The Sydney Morning Herald
Skin cancer linked to evolution of dark skin in early humans
February 26, 2014
Skin cancer may have driven the evolution of dark skin in early humans more than a million years ago, a study has found.
A British scientist has speculated that when early human species shed their body hair their light-coloured flesh would have been highly vulnerable to UV-induced skin cancer.
The skin beneath the fur of our nearest primate relative, the chimpanzee, is fair but early Homo sapiens in Africa were black.
Mel Greaves, a professor at the Institute of Cancer Research in London, said deaths from skin cancer may have provided enough selective pressure to drive the evolution of dark skin pigments.
“Dark or black skin lowers the risk of ultra-violet radiation induced skin cancer by several orders of magnitude,” said Professor Greaves, in an analysis published in the Proceedings of the Royal Society B journal on Wednesday.
People with white skin are about 70 times more likely to develop skin cancer than individuals with black skin.
Professor Greaves’ has based her theory of dark skin evolution on the current high death rate of people with albinism from non-melanoma skin cancers.
The idea that skin cancer could have driven dark skin has previously been discounted by scientists because it was thought it was rarely fatal in young people and so would not affect their ability to reproduce, she said.
But recent studies have found that up to 80 per cent of black people with albinism in regions of Africa near the equator, where UV radiation exposure is high, develop lethal skin cancer before age 30. Similar trends have been shown in albinos outside of Africa, and in mice with the disorder, said Professor Greaves.
“This long-running natural experiment, and its experimental parallel with UVR-exposed albino mice, endorse the plausibility that black skin pigmentation could have arisen in early, pale-skinned hominins as a defence against lethal skin cancer,” she said.
But Jeremy Austin, from the Australian Centre for Ancient DNA in Adelaide, said there were several other explanations for how dark skin evolved in sun-drenched environments.
The evolution of dark skin has been strongly linked to the way UV radiation influences sweat gland function, the permeability of the skin, which affects dehydration and overheating and the depletion of folate, which is critical during pregnancy.
“Albinism is really a special case,” Dr Austin said.
While it made sense that albinism would be a trait selected against in high UV environments that did not explain the positive selection for dark skin.
“Dark skin grades from olive to totally black so there must be some selection pressure acting to balance the amount of melanin in the skin,” he said.
Skin colour is mainly determined by the pigment melanin, which comes in three types, and the way it is packaged in specialised cells, found mainly in the skin, called melanosomes.
In people with dark skin the dark brown to black variety of melanin is packaged in a way that absorbs a large amount of the UV radiation that penetrates the skin, thereby protecting against DNA damage. In fair-skinned people, lighter melanin pigments are not as effective at filtering the sun’s rays, nor do they scavenge free radicals, which are involved in cancer development, like the darker version of melanin. Albinism is caused by genetic changes that prevent melanin production.
Genetic studies have suggested that the appearance of a gene variant that promoted black pigmentation arose around the time early humans started occupying savanna landscapes in East Africa, about 1 to 2 million years ago.
Most scientists believe early humans lost their fur when they became bipedal hunter-gathers roaming savannas for food. Without fur they could sweat, a better method of regulating their body temperature during intense activity, such as chasing down dinner.
While Professor Greaves acknowledged that extrapolating the risk of skin cancers in albinos today as evidence of the evolution of dark skin was “clearly speculative”, she said if early humans were pale-skinned they would likely have faced similar afflictions from skin cancer.
Read more: http://www.smh.com.au/world/skin-cancer-linked-to-evolution-of-dark-skin-in-early-humans-20140226-33hgx.html#ixzz2ufgwVkIo