Human skin color evolved as a finely tuned adaptation to ultraviolet (UV) radiation and diet, with dramatic shifts occurring after the Neolithic transition to farming. While the advent of agriculture in Europe is known to have triggered strong selection for lighter skin and traits like blue eyes—primarily to compensate for reduced dietary vitamin D in grain-heavy diets—folate (vitamin B9) played an equally vital but often overlooked role. As we will see, folate insufficiency due to UV exposure made darker skin the norm. UV radiation rapidly destroys folate in the skin, and folate deficiency triggers neural tube defects such as spina bifida, which were frequently fatal in developing fetuses. This reproductive cost created intense evolutionary pressure towards darker skin tones that complemented vitamin D dynamics, helping explain why light skin emerged so rapidly in northern latitudes, where folate destruction by UV is over twenty times weaker, once farming altered the nutritional landscape.

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