Vitamins and the Evolution of Skin Colour

Nina Jablonski - NASA

Current research suggests the global distribution of skin colour has arisen through an evolutionary process involving UV radiation and vitamins.

According to studies made by the State University of New York College in 1997, the global distribution of skin colouration can be correlated with UV radiation levels. To be more precise, amounts of melanin in the skin were found to be highest at the equator. They then decreased by about 8% for each 10 degrees of latitude in the Northern Hemisphere and by about 4% for each 10 degrees of latitude in the Southern Hemisphere.

Darwin and Natural Selection

Many scientists believe that this range of skin colouring has arisen as a result of evolution. The fundamental idea behind the theory of evolution is that all the species alive on Earth today have descended from a simple common ancestor. In his Origin of Species, published in 1859, Charles Darwin proposed a mechanism for this, called ‘Natural Selection’.

This theory effectively states that in the struggle for existence, only those individuals that are adapted to their surroundings will survive to reproduce. The favourable genetic characteristics that allowed them to survive will then be passed on to their offspring.

How Does Natural Selection Explain Skin Colour?

When contemplating the evolution of skin colour, an initial hypothesis might be that dark skin predominates in equatorial areas because people with light skin were more susceptible to skin cancer. The flaw in this idea, of course, is that most people develop skin cancers later in life, well after they have reproduced and passed their genes for fair skin to their offspring.

Moreover, it doesn’t explain why fair skinned people predominate in northern latitudes. The amount of UV radiation is markedly reduced here, so why should humans with any skin colour, let alone dark skinned people, have died out or been prevented from reproducing in these areas? The answer, according to anthropologist Nina Jablonski , is that deficiencies in Vitamin B folate (folic acid) and vitamin D, rather than skin cancer, have been the evolutionary drivers of skin pigmentation.

Fair Skin and Folate Deficiency

In studies made in 1978 Jablonski found that excessive exposure to UV rays destroys blood folic acid. In equatorial regions, fair skinned people lacking sufficient melanin to block UV radiation may consequently have been at risk of having reduced levels of this vitamin.

Because folic acid is important in embryonic neural tube formation, Jablonski concludes that natural selection favoured people with dark pigmentation in these areas because they could produce healthy offspring. Moreover, a lack of folate reduces sperm production and DNA synthesis, which may have further contributed to the demise of fair skinned people in warm climates.

Dark Skin and Vitamin D Deficiency

Conversely, people with fair skin may be at an advantage in more extreme latitudes where UV radiation is weaker. This is because UV B light is needed to manufacture vitamin D3, important in calcium absorption. Vitamin D is particularly vital during pregnancy for depositing calcium in the fast growing bones of the foetus, and also for foetal growth and development of the nervous system, lungs and immune system.

Jablonski hypothesises that this need for vitamin D during pregnancy may be why women in fact tend to have lighter skin than men, and why reproduction may not have been as successful in darker skinned women in northern latitudes.

Diet and Vitamin D Deficiency

According to some researchers, UV radiation as a source of vitamin D may only have increased in importance over the last 10,000 years. Prior to this, when humans were predominantly hunter-gatherers, vitamin D was also readily available in the meat and fish prevalent in their diet. Indeed, the Inuit Indians, whose diet includes ocean fish, seem to have evolved successfully in Arctic Regions despite having dark skins.

Heather Norton, a molecular anthropologist at the University of Arizona, has made studies of the mutant SLC24A5 gene, responsible for fair skin pigmentation, and agrees with the proposition that it only appeared around 6000 years ago. Careful analysis revealed little to no variation in the DNA surrounding this gene, suggesting that less time had passed since it appeared than previously believed.

Is Vitamin D a Genuine Selecting Agent?

With the introduction of agriculture and sedentary lifestyles, then, dark skinned people must not have received sufficient Vitamin D to successfully reproduce, leading to their ultimate decline in northern latitudes. Interestingly, however, researchers at the University of Wisconsin have found that rats deprived of Vitamin D are still capable of reproducing, albeit with the production of smaller litters and decreased neonatal growth rates.

Is this enough, then, to result in the disappearance of all dark skinned people in extreme latitudes? Perhaps not, but according to researcher Reinhold Vieth, pelvic deformity in women with rickets (a disease caused by Vitamin D deficiency) also makes a vaginal delivery virtually impossible. This would have further decreased the numbers of offspring produced by dark skinned women.

Nina Jablonski’s theory certainly holds some merit, and remains the most logical explanation for the evolution of skin colour throughout the globe. As Jablonski herself states, perhaps Darwin himself would have agreed with her if he’d had access to such modern technology as NASA’s TOM’s satellite (see figure 1), which has recorded the global distribution of UV radiation. At the very least, it may have inspired him to discuss human evolution, a subject that he only briefly touched on in his publications.

References

Halloran, B. and De Luca, H.,1980, 'Effect of Vitamin D Deficiency on Fertility and Reproductive Capacity in the Female Rat', nutrition.org

Jablonski, N. and Chaplin, G., 2000, 'The Evolution of Human Skin Coloration', California Academy of Sciences, yanaiweb.com

Jablonski, N., 2009, 'Jablonski Breaks the Illusion of Skin Colour', ted.org

Norton, H., Hammer, M., 'Sequence Variation in the Pigmentation Gene SLC24A5... and the evolution of Light Skin', physanth.org

Relethford, j.H., 1998, 'Hemispheric Difference in Human Skin Colour', Am J Phys Anthropol, nih.gov

Vieth, R., 'Effects of Vitamin D on Bone and Natural Selection of Skin Colour; How much Vitamin D Nutrition Are We Talking About?' ms.org