Why Skin Comes in Colors
Like society, science has long struggled with skin color. Racist and colonialist attitudes were once thinly disguised behind research, but now anthropology appears poised to make up for past shortfalls. Nina Jablonski, Curator and Irvine Chair of Anthropology at the California Academy of Sciences, has developed a comprehensive theory, bolstered by biological logic and high-tech satellite data, to explain our colorful tapestry of skin tones.
Jablonski presented her ideas at the annual meeting of the American Association of Physical Anthropologists, and she and Academy Research Associate George Chaplin will lay out the full theory in a forthcoming issue of the Journal of Human Evolution. "Like most theories in evolutionary biology, this one has come together in part by accident and in part by design," says Jablonski.
The first accident came about eight years ago when, while teaching at the University of Western Australia, she was asked to give a guest lecture about human skin. In the course of researching the topic she discovered a range of opinions about whether human skin color is an adaptive response to some environmental factor - and if so to which one and whether it fell under the influence of natural selection. Did populations evolve particular skin colors in order to better survive in certain environments?
To answer that question, Jablonski sought evidence that having light skin in some regions and dark skin in others permitted greater reproductive success. The intermittent search for an answer continued for the next several years and involved reviews of existing studies as well as original analysis of some fortuitously available data. It also drew on Jablonski's background in biology and physiology, before she became an authority on the living and fossil primates of Asia.
Two previous studies proved particularly critical to Jablonski's thinking. One study showed that folate (part of the B vitamin complex) in the human body breaks down rapidly during exposure to intense ultraviolet (UV) radiation, especially in light-skinned people; up to half the folate in blood plasma can be lost in under an hour. The key finding from the second study was that too low folate levels can cause debilitating neural tube defects (NTDs) in fetuses (the reason why women should take folic acid before and during a pregnancy). Still other studies showed folate's role in producing viable sperm in mice and rats.
So Jablonski had her answer to how folate can influence an individual's odds of reproductive (and hence evolutionary) success, and a plausible partial answer for skin color. While others have suggested a link between shades of skin color and degrees of exposure to the sun's UV rays, she was the first to link sun overexposure and ntds as part of a mechanism regulating skin tone.
The mechanism works by adjusting levels of melanin, a dark skin pigment that occurs in all vertebrates and acts as a natural sunblock. The more you have, the better your protection from potentially harmful radiation. Dark skin probably became an essential human trait at least 1.5 million years ago, when fossil evidence from Kenya suggests that our ancestors first attained human-like body proportions and perhaps had less body hair, more sweat glands, and lived in more exposed habitats than their predecessors.
But as later ancestors roamed the globe, encountering new climates and environments, their tropical skin tone posed a problem. Sunlight penetrating our skin spurs production of vitamin D3, essential for absorbing the calcium we need to build and bolster the skeleton. Without adequate amounts of this vitamin we risk rickets and osteoporosis.
In the 1960s, biochemist W. Farnsworth Loomis proposed that all skin pigmentation could be explained by the body's need to make this vitamin. Lighter skin on residents of high-latitude regions ensures that enough UV light, diminished by the sun's low angle and a thicker blanket of atmosphere, gets through. But in the hotter tropics, argued Loomis, people needed more melanin to avoid making potentially toxic amounts of the vitamin. Subsequent clinical data show that our bodies possess a pair of mechanisms to avoid natural overdoses of vitamin D. So Jablonski thinks that potential damage from falling folate levels offers a more compelling reason for darkening the skin.
Jablonski and Chaplin happened across a bounty of satellite data that contributed enormously to their study. For 15 years until the early 1990s, NASA's Total Ozone Mapping Spectrometer had measured levels of UV radiation at the Earth's surface. From a small subset of the data, Jablonski and Chaplin calculated for various latitudes the average daily amount of UV radiation strong enough to cause the slightest reddening of light skin.
Another coincidence concerned an ongoing study of the connection between sunlight and vitamin D3 production in northern climes. Medical researchers in Boston and elsewhere exposed circumcised foreskins to UV light on the rooftops of hospitals. At the end of each day, the researchers analyzed the samples for biochemical signs of vitamin D3 synthesis until they had pinpointed the precise day in spring when there was sufficient sunlight to start the synthesis. Knowing the date, Jablonski and Chaplin could match it with the satellite data to obtain the amount of UV radiation reaching Boston on that day. Then, using the city's latitude as a benchmark, they could begin to map the calculated minimum thresholds of sunlight at latitudes worldwide for vitamin D3 to be produced in light-skinned people.
When plotted on a map, the data reveal three distinct skin tone zones. Zone 1 falls within the tropics, including most of Africa, Mexico, Central America, Amazonia, Asia, and Oceania, and contains peoples with highly melanized skin. Zone 2 encompasses large parts of the Northern Hemisphere, including most of the United States and southern Europe.
Historical residents of Zone 2, says Jablonski, have moderately pigmented skin but a tremendous potential to alter their skin tone through tanning; they can increase melanin to prevent folate loss and then lighten their tan in fall and winter in order to take advantage of dimmer, briefer days. Zone 3 includes high-latitude and polar regions, where people face the greatest risk of vitamin D3 deficiency and compensate for the diminished light with paler skin and by eating vitamin D3-rich foods.
Like many things about our patchwork biology, human skin color strikes a delicate compromise between getting too little or too much sunlight. "This is the kind of balancing act that evolution is great at perfecting," says Jablonski. "You never have the luxury in evolution to design something from scratch, [but] for naked mammals this works very well."
Curiously, females tended to have lighter skin than males in every examined population, a "significant biological message" that Jablonski attributes to the need for women to generate extra vitamin D3 at critical times in their lives, particularly while pregnant or nursing an infant.
Jablonski and Chaplin also noticed that some populations had skin tones that differed from the values expected based on their homeland's latitude. Tibetans, for instance, have lighter skin than predicted, but Jablonski points out they have occupied the Tibetan Plateau for fewer than 10,000 years and had to wear sufficient clothing to survive at high elevation. Eskimos, however, have darker than expected skins due to their relatively recent occupation of the Arctic. They more than compensate for their pigmentation with vitamin D3 from a diet heavy in fish oil and marine mammal blubber.
Jablonski believes that the apparent connection between sun exposure, folate, and NTDs may also help explain the recent and rapid global decline in amphibian populations. Frog embryo skin tends to be pigmented above the developing neural tube, and amphibians also have an insulating coat of jelly around their eggs and the ability to quickly repair DNA damaged by sunlight. Nevertheless, cases of sunlight-induced birth defects in tadpoles, such as uneven neural folds, and spine or tail kinks, says Jablonski, are consistent with a critical lack of folate, and immobile eggs would be most vulnerable to increased UV radiation during this crucial phase for neural tube growth.
Whether in frogs or people, however, Jablonski admits that it remains to be demonstrated that UV radiation can result in NTDs. She cites a recent report by a Brazilian researcher about three light-skinned women who had each visited a tanning salon early in her pregnancy. Each woman's infant suffered from a severe ntd, perhaps due to its mother's folate loss while under the tanning lamps.
Says Jablonski, "That's the closest we have to a smoking gun," but she would like to pursue collaborations with more clinically minded colleagues in order to gather more evidence in support of her ideas. Having made a constructive contribution to the socially charged subject of skin color, Jablonski plans to pursue this work further alongside her paleontological research.
Blake Edgar is Senior Editor of California Wild.