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. National Geographic talked with Greaves about his research.

View Images At one time, natural selection may have favored those who had the darkest variations in skin color to protect against ultraviolet radiation. PHOTOGRAPH BY SARAH LEEN, NATIONAL GEOGRAPHIC

You point to skin cancer as a reason that skin color evolved. Among cancers, is skin cancer unique in influencing evolutionary protections?

I can't think of any other cancer and circumstance that would have had a sufficiently large impact on survival and reproduction. You might think that pediatric cancers might have been subject to evolutionary selection, but my guess is that they have always been too rare to provoke protective selection.

Can you explain when and why our human ancestors became black?

The genetic evidence suggests that black skin became the norm in Africa some 1.2 million years ago, around the time that early humans were colonizing the savanna and had lost most of their body hair. Most investigators believe that black pigmentation was an essential adaption to protect naked, pale skin against solar ultraviolet radiation, which is high all year round near the equator.

There has been consensus on some of the life-threatening impacts of UVR via the skin. Ideas have included damage to sweat glands and degradation of folate and other essential nutrients in blood circulating through the skin.

But skin cancer has been universally rejected as a possible selective force for the adaptation of black skin. This is on the grounds that in modern-day Caucasians, it is usually benign or is lethal too late in life to influence evolution. In my paper I suggest this is taking cancer out of the relevant context and that the experience of African albinos illustrates very vividly what the impact of intense UVR might have been on early humans.

Why did some people then evolve back to the white skin that was originally underneath hominids' hair?

As our human ancestors migrated out of Africa, those that moved away from equatorial and tropical regions underwent positive selection for paler skin. This was in part due to the reduced pressure from UVR skin damage, but also because black skin became a disadvantage, possibly because [pale skin is better at generating vitamin D] and dark skin is more susceptible to frostbite.

So you're saying that skin cancer played a part in skin color: Humans were originally white under all their hair, then evolved to black a million or two million years ago, then 50,000 to 100,000 years ago some went back to white as they migrated farther north?

That's exactly what I am suggesting. But unless Jared Diamond and Darwin [two scientists who dismissed skin cancer as a factor in evolution] are right and skin color variation is just incidental and endorsed by sexual preferences, then there has to be an evolutionary logic.