Skin is just a thin covering which protects us from the hostile world outside. It s water and bacteria proof, and dark skin is also also sunproof.
Melanin is a form of colourant that lies in the bottom layers of the skin.
If you ve got a lot of melanin, your skin is dark, and the less melanin you ve got, the lighter your skin is. Melanin is what creates suntans, and biologically it has one particular purpose - it acts as nature s own sunscreen. It stops UV rays from penetrating the skin and burning it. Because we all evolved in the sunny deserts of Africa we needed to protect ourselves from burning, which is why we originally had black skin. You can tell how little protection white skin has against the sun from the levels of skin cancer in Australia.