Extract from In the Blood; God, Genes and Destiny by Steve Jones (1996) (pub Flamingo, ISBN 0 00 255512 3)

From Chapter IV: The Soul Beneath the Skin

What does melanin do? At first sight, evolution has got it wrong. If humans followed the rules that apply to other animals, Europeans ought to be black and Africans white. Most creatures from hot sunny places are not dark but light, for the simple reason that black objects absorb more of the sun's heat. A black person in sunlight absorbs a third more solar energy than does someone with white skin.

Humans are tropical animals, which explains why most people wear clothes most of the time. Give people from the Equator or the Poles the choice and they set the central heating at the same level. Cold is dangerous. The blood gets sticky, veins [primarily arteries - Jones using a common pop-science shorthand here?] contract and blood pressure goes up. All this makes the heart work harder. Deaths due to heart disease and stroke show a dramatic increase - by one-tenth for every three-degree drop in temperature - in chilly weather. In the British midwinter, the number of strokes is twice that during the summer. Even a cool rather than a comfortable living-room increases the risk of death. This alone explains why the increase in winter death rate is twice as great among poor people than among rich. Anything that helps people from cold places to warm up (black skin included) should, it seems, be favoured.

Cold has certainly affected the evolution of other characters. Australian aborigines can sleep naked outside on frosty nights while the effete sheep-shearers who surround them shiver in their sleeping bags. Body shape has also changed to deal with low temperatures and people from the Arctic are (with some exceptions) shorter and squatter than those from the tropics and have smaller surface through which to lose heat.

Being cold is unpleasant, but it is usually possible to do something about it and, even if this fails, to survive a drop in body temperature. Being too hot, though, is lethal. Many animals live on a thermal tightrope, shuffling in and out of the sun to stay at the right temperature. In a Spanish bull-ring, seats in the sun are half the price of those in the shade, showing the value of the right relationship with sunshine. If it goes wrong, and things get too hot, the body has a biochemical fire-brigade. It arrives, bearing emergency blankets heat shock proteins [sic - word "called" between blankets and heat missing?] These proteins (sometimes, rather fetchingly, called chaperonases) cluster around fragile parts of the cell and protect them. People from Siberia summon their chaperones at lower temperatures than do those from warmer climes, so that it takes less to inflame the inhabitants of Novosibirsk than those of Naples.

All this makes it doubly odd that tropical peoples are dark and must pay a penalty for soaking solar energy. However, there is more to staying cool than avoiding the heat. In the moist tropics blacks given an exhausting task to perform in the sun can continue working for longer than can whites. This is because blacks are, for reasons not well understood, able to sweat more than can whites. Sweating - the body's most effective cooling mechanism - more than compensates for the effects of skin colour. A black skin might, indeed, reduce overheating because it protects so well against sunburn (which reduces the ability to sweat). In a society based on hunting, in which the hunter is out all day and needs to keep cool, sunburn would slow him down and might cause him - or his children - to starve.

Ultraviolet light is powerful stuff as anyone who has watched their carpets fade knows. Melanin is good at keeping it out. People without much melanin are at high risk of skin cancer. Because so many of its citizens have pale skin, Ireland has the third highest incidence of the disease in the world (after Australia and South Africa) in spite of the lack of chances of sunbathing. Whites with light skin are at eight times greater risk than those with dark. Thirty times more sun is needed to cause sunburn (the prime cause of skin cancer) in blacks than in whites.

The uncontrolled cell division that is cancer is sparked off because sunlight produces reactive substances - free radicals - when it interacts with oxygen and water. These can break the DNA chain. Much of melanin's protective action depends on its ability to mop them up. By so doing it reduces DNA damage by three-quarters. This is, though, not the whole story. Under intense ultraviolet, melanin is overwhelmed and begins to produce free radicals of its own that damage - rather than protect - the DNA. This is particularly true of the form of melanin found in red hair and freckles, which is why red-heads are particularly liable both to sunburn and skin cancer. The genes for red hair are also responsible for producing a hormone that encourages the skin to produce melanin when damaged by sunlight. Eight out of ten pale-skinned red-heads have a damaged version of this gene, compared to only one in twenty-five of those who have dark hair and tan easily.

Edited paragraphs: 

.....................Albinos in Africa have a rate of skin cancer a thousand times greater than their pigmented brethren. No albino more than twenty years old is free of the disease and in the sunniest parts of the continent (such as Nigeria) only about one in ten lives past thirty.

Ultraviolet light, though, is not all bad. When it penetrates the skin, it makes vitamin D; far more than comes from even a well balanced diet. Without it children suffer from rickets, an illness producing soft bones and deformed skeletons.....................

Although the idea of rickets as the driving force behind the evolution of skin colour is attractive, melanin was probably not lost because of changes in vitamin balance. Before cities and smoke most people spent most of their time in sunlight. Rickets is a disease of civilization..........

Although a change in skin colour is the most striking event in human evolution, nobody really knows why melanin was lost as people emerged from Africa

The chapter continues with further evidence, some highly speculative, for the various roles of  melanin. 

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