In this TED talk, author Nina Jablonski (Skin: A Natural History) suggests that whenever someone demands proof of evolution by natural selection, you should roll up your sleeve and show them your skin color. You carry this evidence around with you every day. You should point out that skin color changes depending on where one's ancestors lived. Check out the dramatic world map image at about 4:20 of the video. Darker skin colors can be found near the equator and lighter skin colors abound in higher latitudes. There is a fundamental and undeniable relation between skin color and latitude. But that is just the beginning of her story. We all have melanin: nature's sunscreen. It protected equatorial people from harmful UVB rays, but nonetheless allowed their skin to produce vitamin D, which is now recognized as essential for proper skin, bones and immune system function. When humans moved to higher latitudes, their skin tones lighted up to allow better production of Vitamin D. Jablonski warns, however, that the angle at which sunlight hits the earth at higher latitudes blocks most UVB rays, allowing only UBA rays which cannot produce Vitamin D. Hence, those of us living in the northern latitudes, no matter what our skin color but especially those of us who work indoors, are at risk for lack of vitamin D. She urges doctors to do a better job warning patients in higher latitudes about the potential damage to skin, bones and immune system caused by their lack of vitamin D.