A new analysis suggests that black skin may have evolved in humans as a protective measure against skin cancer.
Previously, skin cancer has been disregarded as an influence in the evolution of black skin in humans. This is based on the belief that skin cancer only rarely causes death at ages young enough to affect reproduction.
"Charles Darwin thought variation in skin color was of no adaptive value and other investigators have dismissed cancer as a selective force in evolution," acknowledges study author Prof. Mel Greaves of the Institute of Cancer Research in London, UK.
"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 - in eumelanin."
Eumelanin is the brown-black version of melanin - the naturally occurring substance that gives color to hair, skin and the iris of the eye, and that protects the skin from the sun. Eumelanin is much more effective than other variants of melanin at preventing the damage to DNA that results in skin cancers.
How did human skin evolve in response to UV exposure?
Human skin evolved to be rich in melanin between 1.2 million and 1.8 million years ago in the East African Savanna.
It is thought that early humans evolved away from a complete covering of body hair to facilitate heat loss, and that they had pale skin containing pheomelanin - the red-yellow melanin variant found in chimpanzees, our nearest genetic relatives.
Humans migrated north from Africa into Europe 50-80,000 years ago, at a time in the Earth's history when there was substantially less ultraviolet (UV) exposure. There may have been a selective advantage in having paler skin for the early Europeans - possibly to obtain more vitamin D.
Although scientists generally agree that the black skin characteristic of eumelanin thrived in humans due to the ability of eumelanin to absorb UV radiation, the extent to how this could have protected early humans against lethal diseases is debatable.
People with albinism at risk of fatal skin cancer in equatorial regions
In his analysis, Prof. Greaves cites studies that show 80% or more of people with albinism from equatorial African countries - such as Tanzania and Nigeria - develop fatal skin cancers before the age of 30.
Other tropical countries with year-round high UV-exposure, such as Panama, also report high levels of skin cancer in people with albinism.
Albinism is caused by a mutation in the genes that prevents melanin from being produced.
Prof. Greaves thinks that because people with albinism develop cancer at reproductive ages, this means that early, pale-skinned humans faced evolutionary pressure to develop skin rich in melanin that would protect against cancer.
White- or pale-skinned people are approximately 1,000 times more at risk of skin cancer than people with dark skin.
Excessive sun exposure is generally agreed to be the the major cause for the skin cancers basal cell carcinoma, squamous cell carcinoma and malignant melanoma - collectively the most common cancers experienced by humans.
In 2013, Medical News Today reported on a study that found sunscreen shields an important gene for protecting against skin cancer.