The Evolution of Human Skin: How Northern Europeans Get More Sun

Humans have a variety of traits--different colored skin, eyes and hair. While one popular theory holds that Northern Europeans developed light skin in order to absorb more UV light to make more vitamin D, a new study calls that idea into question. It turns out that light skin may have evolved for other reasons.

Vitamin D is important for healthy bones and immune function, and ramping up the skin's capacity to capture UV light to make vitamin D is important. Yet changes in the skin's function as a barrier to the elements makes a great contribution that alterations in skin pigment when it comes to making vitamin D.

"At the higher latitudes of Great Britain, Scandinavia and the Baltic States, as well as Northern Germany and France, very little UVB light reaches the Earth, and it's the key wavelength required by the skin for vitamin D generation," said Peter Elias, one of the researchers, in a news release. "While it seems logical that the loss of the pigment melanin would serve as a compensatory mechanism, allowing for more irradiation of the skin surface and therefore more vitamin D production, this hypothesis is flawed for many reasons."

In fact, earlier research showed that dark-skinned humans make vitamin D just as efficiently as lightly-pigmented humans after skin exposure. In addition, osteoporosis, which has been associated with vitamin D deficiency, is actually less common in darkly-pigmented humans.

In fact, it appears that a specific skin-barrier protein called filaggrin may have more of a role in light absorption that pigment. Filaggrin is broken down into a molecule called urocanic acid, which is the most potent absorber of UVB light in the skin.

In this latest study, the researchers found that there was a higher prevalence of inborn mutations in the filaggrin gene among Northern European populations. In fact, up to 10 percent of normal individuals carried mutations in the filaggrin gene in these northern nations. Higher filaggrin mutation rates also correlated with higher vitamin D levels in the blood.

So if filaggrin is responsible for more vitamin D, why the lighter skin? Pigmented skin provides a better barrier, which was critically important for protection against dehydration and infections among ancestral humans. But the need for pigment to provide this protection waned as humans moved northward. The need to absorb UVB light, though, became greater.

"Once human populations migrated northward, away from the tropical onslaught of UVB, pigment was gradually lost in service of metabolic conservation," said Elias. "The body will not waste precious energy and proteins to make proteins that it no longer needs."

The findings are published in the journal Evolutionary Biology.