ON THE sun-drenched Caribbean island of St Thomas, there is a plant known to the locals as the tourist tree, so called because its red bark peels off in long pale strips. It should be grown more widely, as a cautionary reminder that you don鈥檛 have to travel to the tropics to see fair-skinned holidaymakers burnt to a crisp. The sad fact is that some of us are woefully equipped to deal with even a little sunshine. How could evolution have played such a nasty trick?
People have long assumed that pigments in our skin protect us from the sun. It seems obvious, given that the nearer the equator you go, the darker the skin of the local people. And the discovery of a link between UV and cancer has strengthened that conviction. But if dark skin is such an advantage, why are we not all as dark as possible?
This has puzzled experts for decades and there has been no shortage of explanations, from the suggestion that males prefer paler women to the notion that the lack of sunshine at high latitudes has caused skin colour to fade, in much the same way that cave-dwelling animals have lost their pigmentation. But none is completely convincing.
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Now Nina Jablonski from the California Academy of Sciences in San Francisco thinks she has the answer. 鈥淪kin colour is not the result of a simple one-on-one relationship between UV and melanin,鈥� she says. 鈥淚t鈥檚 a complex of two conflicting functions.鈥� In other words, being dark has its benefits, but so too does being pale, and the huge variety of human pigmentation is the consequence of a struggle between opposing forces favouring dark and light.
It all started millions of years ago when our ancestors became bipedal. Walking upright in the savannah was hot work, so to cool down they needed to sweat effectively. We have more than 2.5 million sweat pores 鈥� proportionately more than almost any other mammal. An average sedentary adult loses some 0.7 litres of sweat daily, and exercise can boost that to2.5 litres an hour. But the cooling effect of sweat relies on evaporation, so to make sweating really efficient our ancestors had to lose their body hair.
Jablonski suspects that early hominids, like modern chimpanzees, would have had pale skin and dark hair. As they lost their protective hairy coat, she says, there would have been strong selection pressure for dark skin, and not just because darker individuals would have been less prone to sweat-reducing sunburn and so more likely to survive and pass on their genes for that pigmentation. Jablonski believes the main reason hominids became darker was to prevent the breakdown of an essential nutrient called folate.
A member of the vitamin B complex, folate is perhaps better known in its related form, folic acid. It plays a crucial role in a developing embryo鈥檚 neural tube: too little can lead to serious neurological disorders ranging from spina bifida to failure to form a complete brain and skull. Folate is also important in biological systems involving cell division, such as blood and sperm production, and a deficiency can lower male fertility. 鈥淭he tie-in is that folate is both biologically vital and UV-sensitive,鈥� says Jablonski. She cites research by Lorenzo Botto from the Centers for Disease Control in Atlanta and colleagues showing that folate breaks down when the body is exposed to the longer wavelengths of UV. Clearly, what our savannah-dwelling ancestors needed was a sunscreen.
Enter melanin. A complex polymer of carbon rings and cross-linking proteins, melanin can absorb and disperse UV light. We all have approximately the same number of melanin-producing cells, or melanocytes, but the amount of melanin they contain varies. For example, a dark-skinned African can have up to 43 times as much melanin as a Caucasian.
Jablonski has recently confirmed the link between human pigmentation and UV exposure. But her study, with George Chaplin, a specialist in geographic information systems from Manchester Metropolitan University, suggests that melanin is not simply there to protect against sunburn, but that protection against folate deficiency is the key (Journal of Human Evolution, vol 39, p 57).
Jablonski and Chaplin used satellite images from NASA鈥檚 Total Ozone Mapping Spectrometer to map the intensity of UV across the globe, and a database of 85 indigenous human groups to model skin colour. As expected, there was a strong link between the two. But a closer look at the seasonal variations in UV revealed something unexpected 鈥� it is not peak summer UV that correlates best with darkening, but its intensity in autumn and winter. That鈥檚 surprising because if melanin鈥檚 main job is to guard against sunburn, anyone living below about 50掳 of latitude should be as dark as possible to have maximum protection in the summer Sun. The fact that most of us are not suggests there is more to it.
So what鈥檚 going on? Outside the tropics, winter sunlight does not directly damage skin, but it does destroy folate. That鈥檚 because folate is particularly sensitive to the longer wavelength of UV that can penetrate the Earth鈥檚 atmosphere in winter, even at high latitudes where there is more atmosphere for light to pass through. 鈥淭his finding greatly strengthens the folate hypothesis,鈥� says Jablonski.
But melanin is not biologically expensive to make, so why should people living in places with less UV have less of it? If melanin is needed to protect against neural disorders caused by folate deficiency, why aren鈥檛 we all as dark as possible? 鈥淚t鈥檚 all to do with vitamins,鈥� says Jablonski 鈥� in particular, vitamin D3, which helps us absorb calcium and deposit it in the bones. We need UV light to make vitamin D3. So the melanin that our forebears acquired to screen out UV 鈥� reducing sunburn, allowing sweating and protecting folate 鈥� would also block vitamin D3 synthesis, which might lead to rickets and worse. 鈥淭here is a second selection pressure working in the opposite direction, promoting paleness,鈥� says Jablonski.
If she鈥檚 right, our skin colour should be a balance of the darkening requirements for folate protection and the lightening requirements for vitamin D3 production. To test this prediction, Jablonski and Chaplin used the NASA UV data again, plus 180 studies of indigenous peoples and the biochemistry of folate and vitamin D3 to predict the skin colour of various populations around the globe.
鈥淲e were pretty much spot-on,鈥� says Jablonski. The data points that didn鈥檛 fit the model well were all from populations that had recently migrated. These included several peoples who had migrated in the past 1000 years. 鈥淲hen the sample was from a group who, like Basques, East Africans and Australian Aborigines, had been in a place for at least 10,000 years, then the match was nearly perfect,鈥� she says.
What鈥檚 more, because vitamin D3鈥榮 role in calcium metabolism means that women need more of it in pregnancy, Jablonski鈥檚 model could explain the observation that women in a given society tend to have fairer skin than men. It could also account for the blond hair common among several far-northern populations. 鈥淲ith the body exposed as little as possible,鈥� Jablonski explains, 鈥測ou need to maximise the amount of light that gets through to the head. Having pale hair allows more light to penetrate to the scalp.鈥�
According to Richard Brand from the University of Vermont, Jablonski鈥檚 ideas have generated great interest among doctors studying birth defects, especially in communities of recent migrants. A recent review by Lorenzo Botto found that folate-related deficiencies tend to be higher in such communities. But not everyone supports Jablonski鈥檚 theory. One criticism made by Maciej Henneberg from the University of Adelaide is that the pressures to be pale are not as great as she suggests because our hunter-gatherer ancestors didn鈥檛 have to make all their own vitamin D3, getting some of it in their diet. Jablonski contends, however, that this applies only to very modern human populations with the technology to engage in fishing or harvesting other foods rich in vitamin D3.
Other experts say skin colour is more a matter of who we find attractive than the vitamins we need to survive. Peter Frost from Laval University in Quebec and Pierre van den Berge from the University of Washington in Seattle have carried out a worldwide survey into people鈥檚 ideal partners. 鈥淚f we look at traditional societies, the ideal woman is 鈥榬ed鈥� in sub-Saharan Africa, 鈥榞olden鈥� in South-East Asia, and 鈥榮now-white鈥� in Europe,鈥� says Frost.
They have also found that, as well as having more melanin, men鈥檚 skin has more blood circulating in it. 鈥淢any cultures associate a brown-ruddy complexion with strength and a pale one with weakness, effeminacy and cowardice,鈥� he says. But Jablonski is sceptical about these criticisms. 鈥淭hey apply only if males are the ones choosing their mates 鈥� something that many studies have shown is not the case in humans,鈥� she says.
Despite all this, Jablonski and Chaplin have been applauded for their efforts to understand skin colour as a practical adaptation based on our nutritional needs. Dermatologist James Cleaver from the University of California in San Francisco describes their work as 鈥渙ne of those rare, sweeping surveys that illuminates and clarifies many hitherto puzzling factors鈥�. In his view the most striking implication is that human pigmentation is such a fickle entity, changing according to latitude, elevation, humidity, diet and migration of people.
There is also a sense in which Jablonski鈥檚 theory defuses racial tension, because skin colour is merely an evolved biological necessity allowing us to achieve a delicate balance between the conflicting needs to absorb and repel UV. Because it is subject to so many evolutionary forces, skin colour itself is worthless as a characteristic for establishing genealogical or racial groupings of people. 鈥淪kin colour, through its visibility, is of serious social significance,鈥� says Henneberg. 鈥淭here is but one step from 鈥榗olour classification鈥� to racism. So a scientific explanation of the origins of the variation in human skin colour is most welcome.鈥�