Skin colour is determined by the amount and type of the pigment melanin within it. This can be generated in small amounts by sun exposure but is nearly exclusively genetically influenced and therefore ancestry is the major contributor to the shade of an individual’s skin. Melanin in the skin absorbs and disperses 99.9% of UV in sunlight as heat. Without it, the UV radiation would generate direct and indirect damage to the DNA of living cells and the malfunction of their gene product, potentially damaging the health of the organism, namely through sunburn and skin cancer.
It is useful to consider the evolution of human pigmentation into the pale skinned populations of Europe, Asia and Native America as occurring in two stages: the first being from the pale pigments of apes and primates into the dark skinned early African humans and then paler again as humans migrated north, away from Africa. The early hairless humans evolved around 4.5 to 2 million years ago in the rain forests of Africa. They first became hairless to combat extreme heat through sweating and sweat evaporation. They moved out onto the East African savannah where they were exposed to much more sun and so more to the risks of UV exposure. This appears to be the first time a major selection pressure was imposed of the skin colour of the human race, although many different theories have been suggested as to the key selection pressure, initial thoughts were varied. One such was the risks of skin cancer and its development causing a selection pressure. However most skin cancers do not develop until later life (after many years of exposure) certainly after the age of child bearing and so would not reduce or remove the low melanin genes from the gene pool. Another suggestion was that sun burn to the nipple area prevented sufficient breast feeding for the young of low melanin individuals and thus their survival was less likely than high melanin individuals. However the greatest and most accepted selection pressure that resulted in the darkening skin of early humans is to do with sunlight’s affect on Folate levels. Very little sun exposure in pale skinned individuals can reduce Folate levels in the skin significantly. Folate has a large role in the development of foetal nervous system and bone marrow, and in sperm and red blood cell production. Pale skin, high sun exposure and therefore low Folate levels, decreases the fertility of males and developing embryos are less likely to reach full development; a huge selection pressure. The advantage of being dark skinned in Africa evolved quickly in early humans; and the trait is still seen today in African ancestries.
However, as well as being dangerous to our health, sunlight is also necessary for the production of Vitamin D in our skin. Vitamin D plays a vital role in the regulation and control of calcium and phosphate uptake and bone growth. Human skin cells have the ability to produce Vitamin D endogenously using energy from sunlight. In the diet of the early hunter-gather humans in Africa there was no need to allow sunlight into skin for the biosynthesis of vitamin D, as there was enough consumed from fish caught to maintain life processes. In fact it has been hypothesised that settlements in equatorial areas such as Africa could not have established without man’s ability to fish, and so maintain exogenous supplies of Vitamin D as skin evolved darker. This was also the reason early European settlers were able to survive until human skin pigmentation return to a paler shade around 6,000 to 12,000 years ago with dark skin, when man migrated north 40,000 years ago. It was only at this time, when farming arose as the main source of food, that levels of vitamin D consumed in the human diet decreased significantly. This presented a trade-off between reducing melanin in the skin allowing more sunlight and UV into the skin cells to produce Vitamin D, with the problems this presented in fertility and foetal development; a balancing act if you like. In Europe, where the average exposure to sun was much less than Africa, the paler skinned individuals of the population allowed more sunlight to penetrate the skin and therefore could internally produce more Vitamin D. This made them healthier, with stronger bones, and gave them a survival advantage over darker skinned individuals. The balance only swung in favour of paler skin here because the sunlight intensity was less than in more southerly areas and the ability to produce Vitamin D greatly outweighed the negative effects of reducing melanin in the skin. In fact, the further north the population was, the less sun exposure they experienced and the further the balance swung. In the far north there were nearly negligible risks of reduced Folate due to the lack of sunlight, and the biggest selection pressure was on those who could extract the most energy, from the little sunlight there was, for Vitamin D production; i.e. the paler in the population. This is reflected in modern European ancestry patterns of skin pigmentation. Celtic and Scandinavian population are very pale with low melanin as their environments had low sun exposure most of the time; whereas the lower European populations, such as the Spanish and Italians, where sun exposure is greater, have darker skin with more melanin enabling them to produce Vitamin D but also giving them greater protection against reduced Folate.
Northern Asians and Native Americans reside in a similar, or the same latitude, as Europeans (around 47O N) and their origins from the common early ancestors in Africa mean that the explanation for the pale colour of their skin is almost identical: the balance between using UV for Vitamin D production against the risks of reducing Folate. However, although pale compared to early humans and more southerly populations, Northern Asians and Native Americans are still darker than the people of Europe. One suggested reason for this has been that Asians and Native Americans consume more fish in their diets than Europeans. This is certainly the believed wisdom for the explanation of the dark skin of the Inuit population, who live at more northerly latitudes than almost all Europeans and yet have darker skin. The high fish diet appears to provide enough Vitamin D to compensate for the lesser UV sunlight without the need to evolve lighter skin. However, many Northern Asians and Native Americans settle in areas that are land locked and have very little fish in the diet, and vice versa many of the palest of the European population, for example Swedes and Danes, have very high fish consumption in their diet, yet have still evolved pale skin.
One suggested reason I found to be plausible is based on the central premise of sexual selection in mates. Research into mating preferences and perceptions of attractiveness pre-industrially has shown a “cross-cultural preference for lighter (skinned) women” (as concluded by van den Berge & Frost, 1986). Where man was capable of farming in the warmer climate of Sub-Saharan Africa, women were largely involved in the cultivation and production of food. This left men with more time and more capability to seek and provide for multiple wives, leading to a highly polygamous culture. This left few unmated individuals and very little preferential choice in mates which ultimately caused the unusual and unnecessarily dark pigment of Sub-Saharan Africans. When humans moved north into colder climes, the land supported less life and hunter-gatherers had to go further and in harsher conditions to get food. This meant that men had to work a lot harder to provide for multiple wives and many children, and for this reason European men became less-polygamous. The increased risks of travelling further in harsh conditions and the increased risk of running out of food also meant the male mortality rate was higher. These factors together increased the number of unmated men and women in the population and hence increased the choice available for preferential mating, which as previously discussed, was for paler women. In the majority of Europe, especially Scandinavia, Britain and North East Europe, the polar caps stretched further south 6000 years ago than today and the conditions for those living here were significantly harsher. This meant that particularly here, the choice in mating was much greater and the population became noticeably paler than in comparable latitudes. This is still evident in the extremely pale population of Celtic and Scandinavian ancestry.
The question still remains though; if the conditions are no longer this harsh in modern Europe, and probably haven’t been for a good few thousand years, why have these populations remained so pale, especially with such an obvious selection pressure as the risks to health of UV damage, such as sun burn and skin cancer?
Epithelioma (a type of skin cancer) prevalence correlates very well with latitude of settlement and therefore UV exposure, and also correlates well with areas of the body which are most exposed, for example the face and hands. The darker a person’s skin is naturally, the less chance they have that UV exposure will cause damage resulting in these epitheliomas. The reason this has not been experienced as a selection pressure may be the point I have already touched on: the average age of cancer onset is post child bearing age and so any pale skinned individuals that die due to UV induced cancers are likely to have already passed on their genetic material. In addition humans are unique in their ability to adapt not just by genetic mutation, but by cultural and social evolution, and it is my view that this is relevant to the pigmentation of these populations. For example humans clothe themselves and so reduce sun exposure to the skin by this method, removing the need to genetically evolve to reduce UV exposure. I feel this may contribute to a widely backed idea that populations reach an adaptive plateau, where they become well adapted enough to their habitat and environment that little further evolution occurs. For example, the early Scandinavians faced extreme challenges in their environment and so adapted very well to the harsh conditions, such as they’re paler skin. When the environment became less harsh they possessed characteristics that posed no disadvantage, and together with the human ability to culturally adapt to any further small changes in conditions, the requirements for them to evolve further were absent.
In conclusion, the evolution of pale skin in European, Northern Asian and Native Americans as humans migrated out of Africa was a result primarily of an evolutionary trade-off between the need for Vitamin D and prevention of the loss of Folate, and secondarily, the sexual preference and ability to choose mates that were of pale skin.