5.3: Evolution of Skin Color

In The Biology of Skin Color, Penn State University anthropologist Dr. Nina Jablonski walks us through the evidence that the different shades of human skin color are evolutionary adaptations to the varying intensity of ultraviolet (UV) radiation in different parts of the world. Our modern human ancestors in Africa likely had dark skin, which is produced by an abundance of the pigment eumelanin in skin cells. In the high-UV environment of sub-Saharan (or equatorial) Africa, darker skin offers protection from the damaging effects of UV radiation. Dr. Jablonski explains that the variation in skin color that evolved since some human populations migrated out of Africa can be explained by the trade-off between protection from UV and the need for some UV absorption for the production of vitamin D.

Biological traits aren’t good or bad. They are features that have evolved within populations because they enhance an organism’s odds of surviving and passing on its genes. Skin color is an easily visible marker of variability. Our lack of body hair and our variable skin color are some of the traits that set us apart from our closest primate relatives. Wavelengths of light are reflected or absorbed by pigment in the skin called melanin. Melanin is synthesized in structures called melanosomes that are produced by cells called melanocytes. There are two primary types of melanin in humans: pheomelanin, which is reddish yellow, and eumelanin, which is brown black.

UV radiation can penetrate living cells and cause mutations in DNA. Melanin protects human cells from the damaging effects of UV radiation by absorbing UV. There is a clear correlation between the intensity of UV radiation and latitude. UV radiation is most intense along the equator and is weakest at the poles. UV intensity predicts the skin color of indigenous populations. Stronger UV radiation is correlated with darker skin color. Data suggest that variation in human skin melanin production arose as different populations adapted biologically to different solar conditions around the world.

Early in human history, our ancestors lost most of their body hair and increased melanin production in skin. Evidence of natural selection can be found in the genome: the MC1R is a gene that codes for a protein involved in the production of eumelanin. Worldwide human genome sampling revealed that among African populations, the vast majority of individuals have an MC1R allele that results in darker skin. Fossil and genetic evidence suggest that all humans were dark-skinned about 1.2 million years ago. UV breaks down circulating folate in the skin’s blood vessels.

UV-B absorption is critical for the synthesis of vitamin D, a process that starts in the skin. Weaker UV-B intensity and greater UV-B variability throughout the year in areas toward the poles put dark-skinned individuals at risk for vitamin D deficiency. Toward the poles, selective pressure for dark skin (to protect folate) decreases and selection for lighter skin shades (to enable vitamin D synthesis) increases. Selection for light-skin gene variants occurred multiple times in different groups around the world. Today, human migration does not take generations. So there is a lot of mismatch between skin color and geography. Skin color is a flexible trait that is inherited independently of other traits.

More About Melanin

“Melanin” is the collective term for a family of pigment molecules found in most organisms, from bacteria to humans, suggesting that melanin has a long evolutionary history and a broad range of important functions. In humans, melanin pigments are found mainly in human skin, hair, and eyes, and they include reddish-yellow pheomelanin and brown and black eumelanins. A related molecule called neuromelanin is found in brain cells. In human skin, melanin pigments are synthesized in organelles called melanosomes that are found in specialized cells called melanocytes in the skin epidermis. Once the melanosomes are filled with a genetically determined amount and type of melanin, they migrate to other skin cells called keratinocytes.

Melanin synthesis involves a series of chemical reactions that begin with the amino acid tyrosine. An enzyme called tyrosinase promotes the conversion of tyrosine into DOPA, and then into dopaquinone. Dopaquinone can either be converted into eumelanin or combined with the amino acid cysteine to produce pheomelanin. Whether eumelanin or pheomelanin is produced depends partly on the activity of the melanocortin 1 receptor (MC1R) protein (Figure).

Eumelanin is a remarkable molecule that can absorb a wide range of the wavelengths of radiation produced by the sun, in particular, the higher-energy UV radiation. UV can damage biological molecules, including DNA. When UV radiation strikes eumelanin, the pigment absorbs the radiation and mostly transforms the energy into thermal energy, without breaking down, making it a powerful sunscreen that protects against UV damage. Pheomelanin is less effective as a sunscreen than eumelanin and can, in fact, produce damaging molecules, known as free radicals, when it interacts with UV radiation.

Genetics of melanin production

Constitutive pigmentation, or the pigmentation we are born with, is a polygenic trait, and many of the genes involved have been identified. These genes code for the enzymes that affect melanin synthesis and for the packaging, distribution, and degradation of melanosomes. Mutations in some of these genes cause an absence of melanin, as seen in human oculocutaneous albinisms and related disorders. For example, one form of albinism is caused by mutations that inactivate the tyrosinase gene.

Figure \(\PageIndex{1}\): The melanocortin 1 receptor (MC1R) is a melanocytic Gs protein coupled receptor that regulates skin pigmentation, UV responses, and melanoma risk.

Let’s look at the MC1R gene. This gene codes for a protein that sits in the melanocyte membrane. It is activated by a variety of stimuli, such as by the melanocyte-stimulating hormone (MSH), and is responsible for determining whether eumelanin or pheomelanin is produced. People of African descent have a version of the MC1R gene that is associated with eumelanin production. There is very little variation in the MC1R gene in African populations, compared to populations indigenous to Europe and Asia. This lack of diversity at a genetic locus is evidence of selection, suggesting that eumelanin production provides an advantage to people living in equatorial Africa.

Scientists have looked for evidence of selection in other parts of the genome and have identified genes involved in skin color in different populations. For example, one allele of a gene called OCA2 results in lighter skin colors and is almost exclusively found in East and Southeast Asian populations. On the other hand, alleles of two genes called SLC24A5 and SLC45A2 are also associated with lighter skin colors and are much more frequent in Europeans than in other populations. These and other data suggest that lighter skin color evolved more than once by different mechanisms. Interestingly, the SLC24A5 and SLC45A2 genes were first discovered in zebrafish and are responsible for differences in the stripe colors.

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