Jablonski, now chairman of the anthropology department at the California Academy of Sciences, begins by assuming that our earliest ancestors had fair skin just like chimpanzees, our closest biological relatives. Between 4.5 million and 2 million years ago, early humans moved from the rain forest and onto the East African savanna. Once on the savanna, they not only had to cope with more exposure to the sun, but they also had to work harder to gather food.
Mammalian brains are particularly vulnerable to overheating: A change of only five or six degrees can cause a heatstroke. So our ancestors had to develop a better cooling system. The answer was sweat, which dissipates heat through evaporation. Early humans probably had few sweat glands, like chimpanzees, and those were mainly located on the palms of their hands and the bottoms of their feet. Occasionally, however, individuals were born with more glands than usual.
The more they could sweat, the longer they could forage before the heat forced them back into the shade. The more they could forage, the better their chances of having healthy offspring and of passing on their sweat glands to future generations. A million years of natural selection later, each human has about 2 million sweat glands spread across his or her body.
Human skin, being less hairy than chimpanzee skin, “dries much quicker,” says Adrienne Zihlman, an anthropologist at the University of California at Santa Cruz. “Just think how after a bath it takes much longer for wet hair to dry.” Hairless skin, however, is particularly vulnerable to damage from sunlight. Scientists long assumed that humans evolved melanin, the main determinant of skin color, to absorb or disperse ultraviolet light. But what is it about ultraviolet light that melanin protects against? Some researchers pointed to the threat of skin cancer.
But cancer usually develops late in life, after a person has already reproduced. Others suggested that sunburned nipples would have hampered breast-feeding. But a slight tan is enough to protect mothers against that problem. During her preparation for the lecture in Australia, Jablonski found a 1978 study that examined the effects of ultraviolet light on folate, a member of the vitamin B complex. An hour of intense sunlight, the study showed, is enough to cut folate levels in half if your skin is light. Jablonski made the next, crucial connection only a few weeks later.
At a seminar on embryonic development, she heard that low folate levels are correlated with neural-tube defects such as spina bifida and anencephaly, in which infants are born without a full brain or spinal cord. Jablonski later came across three documented cases in which children’s neural-tube defects were linked to their mothers’ visits to tanning studios during early pregnancy. Moreover, she found that folate is crucial to sperm development — so much so that a folate inhibitor was developed as a male contraceptive. (“It never got anywhere,” Jablonski says. “It was so effective that it knocked out all folate in the body.”) She now had some intriguing evidence that folate might be the driving force behind the evolution of darker skin.
But why do some people have light skin? As far back as the 1960s, the biochemist W. Farnsworth Loomis had suggested that skin color is determined by the body’s need for vitamin D. The vitamin helps the body absorb calcium and deposit it in bones, an essential function, particularly in fast-growing embryos. (The need for vitamin D during pregnancy may explain why women around the globe tend to have lighter skin than men.)
Unlike folate, vitamin D depends on ultraviolet light for its production in the body. Loomis believed that people who live in the north, where daylight is weakest, evolved fair skin to help absorb more ultraviolet light and that people in the tropics evolved dark skin to block the light, keeping the body from overdosing on vitamin D, which can be toxic at high concentrations. By the time Jablonski did her research, Loomis’s hypothesis had been partially disproved. “You can never overdose on natural amounts of vitamin D,” Jablonski says. “There are only rare cases where people take too many cod-liver supplements.”
But Loomis’s insight about fair skin held up, and it made a perfect complement for Jablonski’s insight about folate and dark skin. …People in the tropics have developed dark skin to block out the sun and protect their body’s folate reserves. People far from the equator have developed fair skin to drink in the sun and produce adequate amounts of vitamin D during the long winter months.