All living human populations belong to a single biological species (Homo sapiens) within a larger group or genus (Homo). Within the human species, a large number of populations may be differentiated genetically through readily observable charasteristics ( e.g., skin, hair, and face and body proportions) and through less obvious but more distinctive biological traits, such as blood type. These biological groupings within species are commonly called races, in humans as well as in other living forms.

Black skin (people from Africa, Native Australians, African-Americans, Caribaeans), is darker than northern European skin because of the increased amount of melanin in the skin. Long before the microscope made it possible to count melanin granules in skin sections, naturalists had come to appreciate the importance of differences in skin (and eye) pigmentation. They observed a direct relationship between sunlight levels and skin pigmentation, at least in Europe and Africa. Later it was detemined that the pigment granules serve as a natural sunscreen, protecting the deeper layers of skin as harmful ultraviolet wavelengths in sunlight are absorbed, and that there is correlation between higher melanin content and slightly lower sweating thresholds.Conversely, low levels of melanin in the skin appear to be an advantage in parts of the world where cloudy conditions are common or where (as in the polar regions) there are prolonged periods of limited sunlight. Lighter-skinned individuals are more able to convert vitamin D precursors in the skin to usable vitamin D . Disorders caused by vitamin D deficiency, are more common among dark-skinned children living in northern Europe, with its generally cloudier climate and shortened hours of sunlight in winter. It has been demonstrated that the rate of skin cancer rises sharply as pigment densities go down. Light-skinned individuals may generate considerable melanin, with continued exposure to sunlight (and so tan), but this does not occur until after damage to the skin has been done. Those who have less melanin in their irises are also more likely to develop cataracts from long-term exposure to ultraviolet radiation.