Cancer may have driven the evolution of black skin early in human history, a study suggests.
Scientists believe dark skins appeared more than a million years ago to prevent our African ancestors dying from skin cancer.
The change occurred after ancient humans shed most of their body hair and ventured out into the sun-drenched African savannah.
Previously, like present-day chimpanzees, they would have had pale skin under their hair.
New evidence indicates that the lethal effects of the sun's rays may have exerted powerful selection pressure on early humans between 1.2 and 1.8 million years ago.
Only those individuals with darker, more protected, skin would have escaped dying young from skin cancer and been free to pass their genes onto future generations.
The theory has been rejected up to now because it was thought skin cancer rarely killed people at a young enough age to affect reproduction.
But new evidence points to the fact that albino black people from parts of Africa with the highest exposure to ultraviolet radiation from the sun almost all develop skin cancer and die young.
Lead scientist Professor Mel Greaves, director of the Centre for Evolution and Cancer at The Institute of Cancer Research in London, said: " Charles Darwin thought variation in skin colour was of no adaptive value and other investigators have dismissed cancer as a selective force in evolution.
"But the clinical data on people with albinism, particularly in Africa, provide a strong argument that lethal cancers may well have played a major role in early human evolution as an important factor in the development of skin rich in dark pigmentation."
The research, published in the journal Proceedings of the Royal Society B, shows that at least 80% of people with albinism from African equatorial countries such as Tanzania and Nigeria die from skin cancer before their 30th birthday.
Albinism is an inherited condition marked by a lack of the pigment melanin which colours the skin, hair and eyes.
The trait is also linked to skin cancer in other tropical countries besides Africa with all-year-round sunshine, such as Panama, the researchers point out.
They believe black skin may have had other benefits in addition to protection from cancer, such as preventing damage to sweat glands and preserving folate, which is vital to foetal development.