Wednesday, June 17, 2009

Why do Europeans, North Asians and Native Americans have pale skins?

The first hominid species, Homo habilis, is believed to have evolved around 2 million years ago in East Africa, The modern human, Homo sapiens, is believed to have evolved from an earlier hominid species, Homo erectus, and quickly colonized much of the African continent. As Homo sapiens emerged, groups of individuals spread slowly around 60,000 years ago to other geographically close by regions through what can be described as the Out of Africa hypothesis. Different tribes adapted to their new habitats through evolutionary changes. Most notable appearance-wise is skin colour, which is at its darkest in individuals living at the equator and palest in those living nearest to the poles. Even if humans did evolve, as suggested by Milford Wolpoff and his associates in their Multiregional hypothesis, separately and independently, the same evolutionary mutations would hold true as to the reasoning behind varying skin colouration. Pigmentation arises because of melanin, a pigment that comes in two types: pheomelanin (red, and found predominantly in freckles and reddened areas such as the lips) and eumelanin (very dark brown). Both the amount and type are determined by four to six genes which operate under incomplete dominance. One copy of each of those genes is inherited from each parent. Each gene comes in several alleles, resulting in the great variety of different skin tones. So what purpose does this pigmentation serve?

Unlike us, many of our hairier mammalian cousins do not have such a pigmented skin; by having a hairy coat, they are protected from UV radiation-induced damage as the hairs themselves absorb or reflect most-short wavelength solar radiation. Non-human mammals that are active in hotter, sunnier climates tend to be sparsely coated in order to facilitate passive heat loss (and sweating is limited to certain areas). Unsurprisingly, they tend to show increased melanin concentrations in the skin along their backs where they are exposed to more sunlight. Our primate ancestors would also have had thick hair and hence pale skin.

Early hominids in Africa were active hunters and gatherers and as such, it was an evolutionary advantage to have less hair and more exocrine glands for increased sweat production, allowing early man to pursue prey and be active in the sun for longer without overheating. Having virtually naked skin, it was imperative that hair loss had to be coupled with increasing melanin production in order to protect underlying skin cells from the effects of over-exposure to UV radiation (which can cause mutation in underlying cells and hence lead to melanomas and cancer) by absorbing and dispersing it as infrared radiation (felt as heat).

A widely accepted hypothesis regarding variations in pigmentation is that of Vitamin D Biosynthesis. It is known that sunlight, particularly ultraviolet rays, stimulate the production of vitamin D, which is partly responsible for calcium uptake in bones. A deficiency of vitamin D can lead to once-common conditions such as rickets, in which bones do not form properly and become misshapen. Heavily pigmented skin in areas of high sunlight exposure not only provides excellent protection from UV radiation, but can also limit vitamin D biosynthesis and hence reduce the risk of developing toxicity. Too much vitamin D can lead to hypercalcaemia followed by nausea, vomiting, weakness, polyuria and polydipsia.

UV exposure decreases the further south or north one travels from the equator due to sunlight having to travel through more ozone and atmospheric gases (because of the tilt of the Earth's axis) and hence, an individual with heavily pigmented skin would struggle to produce enough vitamin D. Such conditions would favour slightly paler skin as adequate protection from sunlight is achieved, whilst maintaining favourable levels of vitamin D3 synthesis. The first European settlers would have been quite dark in complexion, and with a considerably lower exposure level of UV radiation, vitamin D synthesis was impeded. An American study carried out by the the United States Department of Agriculture found “87% of African Americans to be Vitamin D deficient”. To address this issue, some countries (particularly Canada and the USA) have programmes to ensure fortification of milk with vitamin D.

Also contributing to evolutionary adaptations of skin colour is the folate hypothesis. This suggests that folate (or folic acid), which is vital for the synthesis, repair and expression of DNA. It is also essential in red blood cell and sperm production (and as such its presence is quite necessary for human reproduction) Darker skin helps protect folate from UV-induced photolysis and hence helps maintain adequate stores of it. Darker skin however would be of no greater advantage in less sun-intense climates because vitamin D production would be more of a priority and hence, paler skin prevailed.

Interestingly, it has been observed that females tend to exhibit lighter skin pigmentation than males in all populations, and that sexual selection is partly responsible for a reduction in pigmentation in expanding populations. This hypothesis is based on observations that the attraction of human females and human males is partially due to their lighter pigmentation. Thus, lighter-coloured females are perceived to be more feminine than their darker counterparts and therefore would be preferred partners. This idea was further advanced in that paler females would have higher vitamin D production and would be able to meet the calcium demands of pregnancy and lactation. Due to maternal chromosomes, further generations would in theory become gradually paler. This can still be seen today in many Asian countries, particularly in the Far East where it is more desirable for women to be paler. This tends to be for cultural reasons, rather than to bring about any advantageous characteristics in children; for example, in China, having a tan is linked to historically working in fields and being a peasant- having a lighter complexion reflects a higher social status rather than any indication of undesirable genetics and many women practice diligent sun avoidance and take up chemical skin-lightening .

Early migrants to Europe would still have had fairly dark skin, and nowhere near as light as today's modern northern Europeans. However as mentioned, there would have been less exposure to the sun and this would have led to even lower production rates of vitamin D and lighter skin would've proven more favourable. Given that increased vitamin D would give individuals stronger skeletons as well as serving a function in pregnancy, it would have allowed modern man to quickly establish stable and expanding colonies. A superior intellect combined with technologically greater tools and weaponry would have given Homo sapiens a considerable advantage over already present populations of Homo neanderthalis. These factors may have led to the retreat of many neanderthal tribes to the southern tip of Europe (and their eventual extinction). Homo sapiens would prove the victorious conqueror in Europe.

Tribes still exist and have done so (although in considerably smaller numbers ) in the colder regions of Northern Asia, for example in Siberia, for thousands of years after travelling (according to the Out of Africa hypothesis) north east from the middle and near East. Like Europeans, they are also of pale skin because of reduced sunlight exposure. However, populations living nearer to the Siberian Arctic, as well as many other Inuit tribes may be of a darker, more South-East Asian complexion as they gain enough vitamin D from their fishy diet and biosynthesis becomes less important. More melanin in the skin may also serve to prevent over production (even though sunlight is of a lower intensity).

Native Americans share certain phenotype characteristics with the indigenous peoples of North Asia, because it is believed the same people traversed huge ice plains thousands of years ago and colonised North America and consequently South America. Interestingly, many native American tribe populations cannot grow or grow very little facial hair, although whether this provides an evolutionary advantage remains to be seen- it may be through sexual selection, females preferred males with less facial hair and so such genes were gradually removed from the population.

To conclude, inhabitants of Europe, North America and North Asia are genetically paler than their African ancestors who spread and conquered new lands, as an evolutionary response to differing amounts of sunlight (or more precisely UV radiation). Melanin serves to protect underlying cells from the harmful effects of UV. Dark skin provides the best protection from UV-induced cancers and folate photolysis, but leads to reduced vitamin D production. The opposite is true in pale skin. In real world terms, there is essentially a trade off between each factor and an optimum level of melanin concentration is achieved based on basically how strong the sun is. Given today's chemical and technolgical advances, although we can choose to make our skin lighter or darker to appease a personal preference or reflect our socioeconomic status, we were all born a certain colour because of the actions of our ancestors and how they evolved to enable them to prevail wherever they chose to settle.

References: The Evolution of Human Skin and Skin Color - Nina G Jablonski

1 comment:

  1. A nice summary. Particularly pleased to see the idea of evolutionary trade-offs mentioned in your conclusions. Is it not also worth considering the idea that there is no positive selection for light skin, merely a relaxation of selection against, coupled with genetic drift?