By John Arnst
It may seem the sun wages a constant war against our skin. Harmful UV radiation burns us, damages our DNA, and can sow the seeds for melanoma. But the sun is essential to our healthy development and our immune systems, because sun-exposed skin produces Vitamin D. During the long-ranging human exodus from Africa, says anthropologist Nina Jablonski, Vitamin D levels in the body played a key role: driving the evolution of our species' skin color.
“The sunshine vitamin,” as Jablonski calls Vitamin D, is central to understanding the relationship between skin color and geography, she said during a February 16 symposium at the American Association for the Advancement of Science meeting in Boston. She described her thesis in full in her latest book, Living Color: The Biological and Social Meaning of Human Skin Color.
With receptors located in the cells of many major organs and in the immune system, Vitamin D helps in calcium absorption and prevents aberrant cell division. UVB radiation catalyzes its creation in our skin.
According to Jablonski, “there is a conspicuous geographical pattern” between skin color and distance from the equator. At more northern or southern latitudes, the level of UVB rays hitting Earth’s surface decreases due to the planet’s tilt. The equator is bathed year-round in UVB rays, but seasonal variations mean that people in Northern Europe receive virtually no UVB exposure in winters.
As a result, Jablonski said, humans living near the equator developed darker skin tones, while those in northern climates developed lighter hues. High humidity also decreases UVB levels, as marked by the contrast between skin tones of early humans living in dry equatorial Africa and moist equatorial South America.
The dearth of UVB rays in northern climates put positive evolutionary pressure on early migratory humans to ramp up Vitamin D production, Jablonski noted. And indeed, fair-skinned people of European descent are nearly six times more efficient at making Vitamin D from UVB rays than those living near the equator.
Different types of melanin pigments in skin trigger these variations. In dark-skinned people, eumelanin is dominant and acts as a natural sunscreen; fairer-skinned individuals have much more pheomelanin. However, while pheomelanin produces vitamin D efficiently, its reaction with high levels of UVB also makes dangerous free radicals — which damage skin cells over time.
Failing to produce adequate levels of Vitamin D can cause physical deformities, including painful distension of the pelvic bones in women — and severe complications during childbirth. But even with persistent UV exposure, malignant melanoma doesn’t set in until later in life. Its effect on reproductive success would have been almost nonexistent, Jablonski stated.
The process of long-term skin lightening, known as “depigmentation,” occurred due to a series of mutations in a gene called SCL24A5, Jablonski said. That gene controls the size of melanin packets in skin and the types of melanin produced.
“All of us can use this as a wonderful teaching tool,” she said. “We often wonder how we can teach evolution to our students, to our children. What human examples can we bring that will be effective? Well, here’s one: human skin pigmentation. You don’t need any teaching tools, because you’re wearing it."
John Arnst is a senior English and Biology major at the University of Florida. He has interned at the University of Florida’s Global Pathogens Laboratory, loves books and viruses, and would one day like to write books about viruses. Reach him at johnarnst@ufl.edu.
