Evolution of dark skin reconsidered: melanin could have protected early hominids from cancer.
Evidence gathered over the last 40 years shows that albinos in tropical parts of Africa and Central America, where people are constantly exposed to high levels of the sun's ultraviolet radiation, frequently develop skin cancer and die young, says biologist Mel Greaves of the Institute of Cancer Research in London.
Early members of the genus Homo in Africa were probably pale skinned and spent a lot of time 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.
Nonmelanoma skin cancers probably killed many light-skinned early hominids before they could reproduce, he proposes in the April 22 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.
Other researchers have rejected the idea partly because skin cancer doesn't kill many people today.
Ancient, largely hairless hominids probably had skin more like that of living African apes than like human albinos', remarks biological anthropologist Nina Jablonski of Penn State. Apes' pale skin, when exposed to sunlight, develops enough protective melanin pigmentation to enable tanning similar to that of light-skinned people today. Apes possess a gene variant that makes tanning possible, while human albinos don't. Early Homo species probably carried the gene and weren't as prone to skin cancer as Greaves assumes, Jablonski says.
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 development.
Greaves responds that albinos, despite lacking skin-darkening 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, he argues. Albinos represent an imperfect but useful modern analog for ancient hairless hominids living in the tropics, he says.
African albinos in the tropics develop serious or fatal cases of squamous cell and basal cell cancer by their early 30s, Greaves says. Several studies have concluded that less than 10 percent of albinos in equatorial Africa survive beyond their 30s, mainly due to skin cancer. Other investigations have found that nearly all Native American albinos living on islands off Panama develop skin cancers by young adulthood that are fatal without treatment.
Nonmelanoma skin cancer is usually treatable in pale, nonalbino adults. Greaves says that's due to less sun exposure today--partly thanks to sunscreens --than earlier hominids had.
Studies have indicated that a gene needed to produce skin-darkening melanin appeared between 1.8 million and 1.2 million years ago in Africa. Genetic changes that lightened skin appeared as humans left the tropics at least 60,000 years ago.
Analyzing ancient DNA could clarify the timing of hominid skin color changes (SN: 2/22/14, p. 14). But DNA preserves poorly in tropical climates.
Caption: African albinos often develop skin cancer at a young age, studies find.
Please note: Illustration(s) are not available due to copyright restrictions.
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|Title Annotation:||HUMANS & SOCIETY|
|Date:||Apr 5, 2014|
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