Fatal reactions to sunlight may have triggered a protective shift away from pale skin
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.
Early members of the Homo genus in Africa were probably pale skinned and spent much of their days hunting and foraging in direct sunlight, Greaves asserts. Researchers generally agree that the loss of most body hair helped hominids control body temperature in tropical savannas.
Non-melanoma skin cancers probably killed many sun-drenched Stone Age hominids of pallor before they could reproduce, he proposes February 25 in the Proceedings of the Royal Society B. Genes that produce dark skin capable of filtering UV radiation would have spread relatively quickly in populations that had much greater sun exposure throughout life than modern groups do.
“Skin cancer could have plausibly been the most potent selective force responsible for the emergence of black skin in ancient hominids,” Greaves says.
But other researchers have rejected skin cancers as a force in the evolution of black skin partly because such growths don’t kill many people today.
Ancient, largely hairless hominids probably had skin more like that of living African apes than human albinos’, remarks biological anthropologist Nina Jablonski of the Pennsylvania State University in University Park. Apes’ pale skin, when exposed to sunlight, develops enough protective melanin pigmentation to enable tanning similar to that achieved by light-skinned people today. Apes possess a gene variant that makes tanning possible, while human albinos do not. Early Homo species carried the gene and weren’t as prone to skin cancer as Greaves assumes, Jablonski hypothesizes.
In her view, dark skin evolved in Africa around 1.2 million years ago to keep UV radiation from lowering the body’s levels of folate, a B vitamin necessary for fertility and healthy development.
Greaves responds that human albinos, despite lacking melanin, can tan with careful sun exposure. So as with African albinos today, tanning wouldn’t have deterred fatal skin cancers in pale-skinned hominids drenched in UV radiation, he argues. Albinos represent an imperfect but useful modern-day analog for ancient hairless hominids that lived in the tropics, he says.
Albinism affects about 1 in 5,000 present-day black sub-Saharan Africans, making them unable to produce melanin.
African albinos from tropical areas develop serious or fatal cases of squamous cell and basal cell cancer by their early 30s, Greaves says. He cites several studies that have concluded that fewer than 10 percent of albinos in equatorial parts of Africa survive beyond their 30s, mainly due to skin cancer. Other investigations have found that nearly all albinos among Native Americans living on islands near Panama’s Caribbean coast develop skin cancers by their 30s that prove fatal without medical treatment.
Squamous cell and basal cell cancers are usually treatable among light-skinned, non-albino adults. In Greaves’ view, that’s due to lesser sun exposure today — partly thanks to sunscreens — than during early Homo evolution.
Earlier studies of modern human DNA indicated that a gene necessary to produce skin-darkening melanin appeared between 1.8 million and 1.2 million years ago in East Africa. Genetic changes that lightened skin appeared as modern humans migrated to nontropical areas beginning at least 60,000 years ago.
By absorbing more of the sun’s UV radiation in nontropical locales, light skin can make enough Vitamin D to maintain health.
Analyses of ancient DNA may eventually clarify the timing of hominid skin color changes (SN: 2/22/14, p. 14). But DNA preserves poorly in tropical climates.
M. Greaves. Was skin cancer a selective force for black pigmentation in early hominin evolution? Proceedings of the Royal Society B. Published online February 25, 2014. doi:10.1098/rspb.2013.2955.