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A Little Gene Xeroxing Goes a Long Way

Science  14 Sep 2007:
Vol. 317, Issue 5844, pp. 1483a
DOI: 10.1126/science.317.5844.1483a
Extra enzymes.

Hadza diet drives gene copying.

CREDIT: F. MARLOWE/FLORIDA STATE UNIVERSITY

Researchers studying the evolution of starch digestion have uncovered evidence of a surprising adaptation: Rather than relying on mutations in a particular gene to help us digest roots and tubers better, the human genome simply made more copies of the gene in question. The finding is one of the strongest examples yet of evolution affecting gene copy number in humans and sheds light on how our diet split us apart from other primates.

An enzyme called salivary amylase—encoded by the AMY1 gene—helps humans digest starchy food. In a typical evolutionary scenario, natural selection would favor random mutations in AMY1 that caused it to churn out more of the enzyme or a more effective version of it in people who ate a high-starch diet.

But a study published online 9 September in Nature Genetics contends that something else happened. Nathaniel Dominy, an evolutionary anthropologist at the University of California, Santa Cruz, and George Perry at Arizona State University in Tempe analyzed AMY1 in high-starch eaters such as Americans of European descent, Japanese, and Hadza from Tanzania, hunter-gatherers who eat many roots and tubers, as well as groups that eat little starch, such as the Biaka of the Central African Republic and the Mbuti from Congo, both rainforest hunter-gatherers, and Tanzania's Datog and Siberia's Yakut pastoralists. In all, the researchers studied samples from more than 200 people.

The team found that rather than having mutations that boosted AMY1's activity, the high-starch eaters had extra copies of the gene. On average, the high-starch eaters had seven copies of the gene, whereas the low-starch populations had only five. “If you have a gene that's working well, why not just copy it over and over again?” asks Dominy. “Why wait for evolution to just roll the dice?”

For a broader evolutionary perspective, the researchers looked at 15 chimpanzees, which eat little starch. All had only two copies of AMY1. And an analysis of the gene from bonobos, the chimp's closest relative, found that it had mutations that may prevent AMY1 from functioning altogether. “I was very excited to see this,” says Gregory Laden, a biological anthropologist at the University of Minnesota, Twin Cities, who contends that eating starch-rich roots and tubers played a key role in differentiating humans from other apes.

Ajit Varki, who studies human origins at the University of California, San Diego, says the report also suggests that humans may have had access to starchy foods before the advent of agriculture, as is commonly thought. Even populations with low-starch diets had extra AMY1 copies, he notes: “This would imply that first there were some rounds of duplication of the gene in preagricultural humans, and then that went further in agricultural humans.”

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