Extended<unk> red (Afrikaans: Kleurlinge or Bruinmense) family from South Africa showing some spectrum of<unk> ation<unk>  ranges from the darkest brown to the lightest hues. Differences in<unk><unk>  among individuals is caused by variation in pigmentation, which is the result of genetics (inherited from one's biological parents), the exposure to the sun, or both. Differences across populations evolved through natural selection, because of differences in environment, and regulate the biochemical effects of ultraviolet radiation penetrating the<unk> . The actual<unk><unk>  of different humans is affected by many substances, although the single most important substance is the pigment melanin. Melanin is produced within the<unk>  in cells called melanocytes and it is the main determinant of the<unk><unk>  of darker-skin humans. The<unk><unk>  of people with light<unk>  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<unk>  underlying the<unk>  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.<unk>  is not entirely uniform across an individual's<unk> ; for example, the<unk>  of the palm and the sole is lighter than most other<unk> , 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<unk>  pigmentation around the world. Areas that receive higher amounts of UVR, generally located closer to the equator, tend to have darker-skinned populations. Areas that are far from the tropics and closer to the poles have lower intensity of UVR, which is reflected in lighter-skinned populations. Some researchers suggest that human populations over the past 50,000 years have changed from dark-skinned to light-skinned and vice versa as they migrated to different UV zones, and that such major changes in pigmentation may have happened in as little as 100 generations (≈2,500 years) through selective sweeps. Natural<unk><unk>  can also darken as a result of tanning due to exposure to sunlight. The leading theory is that<unk><unk>  adapts to intense sunlight irradiation to provide partial protection against the ultraviolet fraction that produces damage and thus mutations in the DNA of the<unk>  cells. In addition, it has been observed that females on average are significantly lighter in<unk>  pigmentation than males. Females need more calcium during pregnancy and lactation. The body synthesizes vitamin D from sunlight, which helps it absorb calcium. Females evolved to have lighter<unk>  so their bodies absorb more calcium. The social significance of differences in<unk><unk>  has varied across cultures and over time, as demonstrated with regard to social status and discrimination.
Human skin color
Sensitive skin
Body shape
Human body weight
Composition of the human body
Pain in animals
Light skin
Body odor