are currently no recommendations on a safe level of total lifetime sun exposure. according to epidemiologist robyn lucas at australian national university, analysis of lifespan versus disease shows that far more lives worldwide could be lost to diseases caused by lack of sunlight than to those caused by too much, and it is inappropriate to recommend total avoidance of sunlight. over thousands of years, in many climate zones, genetic selection has helped indigenous human populations adapt toward skin pigmentation levels that provide a healthy level of uv exposure. this largely explains the tendency toward darker-skinned populations in the sunniest tropical environments, and lighter skin tones in less-sunny regions and for those who most need vitamin d for rapid bone growth, specifically children and reproductive-age women. the map to the right illustrates the geographic distribution of skin color for native populations prior to 1940, based on von luschan's chromatic scale. these long-term adaptations for optimal health can be confounded by patterns of food, clothing and shelter, especially at a time when large populations have migrated far from the climates for which their skin was genetically adapted.