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actually, melanin in the skin prevents the UV rays from destroying folic acid in prenant women's blood therefor preventing spinabifida which is why it's an adaptation. Humans readapted as they headed up north to having lighter skin because the darker skin would not allow enough rays through and therefore we didn'T get enough vitamine D to help absorb vitamine C and we would die from serious bone malformations. Welcome to natural selection people! :)