When it comes to skin color, the idea that we're really all the same isn't just a utopian dream. A look at skin cancer from an evolutionary perspective suggests that maybe once we were all white; then we were all black; then some of us went back to white.

In a study published in Proceedings of the Royal Society B, Mel Greaves, professor of cell biology at the Institute of Cancer Research in London, looked at some 25 studies of skin cancer in albinos in Africa. Albinos have less melanin, a natural pigment that helps protect the skin against damage from the sun. The more melanin in the body, the darker the skin.

Greaves found that basal cell and squamous cell carcinomas are not relatively harmless diseases of old age. In African albinos, they kill early and quickly. Skin cancer prevention, he concludes, was a driving force in human evolution to dark skin. Other scientists, including Charles Darwin, have long dismissed skin cancer as a force in evolution because it typically strikes those past childbearing age.

Greaves, who studies the role that disease plays in human evolution, believes his study adds credence to the idea that when earlier hominids shed their shaggy hair about two million years ago, exposing their naked, pale skin to the sun on the sun-drenched savanna of Africa, natural selection favored those who had the darkest variations in skin color to protect against the ultraviolet radiation (UVR) that can cause skin cancer.

Much later, about 50,000 to 100,000 years ago, those who migrated to cold northern climates no longer needed that protection, and evolved back to pale skin.