The Sepia Rainbow

The fascinating story of human skin

Artist's rendition of a map created by Penn State anthropologists Nina Jablonski and George Chaplin showing predicted skin colors of indigenous people based on levels of ultraviolet radiation from the sun. The map's predictions correlate very well with observed skin tones. Credit: Cut-paper illustration by Gail McCormick, from map by Nina Jablonski and George Chaplin.All Rights Reserved.

Nina Jablonski’s focus on skin began with a request to give a lecture. It was 1991, and she and her husband and collaborator George Chaplin had recently moved from Hong Kong to take positions at the University of Western Australia.

Jablonski, a primatologist and paleobiologist, was known at the time for her work on Old World monkeys. For this occasion, however, she’d talk about something else, one of the long-standing problems in anthropology: the evolution of human skin and skin color. As she dug in to prepare, she noticed a sizeable gap in the literature. Very little work had been done on skin color since the early 1970s, and most of what she did find was either outdated or just plain wrong.

Since that day, she says, “What started as an innocent, small project has grown into a sort of fractal universe, spinning out in all directions.” Jablonski has authored dozens of papers, scores of talks, and two popular books on the subject of human skin. She has dived enthusiastically into public education, with frequent appearances on NPR and PBS and one memorable spot on the late, lamented Colbert Report. A TED talk she gave has garnered, at last count, over three-quarters of a million views.

Anthropologist Nina Jablonski. Credit: Cut-paper illustration by Gail McCormick, from photograph by Patrick Mansell.All Rights Reserved.

Her curiosity has carried her from the natural history of skin to an exploration of its implications for human health. More recently she has been drawn to reflect on the societal implications of skin color. Since arriving at Penn State in 2006, Jablonski, now Evan Pugh Professor of Anthropology, has devoted much of her considerable energy to understanding the basis, and ramifications, of color-based racism.

The trail has led her into territory where not many academics are tempted to tread. But to her it feels like a natural, and necessary, extension.  “I realized I couldn’t keep talking about just the evolutionary and health aspects of skin color,” she says, “when so much of it has to do with the way people treat one another.”

From Dark to LightOne day in 1981, while teaching gross anatomy to a group of medical students at the University of Hong Kong, Jablonski was struck by an insight that has stayed with her.

As she told a New York Times reporter years later, the students had been presented with a cadaver to dissect, and they were squeamish about the task. But their attitude changed as soon as they managed to cut through the skin. Suddenly, what had seemed dauntingly human was merely a body to examine.  “That moment,” Jablonski said, “showed me how much of what we consider our humanity is imbued in our skin.”

“I realized I couldn’t keep talking about just the evolutionary and health aspects of skin color, when so much of it has to do with the way people treat one another.”

At the time, the evolution of skin color was regarded by many of her peers as an intractable problem. Theory held that darker skin had evolved in order to afford early humans—who had recently lost the cover of fur—a protection against skin cancer under the tropical sun. But skin cancers, Jablonski knew, almost always arise later in life, when an individual is past reproductive age. Blocking their occurrence would offer little or no evolutionary advantage.  

Her preparation for that lecture in Australia hinted at a different scenario.  In a 1978 paper by two American medical researchers, Jablonski found evidence linking exposure to strong sunlight with low levels of folate, an essential B vitamin, in the blood. Other research tied folate deficiency in pregnant women to various birth defects. In men, she learned, folate is vital for sperm production.

These and other observations gradually led her and Chaplin toward a new hypothesis: that humans evolved the ability to produce melanin, the dark-brown pigment that acts as a natural sunscreen, as a way of safeguarding the body’s store of folate.

At the outset, then, living near the equator, all humans would have had dark skin. But that’s only half the riddle. Why and how did lightly pigmented skin come about? The answer, Jablonski reasoned, involves another key vitamin—and the history of human migration.

Anthropologists George Chaplin and Nina Jablonski discovered the connection between levels of ultraviolet light at ground level at different points on Earth and skin color of people native to those areas. Credit: Courtesy of Nina JablonskiAll Rights Reserved.

As she explains, the sun’s ultraviolet rays, in addition to causing cell damage and other forms of harm, play a vital role in human health: They trigger the production of vitamin D in the skin. Vitamin D, as most of us learned in elementary school, is critical for strong bones and healthy teeth. More recent studies show its value in immune function and for fighting off certain cancers and even heart disease.  

In tropical climes, enough UV penetrates even dark skin to provide an adequate dose of vitamin D. However, as our forebears began to migrate, wandering far from the equatorial sun, not enough UV could make its way through the protective melanin. At higher latitudes, particularly in winter, vitamin D levels dropped, to the point where health was compromised. Dark skin became a disadvantage. The evolutionary response, Jablonski says, was loss of pigmentation. Individuals with less melanin in their skin had a better chance of surviving where there was not as much sunlight available. 

In the late 1990s, Jablonski and Chaplin found support for this idea in a set of NASA satellite data, which provided a precise record of surface-level UV radiation at every point on the globe.  When they compared these data with geographical records of skin color variation, they found an overwhelming degree of correlation. Skin color was darkest where surface UV was strongest, the overlay clearly showed, and lightest where surface UV was weak.

After ten years of digging, the two had arrived at what amounts to the first comprehensive theory of human skin color. What Jablonski calls “this beautiful sepia rainbow” evolved as a response to human migration, local UV regimes, and the body’s need for vitamin D.

Skin Color and HealthWith the modern age, of course, the pace of human migration has accelerated tremendously. Humans roam the globe as they will. The majority of them now live indoors, and in cities. “We are in the middle of a huge new experiment in human evolution,” says Jablonski.

The abruptness of change creates what she calls  “mismatches” between skin color and environment: Think of fair-skinned people moving from England to Australia, for example, or dark-skinned people of African descent dwelling in Canada or Scandinavia. The upshot of these discrepancies, Jablonski says, has been a rise in health problems. 

Some of these problems, like skin cancer, are relatively easy to spot and solve with protective measures. Others are more insidious. For dark-skinned people living in areas of high latitude—and spending more and more time indoors—vitamin D deficiency poses a stealthy but growing threat. “We are just beginning to explore the broader health implications,” Jablonski says. For her, the perfect laboratory for this exploration is South Africa.

“This is a country that has been a melting pot of humanity not just for a few decades but for thousands of years,” Jablonski explains. “It has this deep, complex history, a very heterogeneous population—and a fascinating climate. The southern part is strongly seasonal, with a pronounced winter and a pronounced summer.” Last but not least, South Africa is a country that for decades has experienced a high prevalence of deadly infectious diseases, including HIV, AIDS, and tuberculosis. 

“We are in the middle of a huge new experiment in human evolution.”

In a study recently concluded on the outskirts of Cape Town, Jablonski was able to show how these several factors may be intertwined. She and her colleagues tested healthy young individuals of the Xhosa and Cape Mixed ethnic groups, both of whom have darker skin than the area’s indigenous Khoisan people. The darker skin, she suggests, is an indication of recent migration into the region—and a mismatch with local levels of UV radiation.

As expected, the researchers found that their Xhosa and Cape Mixed subjects exhibited vitamin-D deficiency in winter, when UV levels are lowest and people spend most of their time indoors. Then, when the team exposed both winter and summer blood samples from the two groups to cultures of HIV, they found that the cells in the vitamin D-deficient winter samples were significantly more likely to get infected.  Administering high-dose vitamin-D supplements before the samples were taken reversed these effects. 

To her, the study points to a clear progression from UV-pigmentation mismatch,  to vitamin D deficiency, to serious immune-system compromise. “But what’s most exciting,” she says, “is that we might have here the basis for a public health mediation in the form of seasonal vitamin D supplementation for at-risk groups.”

Evolution and PrejudiceAfter publication of her first book on the subject, Skin: A Natural History, in 2006, Jablonski began to lecture frequently about the evolution of skin color. She found her audiences eager to hear more about how that story fit with our society’s concept of race, if it fit in at all. It’s a topic many anthropologists shy away from, but Jablonski realized that her vantage point could be uniquely valuable.

“I am talking about humans from the perspective of deep evolutionary time,” she says. “There are certain things that we can look at—patterns of pigmentation, distribution of fossils and of populations of ancient humans—that are pretty much beyond question. By looking at them, we can begin to understand how humans came to be as diverse in appearance as they are today. Then, having understood the mechanisms, we can look at what’s happened in the more recent past.”

The range of human skin tones derives from our evolutionary responses to levels of UV radiation and our need for vitamin D and vitamin B9 (folate). Credit: Cut-paper illustration by Gail McCormick, from photograph by Patrick Mansell.All Rights Reserved.

One of the most important findings from recent genetic work, she says, is that the same skin colors have evolved multiple times in human history. In South America and Polynesia and the Mediterranean, for example, people have evolved highly tannable skin, and they’ve done so independently, in each instance involving different genes. Similarly, lightly pigmented, or what Jablonski calls de-pigmented, skin has evolved not just once or twice, but probably at least three times. “For an evolutionary biologist, any time you find evidence of this kind of convergent evolution it’s very exciting,” she says. “The same mechanisms are operating in a repeatable and predictable fashion. You may have two people of identical skin color whose pigmentation genes are very different.”

Other studies have shown that the genes that determine pigmentation are different from those that code for things like facial features. “That means there’s no such thing as a racial complex of color, hair, eyes, nose shape, or whatever,” Jablonski says. “These things travel independently of one another in human evolution.”

Biologically, in fact, there is no such thing as race at all. Which is not to say, she hastens to add, that race isn’t a real and enduring force in people’s lives—a matter, too often literally, of life and death. What’s left to understand, she argues, is the social construct. And for Jablonski, the initial question to be answered was how that construct arose in the first place. Her 2012 book, Living Color, documents her exploration.

“There’s no such thing as a racial complex of color, hair, eyes, nose shape, or whatever,” Jablonski says. Biologically, in fact, there is no such thing as race at all.

Jablonski traces the class meaning of skin color in traditional agrarian societies, where light skin was the privilege of those not forced to work in the sun. She recounts the first meetings between European explorers and the native peoples they encountered—the astonishment at seeing human beings who looked so different, which soon gave way to disapproval and condemnation of the Other.  “The negativity of the explorers’ written accounts had a disproportionate effect on how Europeans framed their ideas of dark-pigmented people,” she says.

More disturbing, in her eyes, is the history of scientific racism. By the mid-18th century philosophers like Immanuel Kant were pondering the origins of human diversity. “Kant defined four races and grouped them hierarchically, with the European race at the top,” Jablonski says. “His ideas have no basis in experimental observation—they reflect his own, emotionally-based agenda.” But Kant’s prominence gave his writings extraordinary influence on other Enlightenment thinkers, some of whom in turn reinterpreted Biblical allusions regarding light and dark in order to justify their views. 

“So you have this whole set of cultural memes that begin to take on a life of their own,” Jablonski says. “Not coincidentally, these were really crystallized at the height of the transatlantic slave trade. What you have is a nexus of unfavorable forces that promoted the rigid definition and hierarchical installment of races based on imperfect and emotionally based data and commercial gain.” 

De-Installing RaceJablonski calls the historical linking of skin color with character “humanity’s greatest intellectual fallacy.” The question now, she says, is “How can we retrieve this? How do we begin the long-term process of de-installing race from our national and, indeed, global consciousness?”

The answer, she firmly believes, is to make race education prominent in the classroom.  In a recent interview on NPR she talked about introducing discussions of race into primary schools—not bogging down in details of evolutionary biology, but teaching the basics of how human diversity came about, and how people have thought about race through the centuries.

“It’s taken us over two hundred years to get into this mess, and it’s going to take us a long time to get out of it. But we have to start.”

“It needs to be part of the educational landscape,” she says. “And it has to begin early. Often by the time we talk about these things, when kids come to college, the horse is out of the barn. We’re trying to reverse years of prejudice. We need to try to prevent many of these misconceptions and racial biases from being installed in the first place.”

She is currently working with historian and filmmaker Henry Louis Gates Jr. to develop a national curriculum focused on human diversity, with the dual purpose of getting kids thinking about race and sparking their interest in science. The three-year program recently received funding from the Robert Wood Johnson Foundation, and will be piloted in summer camps including Penn State’s popular “Science U.” 

In South Africa, where she has worked for over a decade, Jablonski is collaborating with a writer and an illustrator to produce a graphic novel on the evolution of skin color. The target audience is middle-school students. She has already landed a South African publisher, and hopes eventually to bring out an edition in the U.S.

Jablonski has also convened the “Effects of Race” initiative, bringing together an international cast of scholars and artists at Stellenbosch Institute for Advanced Study in Stellenbosch, near Cape Town, to look for fresh insights into issues of race. One current project hopes to inform the South African government’s efforts to update its categories for the classification of its people, which persist from the apartheid era.

Back home, she is proud to be talking and working with colleagues across Penn State. “With Paul Taylor and Robert Berlusconi in philosophy, Sam Richards in sociology, Gary King in biobehavioral health—among others—we have quietly built up a group of really influential scholars here who are working on complementary projects, coming from different perspectives to look at racial inequality, how it came about, and what can be done to change it.  They are also expert public communicators.”

Jablonski herself feels a strong responsibility to continue with her busy public schedule. “The buck has been passed on this topic for generations,” she says. “I think everyone who has the ability to speak outside the academy should do so.

“I’m under no illusions,” she quickly adds. “It’s taken us over two hundred years to get into this mess, and it’s going to take us a long time to get out of it. But we have to start. This has to be a national priority. If we don’t begin to grasp this nettle now, I’m afraid we’re doomed.”



This story first appeared in the October 2015 issue of Research|Penn State magazine.

Last Updated July 29, 2017