Human skin color ranges from the darkest brown to the lightest hues. Differences in skin color among individuals is caused by variation in pigmentation, which is the result of genetics (inherited from ones biological parents and or individual gene alleles), exposure to the sun, natural and sexual selection, or all of these. Differences across populations evolved through natural or sexual selection, because of social norms and differences in environment, as well as regulations of the biochemical effects of ultraviolet radiation penetrating the skin.The actual skin color of different humans is affected by many substances, although the single most important substance is the pigment melanin. Melanin is produced within the skin in cells called melanocytes and it is the main determinant of the skin color of darker-skin humans. The skin color of people with light skin is determined mainly by the bluish-white connective tissue under the dermis and by the hemoglobin circulating in the veins of the dermis. The red color underlying the skin becomes more visible, especially in the face, when, as consequence of physical exercise or sexual arousal, or the stimulation of the nervous system (anger, embarrassment), arterioles dilate. Color is not entirely uniform across an individuals skin; for example, the skin of the palm and the sole is lighter than most other skin, and this is especially noticeable in darker-skinned people.There is a direct correlation between the geographic distribution of ultraviolet radiation (UVR) and the distribution of indigenous skin pigmentation around the world.
Different populations have different allele frequencies of these genes, and it is the combination of these allele variations that bring about the complex, continuous variation in skin coloration we can observe today in modern humans. Population and admixture studies suggest a 3-way model for the evolution of human skin color, with dark skin evolving in early hominids in sub-Saharan Africa and light skin evolving independently in Europe and East Asia after modern humans had expanded out of Africa.For skin color, the broad sense heritability (defined as the overall effect of genetic vs. nongenetic factors) is very high, provided one is able to control for the most important nongenetic factor, exposure to sunlight. Many aspects of the evolution of human skin and skin color can be reconstructed using comparative anatomy, physiology, and genomics. Enhancement of thermal sweating was a key innovation in human evolution that allowed maintenance of homeostasis (including constant brain temperature) during sustained physical activity in hot environments. Dark skin evolved pari passu with the loss of body hair and was the original state for the genus Homo. Melanin pigmentation is adaptive and has been maintained by natural selection. In recent prehistory, humans became adept at protecting themselves from the environment through clothing and shelter, thus reducing the scope for the action of natural selection on human skin. Credit for describing the relationship between latitude and skin color in modern humans is usually ascribed to an Italian geographer, Renato Basutti, whose widely reproduced "skin color maps" illustrate the correlation of darker skin with equatorial proximity.