All over the world, millions of people see skin color as a symbol of superiority or inferiority, whether they are conscious of it or not.

Others see humanity’s array of skin tones, from white to ocher to black, as a positive mark of our remarkable diversity.

But Nina Jablonski sees skin color, first and foremost, as an evolutionary gateway to vitamin D.

Jablonski, the Evan Pugh professor of anthropology at Penn State University, has made a name for herself with her research on how human skin color evolved from the earliest humans to today’s multiple hues.

The short version: Our skin color is primarily designed to regulate how much sunlight we let into our bodies to produce vitamin D, which is important for bone health, safe pregnancies and a strong immune system. In a related way, it also keeps too much ultraviolet radiation from destroying folate in women’s bodies, which can lead to certain birth defects.

In general, she has found, the skin color of ancient people matched up well with the amount of sunlight that bathed the regions in which they settled. Those in the tropics had darker skin; those in temperate zones had lighter skin. That worked fine for the thousands of years when people spent most of their time living and working outdoors, she said.

But then two things happened. First, modern technology allowed people to settle all over the world, creating mismatches between skin color and sunlight.

“You’ve got people from England moving to Australia; people from west Africa moving to Finland. You have this dramatic movement of people to environments to which they are poorly adapted from a solar perspective.”

Another modern development, urbanization, means people are spending more and more time indoors.

From a health perspective, these trends have had a bigger effect on darker-skinned people, she said. Lighter-skinned people can adapt to sunny climates by using sunscreen to prevent skin cancer and folate problems and yet still get enough of the ultraviolet B radiation that triggers vitamin D production.

“If you’re a darkly pigmented person living in a far northern place or living in a city and not getting much sun exposure, though, then we are not addressing the problem of likely vitamin D deficiency,” Jablonski said.

Lisa Bodnar, a researcher at the University of Pittsburgh’s Graduate School of Public Health, has found this trend in her studies of pregnant women.

Bodnar, who has a doctorate in nutrition, has found that women who have vitamin D deficiencies are more likely to deliver babies early or get the dangerous condition of preeclampsia, which causes a woman’s blood pressure to spike and often leads to premature delivery of her child.

Even using the most conservative guidelines of how much vitamin D women should have, Bodnar said, nearly half of African-American mothers have vitamin D deficiencies, compared with just 10% of Caucasian mothers.

Adding to this picture is a study Jablonski and colleagues in South Africa have been conducting among residents of Cape Town.

Among darker-skinned South Africans, she said, the more time they spend indoors, the lower their vitamin D levels, and the weaker their immune systems.

The good news, she said, is that vitamin D supplements can reverse these trends.