As the darkest days of the year close in, some of us crave the feel of sunlight on our skin. That could reflect our evolutionary heritage, since we humans are not well-adapted to live as far north as Philadelphia. We’re essentially an equatorial, tropical species who migrated only recently to places with long, dark winters. There hasn’t been time to adapt.
One major problem with living this far from the equator is that it’s hard to get enough Vitamin D, a hormone that turns out to be essential not only for keeping our bones strong but also for running our immune systems and just about everything else.
Humans have evolved a nifty ability to make Vitamin D in our skins, but the process requires an intensity of ultraviolet light that doesn’t exist here in winter outside of a tanning booth.
It may look sunny outside in November and December, but the angle of the sun is too low, and the intensity too weak, to make any Vitamin D, said Pennsylvania State University anthropology professor Nina Jablonski, author of the book Skin: A Natural History. Even if you go skiing in the Poconos and come back with a burn, you haven’t gotten enough of the right wavelength to make Vitamin D, she said.
That’s why some doctors disagree with a recent government panel’s statements that most Americans get enough Vitamin D from a combination of sunshine and fortified foods.
Since then, a Stanford study of 6,000 people showed that more than half of those who avoided sun were deficient. Among dark-skinned people, deficiency was common even among those who didn’t avoid sunshine.
Pediatrics professor Bruce Hollis has sampled blood levels of Vitamin D and found that nomadic people in Africa have about four or five times as much as a typical African American person living in the United States.
Some scientists have proposed that white skin evolved to solve the problem of Vitamin D production, but it’s just a partial solution. Light skin just extends the portion of the year when it’s possible to make Vitamin D from sunlight, said Hollis, who works at the Medical University of South Carolina.
The government panel was commissioned just to determine whether Americans got enough to keep our bones healthy, but Vitamin D has dozens of other jobs. It’s essential for our hearts, brains, and nervous systems to function, Hollis said. “It affects everything.”
Penn State’s Jablonski sees light skin as an indication of the importance of Vitamin D. In exchange for the ability to get a little more Vitamin D, people traded away protection from sunburn, skin cancer, and certain birth defects tied to folic acid deficiency. Folic acid is depleted by too much sun, she said.
The connection between Vitamin D and skin-color evolution is called the “Vitamin D Hypothesis” and was first proposed in the 1930s.
It is possible to get enough Vitamin D from diet alone, but not with a typical American diet. The Inuit did it by eating lots of whale blubber.
Why did evolution make us so high-maintenance and dependent on all these vitamins and minerals? That’s a question that intrigued Michael Holick, a professor at Boston University School of Medicine. He found that it all started around 500 million years ago, just as complex organisms were starting to evolve.
Complex life started in the ancient seas that were rich with plankton. Plankton make Vitamin D from the sun so it infuses the food chain. While it’s not clear what they needed it for, Vitamin D was everywhere and apparently useful, and so it became woven into the biology of the animal kingdom at its roots.
“We’ve looked at everything from zooplankton to brine shrimp to frogs and lizards,” said Holick. They all need Vitamin D. Animals either make their own with sunlight, or they eat animals that contain it.
Nocturnal animals and polar bears get their D from their prey. Even vampire bats need Vitamin D, Holick said; it’s in the blood they suck from other animals. Sheep make their own Vitamin D in their wool and then get it into their systems when they lick themselves.
Originally, humans made more than enough in skin, Holick said. When some of our ancestors left Africa, they adjusted their skin tone to allow in more sunlight. Penn State’s Jablonski found that around the world, the skin color of native people maps almost perfectly onto a map of UV radiation; the more UV, the darker the average skin.
In 2005, Penn State professor Mark Shriver and colleagues isolated a genetic mutation that contributed to Europeans’ having white skin, a mutation that in zebra fish leads to absence of the characteristic stripes.
Shriver, a professor of anthropology and genetics, said the original human skin color was probably light, because other apes are light-skinned under their fur. Dark skin became advantageous in Africa when we lost our fur.
Genetic evidence suggests that light-skin-related mutations arose recently, less than 15,000 years ago, and spread fast through Europeans.
Jablonski said that when scientists scraped bits of DNA from Neanderthal skeletons, they found a mutation of another skin-color-related gene. Neanderthals lived in Europe and the Middle East long before our ancestors left Africa, and apparently they independently evolved light skin.
In the Arctic, the Inuit never developed light skin, which scientists at first considered a paradox until they discovered how much Vitamin D is in a traditional Inuit diet, which included oily fish and whale blubber.
Hollis, however, believes light skin was only a partial solution to the problem of living at high latitudes. That’s why white people still succumbed to the horrible bone-softening disease known as rickets back in the 17th and 18th centuries. Now rickets is uncommon thanks to the fact that we fortified milk and other foods with Vitamin D.
Until late 2010, the government’s minimum daily allowance for children was the same one set in the 1960s, based on roughly the amount needed to prevent rickets.
That was tripled following an Institute of Medicine report, but Hollis said this was based only on skeletal health, since panel members admitted it was beyond their scope to look at other ways Vitamin D influences the body. Hollis thinks some people would be healthier with Vitamin D supplementation beyond what we already get from fortified foods.
Humans remain an equatorial species struggling to adapt to life in temperate zones like Philadelphia, he said. “This is an ongoing experiment.”