Black skin, a response to skin cancer risk

The ability of ultraviolet radiation to cause skin cancer is well established. Yet, it is only “generally believed” that the development of dark skin by people in Africa was an adaptive response to protect them from the damaging effects of UV rays. Also, the harmful impact of UV rays in the survival and/or reproductive fitness of individuals is “uncertain.”

But a paper published on February 26 in the Proceedings of the Royal Society B reports evidence that dark skin evolved in early humans living in Africa to protect them from the damaging effects of UV radiation. It provides proof that skin cancer can affect young people in the reproductive age.

Unlike light-skinned people, dark-skinned people have relatively lesser risk of suffering from skin cancer. And even if they do have skin cancer, it is typically restricted to the soles and palms that are less pigmented. The reason — the presence of brown/black eumelanin in dark-skinned people that filters out the UV radiation. So much so that dark-skinned people enjoy a 500- to 1,000-fold protection compared with light-skinned people.

But hominins who lived some 2-3 million years ago in the East African savannah were probably not black skinned. The loss of body hair to facilitate sweating and heat loss (thermoregulation) for the first time exposed the bare skin, which was about white or pale. The skin of chimpanzees — our nearest primate relative — under its thick fur of hair is pale or white.

Genetic evidence suggests that about 1.2 to 1.8 million years ago Africans living in the hot, open savannah that has highest levels of UVB radiation throughout the year were under selective pressure to retain a variant of MC1R gene that encodes for effective dark colouration of the skin (melanization).

But the key notion that skin cancer served as a selective force for the development of dark skin has been dismissed on the grounds that lethal skin cancer is rarely seen in young people during the reproductive years.

The author of the study Mel Greaves from the Institute of Cancer Research, Sutton, Surrey, U.K., cites the high incidence of skin cancer at a young age in albinos — who are white-skinned due to a disorder of melanin pigmentation — to question the misconception. Albinos are seen in black ethnic groups throughout sub-Saharan Africa (1 in 5,000), Ibo in Nigeria and the Tongo in Zimbabwe (1 in 1,000).

The prevalence of skin cancer in African albinos in low latitude countries like Tanzania, Cameroon, and Nigeria is high and it occurs at an early age. While focal skin lesions can be seen in children as young as five years, overt skin cancer can be seen in most albinos by the age of 20 years.

Albinos in several other low-latitude countries like India, Papua New Guinea and Panama run the same high risk of developing skin cancer. Albino mice exposed to UV radiation suffer similarly.

These examples “endorse the plausibility that black pigmentation could have arisen in early pale-skinned hominins as a defence against lethal skin cancer,” the paper notes.

Published in The Hindu on February 27, 2014