Some dark skin tones have evolved relatively recently from paler genetic variants.
A study of genetic variation and skin colour in African populations has illuminated a number of gene variants that affect the levels of pigmentation expressed in the skin, and their likely evolutionary basis.
'People have thought it was just light skin that has been evolving,' said Dr Tishkoff to New Scientist. 'I think dark skin continues to evolve as well.'
A team of researchers lead by Dr Sarah Tishkoff of the University of Pennsylvania, Philadelphia, genotyped nearly 1600 volunteers from ethnically and genetically diverse African populations. Using a measure of light reflectance from their skin as a proxy for pigmentation levels, the team conducted a genome-wide association study (GWAS). Their results identified four genomic regions associated with skin colour, accounting for approximately 30 percent of the variation in skin pigmentation between the volunteers.
Previous work focusing on the MC1R gene had lead to the general belief that dark skin colour was a consistent phenotype throughout Africa – largely due to its ability to protect against harmful levels of UV radiation.
'They thought [MC1R] shows that there has been selection for dark skin in Africa and therefore there’s no variation,' Dr Tishkoff told New Scientist.
This new work identified mutations in the gene MFSD12 which lead to reduced pigment expression, variations of which are more frequently observed in darker skinned individuals. These variations are thought to have emerged more than 300,000 years ago suggesting that ancestral humans had moderate pigmentation, instead of the very dark skin tones seen in populations such as the Dinka in South Sudan.
In addition, many of the other variants examined in the study also appeared before the emergence of modern humans. In many cases the older variants were associated with less pigmentation and hence lighter skin. This could suggest that darker skin tones have evolved from a lighter-skinned ancestral state.
The study also belies the idea of distinct biological races. Dr Tishkoff said: 'There is so much diversity in Africa that's not often appreciated. There's no such thing as an African race.'
The study was published in Science.