Dr. Nina Jablonski, an Atherton Professor and Evan Pugh University Professor Emerita of Anthropology at Pennsylvania State University, has spent her career researching one central question: why do humans have such a wide range of skin colors? Her work demonstrates that variations in skin color are adaptive traits shaped by ultraviolet (UV) radiation levels, not markers of biological “race.” This line of research spans evolutionary biology, health, and education.
The Core Question: Why Skin Color Varies
Jablonski’s primary research investigates how human skin pigmentation evolved as a direct response to UV radiation in different parts of the world. Using satellite data on UV levels, she tested this idea quantitatively for the first time and found that the gradual shift in skin color observed among indigenous peoples across the globe closely tracks UV radiation intensity. Populations near the equator, where UV is strongest, have the darkest skin. Populations at higher latitudes, where UV is weakest, have the lightest.
This gradient isn’t random or cosmetic. It reflects a biological balancing act between two competing needs: protecting the body from UV damage and producing enough vitamin D.
The Folate and Vitamin D Tradeoff
At the heart of Jablonski’s work is what’s known as the vitamin D-folate hypothesis. UV radiation does two things in the body that pull in opposite directions. It triggers vitamin D production in the skin, which is essential for bone health, immune function, and reproduction. But it also breaks down folate, a B vitamin critical for DNA repair, cell division, and healthy fetal development during pregnancy.
In regions with intense UV exposure, darker skin acts as a natural shield. Melanin, the pigment responsible for skin color, blocks enough UV to prevent folate from being destroyed while still allowing some vitamin D synthesis. In regions with low UV exposure, lighter skin lets more UV penetrate, which helps the body make adequate vitamin D from limited sunlight. Jablonski’s research frames skin color as a “compromise solution” between these two physiological requirements.
There’s even evidence that folate and melanin work together in a feedback loop. Folate helps regulate the production of a compound the body needs to make melanin pigments. So melanin protects folate from UV destruction, and folate in turn supports melanin production.
How Losing Body Hair Changed Everything
Jablonski has also explored the deeper evolutionary timeline behind skin pigmentation. Early human ancestors were likely covered in body hair, much like other primates, which shielded the skin beneath from direct sunlight. As humans evolved to regulate body temperature through sweating (a far more efficient cooling system for long-distance movement in hot environments), they lost most of their body hair. This left skin directly exposed to UV radiation for the first time, creating the evolutionary pressure that drove the development of dark pigmentation in our earliest ancestors in Africa.
Her book, “Skin: A Natural History,” traces this full arc, from the mechanics of sweat and the structure of skin to the evolution of human color diversity and the cultural meanings people have layered on top of it.
Modern Mismatch and Health Risks
Jablonski’s research extends into the present day, examining what happens when people live in UV environments their skin didn’t evolve for. When dark-skinned populations migrate to high-latitude regions with weak sunlight, they face a higher risk of vitamin D deficiency because their melanin-rich skin blocks too much of the limited UV available. When light-skinned populations move to tropical regions or spend long hours in intense sun, they face elevated risks of UV damage, including skin cancer, because their skin lacks sufficient melanin protection.
Indoor lifestyles compound this problem. Even people living near the equator can become vitamin D deficient if they spend most of their time inside, shielded from sunlight. Jablonski views these mismatches between ancestral skin pigmentation and current environment as significant but underappreciated public health concerns. Her work emphasizes that the evolution of skin pigmentation was shaped by genetic, environmental, and cultural variables interacting over hundreds of thousands of years, and understanding that history has real implications for health today.
Challenging the Biological Concept of Race
A major thread running through Jablonski’s work is dismantling the idea that skin color reflects distinct biological races. Genomic analysis of diverse human DNA has shown that racial and ethnic classifications based on skin color have no biological basis. They are socially constructed categories. Yet these classifications persist in census systems, medical research frameworks, and everyday assumptions about human difference.
Jablonski has argued that the scientific systems built on racial classifications never reflected actual biological diversity. Instead, they reflected the cultural institutions and economic systems of the societies that created them. Her evolutionary research provides a clear alternative explanation: skin color is a continuous gradient shaped by UV exposure over generations, not a set of discrete categories that divide humanity into fundamentally different groups.
Education and Outreach
Beyond the lab, Jablonski has invested in translating her research into educational tools. She co-developed the “Finding Your Roots” curriculum, a program designed to spark interest in science by inviting students to explore their own genetic ancestry and genealogy. The curriculum was built by a team of historians, biologists, geneticists, anthropologists, and educators, and it aligns with Next Generation Science Standards.
The program currently runs summer camps primarily for underrepresented minorities and disadvantaged learners of middle school age, led by researchers at Penn State. A parallel effort develops personalized genetics and genealogy modules for undergraduate biology courses at historically Black colleges and universities. The underlying hypothesis is straightforward: students become more engaged with science when the subject is themselves. Practical and ethical considerations of using personal DNA testing with young learners have been a formal part of this research, reflecting Jablonski’s awareness that the social dimensions of human variation require as much care as the biological ones.