It was noticed long ago that people from different parts of the world had different skin colors. For example, people who live near equator usually had darker skin than people who lived in colder regions.
When studied by scientists it was found that person’s skin color is determined by the amount of melanin in his or her skin. Everyone has melanin in their skin but people who live where the sun shines all the year round have more melanin in their skins than people who live in colder areas.
Melanin is a dark brown to black pigment produced by special cells called melanocytes. Melanin is essential to protect the skin from the ultraviolet rays coming from the sun which are harmful to skin and may cause skin cancer also.
The areas that receive higher ultraviolet radiation are generally located nearer to the equator, their lives darker-skinned population. Areas closer to North-South poles have a lower intensity of UVR (Ultra Violet Radiation), their lives lighter-skinned population.
Ultra-violet rays coming from the sun is the main factor in the determination of skin-color. These rays are incisive enough to penetrate and burn light skin. The dark skin stops excessive sun rays at the skin to protect the healthy cells below the skin. This is the reason of Negroes of Africa have dark skin whereas the people of Scandinavian countries (Norway, Sweden, Denmark) almost complete lack of melanin in their skin.
In some northern coastal areas such as Alaska and Canada people have darker skin because they eat seafood that provides all the vitamin D needed by the body, therefore their skin produces more melanin which makes skin darker.