Skin colour is one of the most obvious and most variable traits humans possess. As visual creatures, people readily observe these differences in one another. Anthropologist, Dr. Nina Jablonski, and colleague George Chaplin have unraveled the mystery behind the diversity of this distinguishing characteristic in their article, ‘Human skin pigmentation as an adaptation to UV radiation’. The pair have presented the first complete theory of skin colour as an evolutionary adaptation.
Paeleotologists have long known that human ancestors once had bodies covered with dense hair. Several theories on the evolution of the present ‘hairless condition’ exist. In this case, the authors suggest this was in response to hot conditions and the need to lose body heat upon physical activity. As naked beings, their once-protected skin became exposed to a multitude of environmental elements, including abrasion, infection, various chemicals and weather conditions. At the same time as the disappearance of the majority of body hair, humans evolved skin that was a more resilient barrier to such hazards and also contained the protective pigment eumelanin (melanin).
In order to understand why different groups of people evolved different levels of this pigment, and hence different coloured skin, it is necessary to understand Darwin’s theory of evolution by natural selection. Evolution occurs because the individuals best suited to their environment are able to survive and produce more offspring than those less well adapted. Their offspring make up a larger proportion of the next generation and are, in turn, more likely to pass on these successful characteristics to their children. Over time, the population changes into one in which most, and eventually all, individuals possess the adaptation.
There has been much debate about the specific environmental conditions which made dark pigmentation beneficial in some situations. It was previously suggested that, “the protective effects of melanin against sunburn, skin cancer” made dark skin more valuable. Jablonski explains that both sunburn and skin cancers have insignificant effects on a person’s success in procreating as most survive to reproductive age, so this is unlikely to be the reason for the evolution of dark pigmentation.
Another theory was that dark pigmentation developed to prevent the overproduction of vitamin D. Vitamin D is a nutrient needed in only small amounts by the body. It is produced by a reaction that occurs in the skin upon exposure to the ultraviolet B radiation in sunlight. It was thought that high levels of pigmentation, which partially inhibit vitamin D production, evolved to prevent its overproduction and toxicity. This theory was also disproved when it was found that the human body has an inbuilt system to destroy excess vitamin D in the skin, thus overproduction is impossible.
Jablonski and Chaplin propose another theory for the benefit of dark coloured skin. They suggest that dark pigmentation protects folate in blood vessels from being destroyed by UV radiation. Folate is another vitamin required by the body and destruction by UV radiation could lead to deficiency of this nutrient. Folate deficiency could potentially affect both male and female fertility and can, “lead to potentially fatal birth defects such as neural tube defects”. Since folate deficiency, due to lack of adequate protective pigmentation, affects an individual’s ability to survive and reproduce, this would be a strong evolutionary pressure.
Therefore, it is thought that the evolution of darkly pigmented skin came about as a protective mechanism in populations moving from environments with low UV radiation to high UV radiation. Examples of this are the light to moderately pigmented ancestral North Indians who came to inhabit India, and the east Asians who populated South America.
Lighter, depigmented skin also evolved multiple times in response to environmental conditions. The prevalence of lightly pigmented skin in populations at high latitudes is also related to the amount of sunlight, and hence UV radiation. Groups dispersing to these areas would have received less UV exposure and therefore dark pigmentation would prevent the production of sufficient levels of vitamin D. Vitamin D is necessary for calcium absorption and healthy bone growth, its deficiency has been implicated in diabetes, auto immune diseases and viral and bacterial infection. It is now largely accepted that lighter pigmentation evolved in order to maximize production of vitamin D in the conditions of reduced sunlight.
Thus, darkly pigmented skin became established in areas of high UV radiation; arid or tropical regions near the equator (such as Africa, India and South America) and regions of high altitude (the Tibetan plateau and the Altiplano). Conversely, depigmented skin evolved in populations where negligible amounts of sunlight reached the earth for most of the year; regions of high latitude, closer to the poles (northern Europe and east Asia).
Interestingly, peoples inhabiting the middle latitudes evolved skin that compromised these two conflicting physiological needs; the ability to produce sufficient vitamin D and also prevent folate deficiency (by UVR decomposition). These populations developed moderately pigmented skin which could increase pigment levels in response to the seasonal fluctuations in UV radiation; that is, they were able to tan.
Over millennia, the ancient ancestors of modern day humans have needed to adapt to new environments as they migrated between continents and countries. The level of sunlight, and associated UV radiation, in each region varies enormously and is responsible for the plethora of different skin colours seen today. The level of skin pigmentation seen in various populations results from a delicate balancing act: the need to protect folate from destruction by UV radiation while still producing enough of the “sunlight vitamin”, vitamin D.
These days, humans travel rapidly between countries, continents and hemispheres. We frequently live or holiday in destinations far from the areas our ancestors settled in and adapted to. It is to those intrepid migrants that first left Africa and travelled around the globe that we each owe our unique skin colour.
Reference
Jablonski, N.G & Chaplin, G, 2010, ‘Human skin pigmentation as an adaptation to UV radiation’, Proceedings of the National Academy of Sciences, 107(2):8962-8968.
Image reference
‘Looking forward’ uploaded to flickr.com by ‘NeoGaboX’ on the 19 August 2009, <http://www.flickr.com/photos/neogabox/3837281447/>.
[...] Spencer Wells talks about genetic changes due to migration, concentrating on genes affecting skin pigmentation. He fails to mention, though, that that this story is still not well understood, and that there’s controversy about the classic “melanoma versus vitamin D” explanation. [...]