alleviated to a significant extent if more people, earlier on in life, understood why different skin colors exist in the world.
Some might consider discussions of the evolutionary origins of different skin colors as somehow taboo or off-limits, but if more people understood it sooner, it would become immediately clear that skin color has nothing to do with inherent worth, and everything to do with how different human traits evolved in different places to make it easier for people to survive and have children in those different places. The story of human skin colors is a remarkable story, and it could well lead to significant bridges to understanding in that “people of color” are really just “people of evolution,” which is, well, all of us.
In their book The 10,000 Year Explosion: How Civilization Accelerated Human Evolution, Gregory Cochran and Henry Harpending write:
Not only are there strong reasons to believe that significant human evolution over the past 50,000 years is theoretically possible, and in fact likely, but it’s completely obvious that it has taken place, since people look different. This is especially true of populations separated by great distances and geographical barriers … Differences in appearance have a genetic explanation, so we know that there has been substantial genetic change since modern humans spread out of Africa — change that has not taken the same course in every population.
All modern humans have common ancestors in Africa, which is the part of the Earth near the equator that has the most intense sunlight and consequently supported diverse plant and animal life. And the evolution of bare skin, and its exposure to sunlight, had dramatic results for the life prospects of human beings.
Let’s take skin first. Animals tend to be covered in fur. That fur protects them from the sun’s ultraviolet rays, which can damage cells. But that fur also denies animals a wonderful means of preventing overheating, namely, sweat. Water has a remarkable capacity to absorb heat, as demonstrated here. When you sweat, the water molecules change from a liquid to a gas, which requires a great deal of heat energy to do. And when you sweat, your sweat takes the heat energy from your body, your sweat then becomes a gas, and that gas floats away into the atmosphere, carrying the heat with it and cooling you down. Skin in the game was a game-changer for humans.
But furry animals don’t have nearly as much skin to allow sweat to evaporate off it. As a result, animals generally have to rely on panting to let water evaporate from their mouths and tongues. (If you’re interested in more details on sweat, a nice podcast on “Why We Sweat” is available here.)
As Nina Jablonski describes in this lecture, humans evolved to lose their thick hair and develop the fantastic capacity to sweat over large areas of exposed skin, which gave them a huge advantage in being able to sweat to avoid overheating -- so they could hunt further, and more efficiently.
But as humans lost their fur, they gradually developed darker skin:
Why was that? To protect them from the harm caused by the sun’s ultraviolet rays. And as the chart below shows, ultraviolet radiation from the sun is most intense near the equator, where humanity started.
Too much exposure to ultraviolet rays damage cells, so early humans developed darker skin to replace the ultraviolet reflective effect their all-body hair had formerly performed. Darker skin contains more of a substance called melanin, which is nature’s sunscreen. It absorbs visible light and also harmful ultraviolet radiation before it can harm cells. Early humans evolved this natural sunscreen to protect against birth defects caused by ultraviolet rays that could damage DNA and the reproductive process.
And then early humans began to wander -- all over the world. And as they moved north, they moved away from the highest intensity of the sun along the equator. And as they moved away from the intensity of the sun, why didn’t they retain their darker skin? Because they needed more Vitamin D.
Vitamin D is a hormone that works to benefit almost every organ in your body. Vitamin D especially helps with calcium absorption to produce stronger bones and teeth, and it helps regulate the immune system and control cell growth. And your body uses sunlight to produce Vitamin D in a special way akin to how plants convert sunlight into chemical energy (a process called photosynthesis).
If you live closer to the equator, where there’s lots more sunlight, you can get away with letting less sunlight into your body. But if you lived far away from the equator, and you retained darker skin, you would have lost too much of that scarcer sunlight and you wouldn’t be able to produce enough Vitamin D. And so early humans who moved north developed lighter skins so their bodies could let in more light (so they could conduct more photosynthesis and turn light into Vitamin D), such that today lighter-skinned people make Vitamin D at a rate 5 to 6 times faster than darker skinned people.
Professor Jablonski uses the example of people in ancient Scotland to show just how lighter-skinned people needed to get to survive in far northern low-sunlight areas (and even then, they had to develop additional food staples rich in Vitamin D, including fish such as cod).
So there you have it. The explanation as to why different people have different skin colors. And far from being taboo, it’s essential to an understanding of evolution and the development of humanity in all its skin tone diversity.
As Nina Jablonski says:
This is a discussion that we all have to have. How we all got to look how we are is a fantastically interesting story … When teachers talk about how I can teach evolution in my classroom, I say listen, one of the best ways available now is using the story of your own body, your own skin color.
In the next essay, we’ll move from examining how evolution produced different skin colors to examining the evolution of racist ideas based on skin color.
Easy to understand science. Thank you.