"Human skin naturally produces fumaric acid when exposed to sunlight." quote from https://en.wikipedia...ki/Fumaric_acid
Fumaric acid is used to treat psoriaris and acne topically and orally: http://www.ncbi.nlm....pubmed/17456929
UV light (sun exposure) also produces retinoic acid from Vitamin A. Notice how Accutane is a derivative of Vitamin A. Also some people have success in taking large amounts of Vitamin A supplements. I think they could improve alot more if they got more daily sun exposure. Go to this topic and scroll down about a 1/3rd of the way. Alterntivista talks about how people with acne may have not enough retinoic acid in their skin than normal people and need more sunlight as a result.
I believe that we have evolved to depend on sunlight exposure every day. Notice how most cases of moderate-severe acne are located on the face, neck, back, chest and shoulders, the areas of the body most exposed to sunlight. You never see someone with acne on the bottom of their foot. Back in the paleolithic area, we wouldn't be wearing clothing all the time or have access to air conditioned buidlings for shade.