Without too much thought a person would conclude that human skin colour tends to be darker in populations originating near the equator, and increasingly lighter as one moves into higher latitudes. But why is that? How to explain exceptions to this general rule, such as the darker skinned Inuit? And why are women slightly lighter skinned than their male relatives? To answer these and related questions, science has combined evidence collected in many diverse fields of study. This article is mainly drawn from two sources: an October 2002 Scientific American article entitled “Skin Deep” by Nina G. Jablonski and George Chaplin, and the Wikipedia entry found under “human skin color”. The latter contains numerous links and references for those readers interested in more in-depth details, or related topics.
An early theory, now generally discounted, involved skin cancer. Light-skinned people tend to develop skin cancers of various types when overexposed to sun, such as when living nearer the equator than their ancestors would have. The best example of this situation would be Australia, whose population is largely made up of people of light-skinned northern European origin. Historically they have had extremely high rates of skin cancer, and have developed ingrained cultural habits to counteract this. For example, school children are turned home if they show up at school without hats. Conversely, dark-skinned tropical Africans, and indeed Australian Aboriginals, have low skin cancer rates despite living under harsh and relentless tropical and sub-tropical sun.
The theory went that natural selection would have weeded out lighter-skinned people living in high sunlight regions. However this theory has a simple flaw in that most skin cancers develop later in life, after a person’s child bearing years are over, hence ruling out evolutionary effects.
A recent theory that is gaining acceptance in a broad range of scientific and medical fields goes like this: natural selection over time will result in a skin colour that is (1) dark enough to block lower frequency UVA rays that destroy folic acid, but (2) light enough to allow medium wavelength UVB rays to penetrate the skin and play their role in vitamin D production. In other words Mother Nature is playing a fine balancing act between a damaging sun exposure effect, and a beneficial one. The protection against skin cancers afforded by darker skin appears to be a side benefit.
What kind of time frame is involved for evolution to work its magic? Well, an indication can be found in South Africa, w h e re the indigenous Khoisan people are quite lightskinned, while the recently arrived (~1,000 years ago) Zulus are as dark as the people still living in equatorial West Africa w h e re they came from. Further north, the Arabs on the east coast of the Red Sea arrived about 2,000 years ago. They are lighter skinned than the indigenous peoples on the west coast, who are extremely dark, but perhaps the Arabs have slowed down the development of darker skin through the use of their traditional dress which effectively blocks UV. A t any rate, it can be said that in order for a population to evolve a lighter or darker skin suitable to the ambient sun conditions, it must remain in the same general region for thousands of years, and of course the conditions allowing natural selection to take place must exist: natural mutations resulting in different skin colours and statistically significant population numbers and birthrates.
Skin colour mechanisms
People possess genetically determined amounts and types of melanin in their skin. When exposed to sun, melanocytes in human skin produce melanin. Melanin protects us from the damaging effects of sun in two ways – it physically absorbs UV rays, and it chemically neutralizes harmful free radicals formed by UV skin damage. There are two types of melanin, red pheomelanin and dark brown eumelanin. Each parent passes on four to six genes controlling the type and amount of melanin, and there are several variations (alleles) of each of these genes. The net effect is that the full range of skin colour from pale white to dark black and every shade in between can be genetically determined, and changed via mutation and natural selection.
Folate, in the form of folic acid, is a critical participant in the synthesis of DNA when cells divide, so it is essential for any process that involves the rapid creation of cells. Examples of this are the production of sperm, and fetal development. The fetuses of babies whose mothers experienced overexposure to sun have a much greater chance of developing spina bifida and other neural tube defects. These defects have often been linked to expectant mothers using tanning studios during their first trimester. Males overexposed to sun tend to have lower sperm counts, and indeed males with this condition (whether sun related or not) are commonly and effectively treated with folic acid supplements.
In lighter-skinned people (and to a lesser extent in darker skinned people) UVA rays are able to penetrate through the epidermis (outer skin) and deep into the dermis. Blood vessels in the dermis are exposed to this radiation, and the folic acid irreversibly breaks down. At the same time, DNA in the skin cells can be damaged, eventually resulting in skin cancer. Obviously people within a larger p o p u l a t i o n without enough melanin to block the UVA rays will tend to have lower sperm counts and more birth defects, meaning a lesser chance of passing along their light skin genes; the darker-skinned people will be more fertile and their more numerous offspring will have less birth defects. Over time the statistical mechanism at the heart of natural selection will result in a population with little or no members with skin too light for the local sun conditions.
Vitamin D plays a critical role in human health and survival. It allows our intestines to absorb calcium, which is obviously needed for our skeletal development and other purposes. In other words as vitamin D levels drop, the body becomes increasingly unable to extract calcium from food. Vitamin D also plays a crucial role in our immune systems. Low vitamin D levels lead to a weaker immune system, lowering the barriers our bodies maintain to keep at bay nature’s nasty host of diseases (e.g. rickets), parasites and viruses. Vitamin D deficiencies are also specifically linked to higher incidences of lung, colon and prostate cancer. Furthermore, a lively debate is currently raging within the medical community over whether low vitamin D levels caused by lack of sufficient sunlight are a factor in diseases such as multiple sclerosis. A map of global distribution of MS dramatically highlights its preponderance at higher latitudes. On the other hand, those populations tend to be of northern European heritage, so perhaps that racial group is genetically predisposed to MS, and vitamin D deficiency is a red herring.
If skin is light enough, UVB rays penetrate the epidermis, and reach small bodies called keratanocytes. As pre v i o u s l y mentioned, melanocytes at the same time are stimulated to produce melanin, inside little packages called melanosomes. The melanosomes enter into the keratanocytes and form nuclear caps over the DNA inside, protecting it from the UV radiation. The radiation entering the keratanocytes stimulates them to convert cholesterol into previtamin D. This previtamin D is taken by the bloodstream to the kidneys, where the conversion to vitamin D proper is completed.
Historical development of skin colour
Scientists often look to chimpanzees to get an idea of where specific human characteristics came from. Anthropologists assume that at the time of the chimpanzee – prehuman hominid split, about 7m years ago, our early hominid ancestors were similar to chimpanzees in terms of hair and skin. Chimps have stayed more or less the same, but the hominids have evolved greatly, with one evolutionary branch ultimately leading to us. Chimps have pinkish skin which is mostly covered by thick sunblocking hair. The small hairless patches, such as on the face and hands, can produce melanin in order to protect the chimps from sun damage. The various pre-human hominids that came and went in sub-Saharan Africa (this was the limit of their range) probably kept these traits of body hair and light skin for several million years, even while other aspects evolved such as brain size and bipedalism.
Scientists studying the evolution of lice have concluded that prehuman body hair began to disappear approximately 3.3m years ago. At the same time darker skin would have been evolving, to prevent folate break down in the increasingly exposed skin. The ability to produce melanin was already there, as seen in chimps. Genetic evidence suggests that by about 1.4 to 1.2m years ago this gradual change to characteristic human hairlessness and dark skin was complete.
There is a lot of debate about what selective pressures were at play to cause this loss of body hair. The most popular thinking is that there was a fairly rapid change in pre-humans, loss of hair and darker skin being only a part of it. Suddenly (in anthropological terms) our distant ancestors developed larger and longer limbs. It is believed that they were able to travel farther, faster, and increased their protein intake because their larger brains made them better hunters. This entirely new life style, diet and physiology, coupled with a hotter climate on the East African savannah (they had left the forests so they were now more exposed too), meant that they required a more efficient cooling system. Sweat glands spread over most parts of the body, and hair was dropped, as the two are somewhat mutually exclusive. Then much later, as anatomically modern humans migrated north out of Africa, perhaps around 60,000 years ago, the lower sunlight levels encountered as groups settled further north resulted in appropriate lightening of skin colour.
An interesting side bar involves head hair. These hominids living one to three million years ago were walking on two feet. It would seem obvious that head hair was retained to protect the scalp, mercilessly exposed to the sun’s rays, as any balding middle aged white guy on vacation down south knows. Chimpanzees and indeed most other mammals have straight hair. Scientists have shown that straight hair in fact can act as fiber optic conduits, taking UV rays down to the epidermis. Polar bears have exploited and exaggerated this feature to absorb as much heat as possible while maintaining a thick insulating coat. The pre - h u m a n hominid population went the other way and seems to have evolved characteristically kinky African hair to minimize scalp exposure to UV. When humans eventually left Africa for regions exposed to less UV radiation, they reverted to straight hair.
Obviously nature’s skin colour balancing act has been completely messed up by human migration, especially in today’s world of modern technology. It is commonplace to see dark skinned people of southern Indian extraction living in dimly lit northern England, and Jamaicans in Scotland. Australia is full of people whose skin colour is more suited to the fog bound British Isles of yesteryear. Calgary is full of darker-skinned southeast Asians of various hues. Most big cities of the world are a quilt of different skin colours. If the theories described above hold true, one would expect to see a host of medical problems among people living outside of the UV zone they were “designed” for. And indeed one does. As mentioned before, whites living in extremely sunny places suffer from low levels of folic acid, and thus higher incidences of the associated effects, not to mention skin cancer. These effects can be minimized through sun protection (hats, sun block, clothes, etc.) and mitigated via folic acid supplements. Darkskinned people living in places that receive less UV suffer much higher incidences of effects linked to low vitamin D levels. Similarly, these can be partly remedied through vitamin D supplements – cod liver oil in previous generations, but in more palatable forms recently – tanning salons, and winter vacations in sunny places.
And what of the Inuit and others like them? How do they survive with darkish skin in very low light levels much of the year? Not coincidentally, their traditional diet is largely made up of fish and sea mammals, both high in vitamin D.
And why do females have lighter skin? One theory is that this is a result of an almost universal male preference for women with lighter complexions. Apparently in all cultures studied in this regard, both current and historical, males were found to display this preference. (Stop tanning ladies!) Armed with the more recent folic acid / vitamin D theory for skin colour however, scientists are rethinking this sexual preference theory, or seeing it as the effect of an underlying cause. That theory is that because women have higher calcium requirements than males during their fertile years, for fetal development and lactation, evolutionary pre s s u res have pushed the skin colour balance slightly more towards increased calcium production via lighter skin, at a cost of slightly more folate damage. Asecondary or derivative natural selection mechanism would have happened in lockstep with the primary one. Males with a mating preference for slightly lighter-skinned females (within a given population) would have experienced slightly higher birth rates.
Their more numerous male offspring would have tended towards the same lighter skin female preference, and so on - an explanation why males worldwide seem to prefer women with slightly lighter skin than themselves.
One unpleasant topic is the role of skin colour in racism. Humans throughout the ages have found skin colour to be the most convenient way to distinguish between “us and them”. Immediately following Barack Obama’s recent election win, it was astounding to hear the number of people who commented on it being a victory for people of colour, ahead of so many other things the election re s u l t seems to signify. But obviously skin colour is a huge and very real issue that affects millions of Americans in a negative way. However Obama is half Caucasian, his African roots did not come via the slave trade, and the dominating cultural factor at play in his election victory is that he is a member of the Harvard-educated ruling elite. Science tells us that skin colour is an extremely fickle, changeable characteristic, and all it shows really is what latitude a person’s ancestors came from. If the white Australians are isolated for a few thousand years (and run out of sun block and hats), they will gradually become as dark as the Aboriginals.
This general topic is a great one for scientific reading, as it touches on so many areas of science and medicine. In particular there exists a great body of work combining genetics and anthropology. For example, can tight curly African hair, straight black Aboriginal hair, similarly dark skin, and their respective genetic markers tell us something about the timing of the human migration path all the way to Australia? Another area that could be interesting to know more about is this breakdown of cholesterol into vitamin D. Is that the “good” or the “bad” cholesterol? Is more sunlight a treatment for high cholesterol? How do albinos figure in this, and how do they fare in sunny climates? Did pre-humans develop sweat glands all over their body rather than develop better panting to cool themselves as other African mammals - because language had already become a crucial characteristic, and the tongue couldn’t be modified for cooling purposes? How can scientists infer when humans lost their body hair by studying lice?? And why am I going bald – do I need more vitamin D??? I’ll leave these questions and more to the interested reader to pursue.