02-19-2017  3:18 pm      •     

(CNN) -- From the darkest brown to the pastiest white and every shade in between, humans display a tremendous variety of skin colors. Human skin color is directly linked to our survival as a species as we lost our fur and developed a need for protection from the sun, and then migrated into cloudier regions of the globe. Over the course of evolution, scientists argue, skin color was influenced, among other factors, by our need for healthy bones.

To begin to explain this, we turn to Nina Jablonski, professor of anthropology at Pennsylvania State University. She is a well-recognized researcher in primate evolution, and specifically the evolution of human skin, and she was the subject of a Science Seat on CNN Light Years.

The story of human skin color begins with our furry ancestors about 6 to 7 million years ago in Africa, the last time that humans and chimpanzees shared an ancestor. Jablonski says that these ancestors, called Australopithecus, still had ape-like body proportions: fairly long arms and relatively short legs.

"When we look at their skeletons in detail, it's pretty clear that they were not active runners," she said. "They could walk on two legs but they weren't running or striding purposefully across the savannah most of the time, they were sort of living lives that are much like those of chimpanzees: fairly close to the edge of the forest, sometimes going into trees for protection, and then walking for short distances in the open to forage."

By about 1.2 million years ago, humans ancestors had lost their fur and were able to sweat more efficiently to avoid overheating. The fossil record shows this in the species Homo ergaster, for example. During this time, individuals began to walk across the hot savannah, so there was a need for more internal body heat regulation.

Without fur, however, our skin was exposed to the strong equatorial sun. The skin pigment melanin, which is responsible for most of the color of our skin. Melanin is a terrific sunscreen, and darkly pigmented skin became a substitute for fur.

But as our ancestors migrated away from the Earth's equator, which has lots of UV exposure, it became less and less beneficial for those populations to have so much pigmentation as protection from the sun. Why? For answers, we must look at vitamin D.

"Vitamin D is produced at high levels in the skin when it is exposed to ultraviolet light from the sun," says Dr. Michael Holick, a professor of medicine, physiology and biophysics at Boston University Medical Center. He is a leader in vitamin D nutrition, and among countless other works, published the book "The Vitamin D Solution."

According to Holick, back in the 1930s to 1950s, it was thought that the main reason for skin pigmentation was to prevent having too much vitamin D being produced in the skin. Too much vitamin D leads to vitamin D intoxication, which can result in death. However, in the early 1980s, Holick and his colleagues published a paper that disproved that theory.

"It turns out that Mother Nature was quite clever, in that any excessive exposure to sunlight destroys any excess vitamin D produced in the skin," Holick explained.

Vitamin D is produced in skin that's exposed to the sun, and it's involved in helping the intestines absorb calcium, which is a critical nutrient in our bones. However, heavily pigmented skin reduces a person's ability to produce vitamin D in the skin "probably by 90-95 percent," according to Holick, meaning they were more likely to be deficient in the vitamin.

As our ancestors migrated to areas away from the equator, with lower UV radiation, pigmentation became a problem. For example, Holick explained, a person from Africa who is very darkly pigmented has a sun protection factor of around 30. That person would have to be out in the sun at least 10 to 15 times longer to produce the same amount of vitamin D as a lightly-pigmented person from Europe.

Vitamin D is critical for healthy bones, which have always been essential to human survival. Not only are healthy bones important to movement and holding our bodies upright, but they are essential for reproduction. A pregnant mother who is vitamin D deficient can have a baby born with infantile rickets syndrome, a disease that leads to severe bone abnormalities. If the mother remains vitamin D deficient, she is also calcium-deficient. If breast milk is the main food source for the infant, the infant will not receive enough calcium to build healthy bones.

The female infant "will have a flat, deformed pelvis with a small pelvic outlet, making child delivery impossible," Holick said.

While rickets may not have affected male reproductive abilities as much, it would have given them weaker, more brittle bones, increasing their risk of fracture, and therefore affecting their ability to survive.

Vitamin D deficiency is associated with a host of other health problems, such as preeclampsia, asthma, upper respiratory infections in children, and even multiple sclerosis, Holick said. This is because of the vitamin's role in immune function, as it is thought to decrease risk of developing autoimmune diseases.

On the other hand, scientists know that too much sun can deplete a person's reserve of folate, a B-vitamin that is necessary for cell division and repair and known to reduce risk of fetal birth defects. A big source of folate comes from leafy green vegetables, but Holick argues that our hunter-gatherer ancestors would have eaten plenty of those. Today, many people do not eat enough of them.

So how much sun do modern humans actually need? Dr. Holick says that it's not possible to give an absolute recommendation because of the time of day, latitude and skin pigmentation, which all influence vitamin D production.

"For example, a lightly pigmented person in Boston in June at noontime, 10 to 15 minutes of sun exposure between the hours of 10 AM and 3 PM on arms, legs and abdomen and back when appropriate 2 to 3 times a week is usually sufficient," Holick said.

Now that most humans in modern society spend most of their days indoors, away from sunlight, you might assume that humans will eventually become homogeneous in terms of skin color and may lose darker pigmentation.

But Jablonski offers a different explanation for why human skin color will become more homogenous:

"Because of human migrations, especially in the world's largest cities, skin color will become somewhat more homogeneous because of increased admixture of people, while depopulated rural areas will tend to remain much as they are."

She emphasizes that that increased color homogeneity will result as a product of the intermingling of populations, not natural selection.

"There is no significant evolution acting on people who are either 'too light' or 'too dark' for their solar conditions because we have layers of protection in the form of complex culture, like buildings, clothing and diet," she asserted.

Any city-dwelling person, regardless of skin color, can become vitamin D deficient if he or she doesn't receive enough sun exposure. As Jablonski explained, just as lighter-skinned people are more likely to suffer from sunburn and therefore skin cancer, darker-skinned people experience more vitamin D deficiency because short periods of outdoor exposure aren't sufficient to allow them to produce enough vitamin D in the skin.

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