Common forms of skin cancer were Stone Age killers that prompted the evolution of black skin among human ancestors in Africa, a controversial new analysis concludes. Evidence gathered over the last 40 years shows that albinos living in tropical parts of Africa and Central America, where they are constantly exposed to high levels of ultraviolet radiation from the sun, frequently develop skin cancer and die young from it, says biologist Mel Greaves of the Institute of Cancer Research in London. “Skin cancer could have plausibly been the most potent selective force responsible for the emergence of black skin in ancient hominids,” Greaves says.