The amylase enzyme studied by Dr. Perry’s team exists in the saliva, where it predigests starch and lets glucose get absorbed from the mouth into bloodstream. The evolutionary advantage of this strange arrangement is not clear, but it could provide the body with energy during episodes of diarrhea, or might protect against diarrhea. Or it could just make the digestion of starch more efficient.

Whatever the exact mechanism, the extra copies of the amylase gene seem to have arisen through positive selection, the researchers said. Their conclusion is based on comparing the genomes of the Japanese and the Yakut, a Siberian people who eat mostly reindeer. Dr. Perry, a geneticist, said he could not tell whether the Japanese, who have a high-starch diet, including rice, had gained the extra copies of the gene or whether the Yakut had lost theirs.

Geneticists realized only in 2004 that having extra copies of genes was a widespread form of variation in the human genome. Many of the extra copies seem to have arisen through mistakes in the duplication process that doubles the number of chromosomes in dividing cells. The effect of these extra copies is largely unknown and the story of the amylase gene is one of the first to be understood, at least to some degree.

Dr. Perry and his team started their research by having undergraduates at Arizona State University give samples of saliva, which were analyzed for amylase. The researchers found the amount of amylase a person produced was correlated with the number of copies they possessed of the amyloid gene, which ranged from 2 to 15. The copies are arranged in the genome like a string of beads, with each gene being about 120,000 units of DNA in length.

Wondering whether the copy number varied with diet, the researchers then collected saliva and blood samples from the Yakut and other low-starch eating populations, showing that this was indeed the case.