Spit might have helped human evolution by enabling our ancestors to harvest more energy from starch than their primate cousins. Compared with chimpanzees, humans boast many more copies of the gene that makes salivary amylase — a saliva enzyme that breaks down starch into digestible sugars. And carbohydrate-loving societies carry more copies of the gene than those that follow low-carbohydrate diets, claims a new study in Nature Genetics.This strongly implies that people have adapted to their local environment. “High starch foods and a high starch diet have been an important evolutionary force for humans,” says George Perry, an anthropologist at Arizona State University in Tempe, who led the new analysis. The change could possibly have supported the growth in hominid brains that occurred some two million years ago, says Nate Dominy, an anthropologist at the University of California in Santa Cruz involved in the study. “Our diet must have had some shift to feed that brain,” says Dominy, who thinks root vegetables like African tubers allowed large-brained humans to flourish.
Starch, which helps to make a baked potato mushy, is an important source of food for modern humans. But without amylase in the saliva, man can make little use of such complex carbohydrates – enzymes elsewhere in the body are not as good at breaking the compounds down. Previous studies suggested that some people have more copies of the gene for amylase than others, but little was known about the importance of the extra copies. They could have been insignificant: the duplication of some genes has little or no effect on gene expression, says Dominy. To investigate, the team tested people with different numbers of amylase genes. “We took a population of undergraduates and asked them to spit into tubes, then measured the amount of amylase in their saliva,” says Dominy. Cheek swabs were used to measure the number of amylase genes. The conclusion: extra copies of the gene make more amylase — and so an enhanced ability to break down starches. When the researchers ventured beyond university campuses to sample populations in Africa, Asia, Europe and the Arctic, they noticed a trend. Cultures with diets that included high levels of starch tended to have more copies of the amylase gene than cultures that consumed few starches.
Starch-loving cultures such as the Hadza of Tanzania who rely heavily on tubers and other root vegetables, have 6.7 copies of amylase, on average. While people like the Mbuti, pygmy rain forest hunter-gatherers from central Africa who eat little starch, have 5.4 copies on average. In contrast, chimpanzees, dining on fruit and little else, have just two copies of the salivary amylase gene. Comparing the human and chimp genomes hints that the multiplication of this gene in humans came hundreds of thousands of years ago, or more. Dominy speculates that perhaps the change propelled our ancestors to new heights by fuelling the evolution of large brains more than two million years ago. Alternatively, the new copies may have coincided with the rise of agriculture 150,000 years ago, he says. More complete human genome sequences from diverse cultures are needed to firm up when and why this change took place, Dominy says.
The ability to digest starch may have had the added benefit of cutting down on diarrhoea — still a major cause of death in children. “It might pay to start digesting things a bit earlier in the process to get what you can before it’s shot out of your body,” says Dominy. Such studies, linking human evolution to genetic changes, are certain to become more common, says James Sikela, a biologist at the University of Colorado Health Sciences Center in Aurora. “It’s a great example of what can be learned about our past via evolutionary genomics,” he says.
September 17, 2007
Original web page at Nature