Jan. 31, 2010 -- A mutation that causes some Asians to flush red when they down a beer may have evolved to help their ancestors cope with rice wine. A genetic study suggests that the mutation evolved around 10,000 years ago, about the same time as Asians were starting to farm rice and figuring out how to ferment it into boozy drinks.

Bing Su, a geneticist at the Chinese Academy of Sciences in Kunming, and colleagues, studied the genes of 2275 people from 38 east-Asian populations, looking for a mutation that modifies the gene that codes for the enzyme alcohol dehydrogenase.

The mutation causes alcohol to be metabolised at 100 times the speed that it otherwise would be.

As the enzyme removes alcohol so quickly from the blood stream, it protects people from the harmful effects of alcohol, and Su believes it confers an evolutionary advantage: a study in the Han Chinese suggests that those carrying the mutation have the lowest risk of alcoholism (American Journal of Human Genetics, vol 65 p 795).

The mutation also causes a by-product of the alcohol's metabolisation to accumulate in the body, which makes those who have the mutation flush red when they drink.

Follow the rice

The mutation is found most frequently in Asia and least frequently in Europe and Africa, but the reason for this has remained a mystery. Su's explanation is that the mutation spread across Asia and towards Europe in lockstep with rice cultivation.

Genetic analysis shows that the mutation cropped up between 10,000 and 7000 years ago. Su's team found that the mutation is virtually ubiquitous in the Zhejiang province in south-eastern China but becomes less common further north and west.

These dates and locations square up with archaeological evidence of early rice cultivation, which suggests rice was first domesticated in south-eastern China between 12,000 and 8000 years ago, and then spread west.

Nine-thousand-year-old Chinese pottery shards in south-central China bear traces of alcohol. Su believes the earliest rice cultivators began fermenting rice to help preserve it and break it down to release more nutrients.

Nefarious Effects

The mutation in alcohol dehydrogenase would have protected those who had it from some of the nefarious effects of alcohol and alcoholism. As a result, Su says, natural selection for the mutation caused it to spread west in near-synchrony with rice paddies.

Others think the case is not as cut-and-dried as Su claims. "I think the interpretation is too simplistic," says Kenneth Kidd a geneticist at Yale University. But he agrees that cultural forces probably shaped the evolution of the alcohol dehydrogenase mutation among Asians.

Kidd is not convinced that there is proof that the dehydrogenase mutation confers a significant enough advantage to spread through natural selection. Instead, he says, it may simply have spread as the population of rice farmers themselves spread west.