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Did a Starchy Diet Fuel Gene Copying in Early Humans?
11 Dec, 2007 10:45 am
Biologists usually think of gene mutations as the raw material for evolution. But a wide-ranging analysis of a correlation between genes and diet across many different cultures has unveiled a second starting point for natural selection: extra, unmutated copies of an existing gene. A particular set of gene copies helped early humans thrive on starchy foods that other primates ignored, possibly explaining the energy source for the growing brains of our ancestors.
To figure out what the extra gene copies were doing, Dominy’s team compared dietary data, saliva samples, and DNA from chimpanzees and people. Initial tests showed American college students had anywhere from 2 to 15 copies of the salivary amylase gene, and that students with more gene copies had more of the amylase enzyme in their saliva. The team then looked to other populations for a link between gene copies and diet.
They studied seven populations with low-starch or high-starch diets. Groups with low-starch diets – an unusual pattern for most modern humans – were two African hunter-gatherer tribes and a tribe of African pastoralists, all with meat-heavy diets, as well as fish-eating Yakut people of the Asian Arctic. The high-starch groups were Japanese people and European-Americans, eaters of starchy rice and bread, and an African tribe that eats starchy plant parts such as wild yams. The researchers counted copies of the salivary amylase gene in the subjects’ DNA.
The results showed that chimpanzees, which subsist mostly on fruit, have only 2 copies of the gene and low enzyme levels. In contrast, most humans have several additional gene copies. If a population eats more starch, its members have more copies of the gene, on average. For instance, individuals with at least 6 gene copies were more than twice as common in starch-eating groups compared to groups with low-starch diets.
This strong link between gene copy number and diet is noteworthy for two reasons. First, it shows a new route for evolution: Instead of waiting for a beneficial mutation to pop up, evolution can favor duplicating existing genes with useful functions. Extra gene copies can ramp up the gene’s function in the body.
Second, the study could explain how humans grew such big brains. Brain tissue is the body’s biggest gas guzzler. Becoming the brainiest primate, as our ancestors did, required a reliable supply of sugar. Some scientists have doubted whether meat alone could have fueled brain growth, since early humans probably ate meat sporadically. With efficient methods for digesting starchy plant parts hidden underground, pre-humans tapped an unused energy source with an abundant supply.
Not only were the new foods readily available, they had the advantage of being less fibrous than fruits. Reducing dietary fiber seems backwards now. For modern humans, it’s a struggle to eat roughage and “stay regular.” But our ancestors had the opposite problem: they ate so much indigestible plant matter that their bodies had to spend vast amounts of energy just grinding away at their food. The switch to starchy foods saved calories, let early humans grow shorter guts, and left more sugar to feed their brains.
Digesting more starch obviously was a good thing, but why did humans end up grinding starch in their mouths? The blood vessels in our cheeks can absorb a bit of sugar, but most nutrients cross to the bloodstream at the small intestine. Dominy’s team thinks the mouth became a more important place to pick up nutrients when food became scarce. By breaking down starch there, humans could eat marginal foods. Stringy plants like sugar cane became accessible foods because humans could extract nutrients by sucking on the plants without swallowing, and plants containing mild toxins were edible if humans could get calories out of them quickly, before the toxins caused a gut-emptying bout of diarrhea.
Starch is everywhere today, from Pop-Tarts to French fries. But in the Congo, tribes of African pygmies still dig up some of the original starchy tubers that apparently fed human evolution – wild yams as small as your thumb or nearly as big as a VW Beetle.
Perry GH, Dominy NJ, Claw KG, Lee AS, Fiegler H, Redon R, Werner J, Villanea FA, Mountain JL, Misra R, Carter NP, Lee C, Stone AC (2007). Diet and the evolution of human amylase gene copy number variation. Nature Genetics 39: 1256-1260.