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Human Adaptations

Important Conclusions About Skin Color- Suggestions For Further Research

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            Researching human skin color variation allows us to begin to understand the selective pressures and evolutionary forces which acted upon human populations in the past to produce the great degree of diversity in pigmentation across the globe that we can see today.  Understanding human skin color adaptation reveals the significant role that skin color plays in survival in different parts of the globe in various environments.  For many years the consensus was that darker skin evolved to protect people from the deleterious effects of skin cancer. However, it is commonly argued now that the effects of skin cancer occur later on in life and after the time when it would greatly affect reproductive success.  Through recent research and discoveries, it has become clear that skin color adaptations have a different evolutionary basis.  According to current epidemiological and physiological evidence, human skin pigmentation variation results from natural selection “acting to regulate the effects of the sun’s ultraviolet (UV) radiation on key nutrients crucial to reproductive success” (Chaplin et al, 2003).  In other words, because humans are basically hairless apes, our skin color must both protect us from the harmful effects of the sun, while still allowing for adequate synthesis of vitamin D by exposure to enough UVB rays.

            An article written by Chaplin et al. 2003 entitled “Skin Deep” points out that one benefit of darker skin is the potential to protect against the breakdown of folic acid in our body by UV radiation.  Folic acid is a very important nutrient for fertility as well as fetal development. Obviously, decreased levels of folic acid lead to problems in reproductive success.  Notably, the article references a report published in 1996 by Argentine pediatrician Pablo Lapunzina. This report discussed the fact that he had three women patients, who were young and healthy, give birth to babies with neural tube defects after having used sun beds at the beginning of their pregnancy. Folic acid is essential in the prevention of neural tube defects and various other biological and developmental processes.   On the other hand, UVB rays are also essential for “initiating the formation of vitamin D in the skin” (2003), a process critical to bone development.  Rickets results from a severe deficiency of vitamin D and can potentially reduce survival and reproduction (Bamshad et al, 1999). Therefore dark-skinned people living in areas with low UV radiation, and a lack of it in their diet, suffer from high rates of rickets and “other diseases related to vitamin D deficiency” (2003).  To be sure, skin color adaptation is affected by both environmental and cultural factors. For instance, cultural factors became more important in the past for surviving migrations into new climates. This is exemplified by the Inuit diet that is rich in vitamin D and the use of tents and protective clothing by European peoples in the Arabian Peninsula 2000 years ago (2003).

            Human skin color variation, in general, is based upon the levels and ratio of two types of melanin (Birch-Machin et al, 2000; Bamshad et al, 1999). Brown-black (eumelanin) and red-yellow (pheomelanin) melanin are linked to the melanocortin 1 receptor (MC1R), the only gene thus identified that “explains substantial phenotypic variance in human pigmentation” (Birch-Machin et al, 2000).  Human pigmentation is a polygenic trait but the relative importance and role of other genes which may affect skin color have yet to be discovered (Box et al, 2001). 

Dark skin in Africa can be explained by the effects of the environment.  Selective pressures maintained “high levels of eumelanin pigmentation,” resulting in the majority of people having varying degrees of dark skin (Birch-Machin et al, 2000).  As evidence, one article points out that there is an:

absence of amino acid variants in Africa- as well as [a ] low frequency in African Americans and in Asians from Papua New Guinea and India, where skin pigmentation is typically very dark- impl[ying] strong functional constraint on MC1R, probably as a means to minimize sensitivity to UV radiation. The presence of a small number of nonsynonymous variants in the African American and southern-Asian sample populations can be explained by admixture. The other non-African samples, in contrast, reveal high frequencies of a large number of nonsynonymous variants” (Birch-Machin et al, 2000).

In other words, outside of Africa the restraints on dark skin became ‘relaxed’ and this is apparent in the variation in these particular amino acids that is absent in Africa, but not elsewhere. When modern human populations migrated out of Africa to areas with less sun exposure dark skin was not necessarily advantageous any longer, possibly explaining why there is more variation of the MC1R gene in non-African populations (Bamshad et al, 1999).

            Despite all that is known about human skin color adaptation, relatively little has yet to be understood concerning all of the variables which combine to produce skin color. For example, there is a clear lack of data on patterns of human diversity world wide, which make it difficult to examine selective pressures and histories and resolve them statistically (Birch-Machin et al, 2000). Therefore, more data is essential. Additionally, there is a definite need for researchers to take a step towards standardizing their terminology and equipment, or at least their modes of reporting data, so that it is possible to compare studies and evidence from different researchers. Currently, this is a real problem.  All in all, continuing to research skin color adaptation and how it evolved can allow us to better treat and prevent diseases and defects which result from the combined factors of skin color, environment and culture.

Human Skin Color Variation

Interesting Links

Works Cited

 

Bamshad, Michael et al. 1999. High Polymorphism at the Human Melanocortin 1 Receptor Locus. Genetics 151:1547-1557

 

Birch-Machin, Mark A. et al. 2000. Evidence for Variable Selective Pressures at MC1R.  Am J. Hum Genet 66: 1351-1361,

 

Box, Neil F. et al. 2001. Human pigmentation genes: identification, structure and consequences of polymorphic variation. Am J Hum Genet 277, Issues 1-2: 49-62

 

Chaplin, George and Nina Jablonski.  2003. Skin Deep.  Scientific American 13, Issue 2