Dark-skinned Kenyans, lighter-skinned Tunisians, and fair-skinned Norwegians. Everyone knows that as you travel from the equator to the poles, human skin color tends to go from darker to lighter.
But why should this be so?
Since the 1960s, researchers have hypothesized that vitamin D's connection with sunlight has pushed human beings toward developing lighter skin. Why? The pigment that turns human skin dark, melanin, has the effect of blocking UV light absorption.
But what other factors might be at work in pushing human beings toward darker coloration? For some years, scientists had speculated that folic acid levels might have something to do with darker skin, but how could these two things be linked?
Jablonski thought she knew; Sunlight could lower folic acid levels, and low folic acid levels could bring about neural tube defects. As an evolutionary anthropologist, she realized that these two factors could easily work with a third to bring about different human skin colors.
What was the Third factor? Natural selection
Jablonski had arrived at the conclusion that skin color is a result of a push-pull between retaining enough folic-acid (for nervous system development) and making enough vitamin D (for proper bone development).
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