There is a direct relationship between UV radiation (UVR) and the distribution of indigenous skin pigmentation around the world. Areas that have more UVR also have darker-skinned populations. Areas that are far from the tropics and closer to the poles have lower concentration of UVR, and lighter-skinned populations. Human populations have changed from dark-skinned to light-skinned as they left Africa about 100,000 years ago, and some have changed back to darker skin as they returned to higher UV zones.