Is Rice Domestication to Blame for Red-Faced Asians?
If your face turns red after drinking just one glass of wine, blame ancient Chinese farmers. Researchers are reporting that the "Asian Flush" mutation cropped up just as rice was first being domesticated, and it may have protected early farmers from the harms of drinking too much. But some other scientists urge caution, saying that the dates may not match up.
When you drink, enzymes in the liver known as alcohol dehydrogenases (ADHs) convert alcohol to an organic compound called acetaldehyde; another enzyme then converts acetaldehyde to acetic acid. But about 50% of Asians and 5% of Europeans have mutations in these enzymes that can increase the rate of alcohol metabolism up to 100-fold. This leads to a rapid accumulation of acetaldehyde, which can cause capillaries in the face to dilate—--and the face to turn red. Other unpleasant symptoms can include nausea and headaches. In 2008, a team led by geneticist Kenneth Kidd of Yale University found that one of these mutations--known as ADH1B*47His--may have been favored by natural selection in many East Asian populations.
A team led by Bing Su, a geneticist at the Kunming Institute of Zoology in China, set out to find the source of this selection. The researchers searched for the ADH1B*47His mutation in 2275 people across China representing 38 ethnic groups. They found that it was highly prevalent, up to 99%, in ethnic groups from southeast China; a bit less prevalent, 60% to 70%, in western China; and relatively uncommon, 14%, among Tibetans. Moreover, the team found a strong geographical correlation between regions with a high prevalence of the mutation and archaeological sites in China where rice had been domesticated thousands of years ago.
When Su and his colleagues calculated the age of the mutation, it came out at between 7000 and 10,000 years ago. That corresponds roughly to the earliest known evidence for rice farming, the team reports online this week in BMC Evolutionary Biology. "The [mutation] rose to extremely high frequency in a relatively short time, implying that the selective force was quite strong," Su says.
As for what the selective pressure was, the team concludes that the mutation was favored because it protected early farmers from the potentially fatal harms of drinking too much. The researchers cite two additional pieces of evidence for this hypothesis. First, recent archaeological evidence suggests that Chinese farmers concocted an alcoholic brew of rice, honey, and grape or hawthorn as early as 9000 years ago. Second, the drug disulfiram, which causes acetaldehyde to accumulate in the body, discourages alcoholics from drinking by causing nausea, vomiting, and other severe alcohol flush reaction symptoms.
"This study is the latest in a growing body of research showing just how important human culture has been as a transformational force in human evolution," says Darren Curnoe, an anthropologist at the University of New South Wales in Sydney, Australia. Indeed, the rise of farming has been linked to evolutionary changes in genes for other enzymes, such as amylase, which breaks down starch, and lactase, which breaks down lactose in milk.
Other researchers are not entirely convinced. Kidd says that the hypothesis is "quite reasonable" but that it's still speculation at this point. He also questions whether the team has determined the age of the mutation correctly, because the estimates range over at least 3000 years. And Dorian Fuller, an archaeologist at University College London, argues that the team may be wrong to pin the mutation's origins solely on rice cultivation. The archaeological sites the researchers chose, he says, included settlements where rice had just begun to be farmed and those where rice farming was in full flower. Fuller adds that if the team had restricted its analysis to those later sites where rice had become a predominant crop, beginning about 8000 years ago, then alcoholic beverages could also have been made from grapes--and rice might not be solely responsible for the Asian Flush.