The Origin of Skin Color

As I have watched, listened, and participated in our collective conversation about politics and race during the past few months, more questions have been raised than have been answered. I’m not sure of much, but one thing that I am sure about is that we are not as far along as maybe we thought we were.

Well, here is a question I can answer: why do we humans come in a variety of skin color? The following is an excerpt from the book I just published, Ecological Identity: Finding Your Place in a Biological World. This excerpt provides the most recent understanding of how we came to be a species with a wide degree of skin color and pigmentation.

We are a young species. Yet within the 150,000 – 200,000 years of ours species’ existence
one readily visible physical variation has evolved, and it is a physical variation that has led to some dark chapters in the human story: skin color. Within that time, the phenotype for skin color has evolved into a broad spectrum of pigmentation. The phenotype of pigmentation is the result of two forms of the pigment melanin: eumelanin and pheomelanin. The different shape of these two molecules means that they are different colors. Eumelanin comes in two types, black and brown. Pheomelanin has a pink or red hue.

This pigmentation has a direct use. Melanin absorbs ultraviolet radiation. The melanin pigment is found in structures called melanosomes which form sort of an umbrella above the nucleus of the cell. This protects the DNA in the nucleus from the UV radiation that can cause structural damage and mutations. The darker the pigmentation, the more UV radiation is blocked. Therefore, it is easy to see that darker skin is an advantage because it can reduce the likelihood of skin cancer. It is hypothesized, based on genetic and fossil evidence that our ancestors lost the majority of body hair and then developed darker skin 1 to 1.5 million years ago. So it would seem that skin pigmentation should be darker in populations that evolved in areas where UV radiation is more intense. The amount of UV radiation is directly proportional to the latitude on the earth. The closer to the equator, the more UV radiation. As you move toward the poles, the UV radiation reduces.

This can be tested by measuring the amount of pigmentation in the skin of individuals from indigenous populations in a variety of locations around the planet. It is necessary to measure the skin pigmentation of individuals with direct ancestry tied to that specific location. In today’s world of easy migration, this is not as easy as it sounds. However, Nina Jablonski from Penn State University was able to collect such data and did find a direct correlation between UV intensity and skin pigmentation. Along with this data, Rich Kittles from the University of Illinois compared the variation in a gene connected to skin pigmentation, the MC1R gene. He found that within individuals indigenous to Africa (with the longest evolutionary history in one location), there was the least amount of variation of this gene. This indicates that there was strong “negative” natural selection pressure on this characteristic—meaning the natural selection pressure was for this trait to remain unchanged. But something does not add up here. The obvious advantage to darker skin is protection from skin cancer. However, skin cancer does not usually manifest itself until later in life—after reproductive years for adults. What that means is that while it may be an advantage to a longer life, it is not an advantage during one’s reproductive years. We have seen previously that for a trait to be selected for through natural selection it has to be an advantageous trait that leads to producing more and healthier offspring with that trait. Because skin cancer strikes after reproduction, protection from skin cancer is not an advantageous trait, at least in terms of natural selection.

The reproductive advantage connected to skin pigmentation has to do with the vitamin folate. Folate is a needed vitamin in females during pregnancy for healthy embryo development and in males for healthy sperm production. However, strong sunlight breaks down folate. This is the evolutionary advantage for darker skin pigmentation. But if more eumelanin results in melanosomes that protect the cell from skin cancer, and also protects the necessary supply of folate which is a reproductive advantage then why doesn’t everyone have dark skin?

The answer to this question provides an example of competing selection factors, which is common in natural selection. Life is often about compromises. Ultraviolet-B radiation is needed by the body to produce vitamin D. Individuals with darker skin actually block most of the UV-B radiation. However, if they live in a climate with very high levels of UV radiation, such as near the equator, they would still get enough exposure, even with the natural protection to the UV radiation, to produce adequate vitamin D. However, populations that evolved in climates further from the equator require more exposure to UV radiation to be able to adequately produce vitamin D, thus the evolution of lighter skin in these populations. This is a compromise, however, because of the lower levels of UV radiation in these latitudes, the body was still able to preserve the needed folate for reproduction. The exception to support this hypothesis is in the population of indigenous peoples living in the Arctic Circle. Despite very low levels of UV radiation exposure, their skin pigmentation is darker than other populations from northern latitudes. The difference is that the Inuit population traditionally survived on a diet that was very rich in vitamin D. Therefore, their bodies got vitamin D from their diet, not by relying on UV radiation to trigger their body naturally producing it and also have retained the advantageous trait of more pigmentation protecting the supply of folate in their cells.

Of course, now the human population is not geographically bound to where their direct ancestors have lived for generations. This means that individuals with very light skin now living in latitudes with high UV radiation need to protect themselves from the sun and conversely individuals with darker pigmentation living in latitudes with low UV radiation may need to adjust their diet to include vitamin D rich foods or take vitamin supplements. One thing is abundantly clear. Skin pigmentation is simply a product of natural selection and nothing else. The production of eumelanin versus pheomelanin is not any indication of other characteristics. Additionally, it is the consensus of geneticists and evolutionary biologists that from a biological standpoint, there is no such thing as “race” in the human population. The variation in genotypes that we see is literally skin deep. Genetically, we are a very young species with very little genetic variation among the population. We are all of one tribe.

Maybe we could start acting like it.

Advertisements
This entry was posted in Written Blog. Bookmark the permalink.