The report in Nature Genetics by University of California Santa Cruz authors says that as against primates, humans have many more copies of a specific gene crucial to the breakdown of calorie-rich starches. They speculate that these extra calories may have been essential for fuelling the larger brains of humans.
Experts have always pondered if meat in the diet was the answer to man's intelligence.Contradicts lead researcher Dr Nathaniel Dominy: "Even when you look at modern human hunter-gatherers, meat is a relatively small fraction of their diet.
"To think that, two to four million years ago, a small-brained, awkwardly bipedal animal could efficiently acquire meat, even by scavenging, just doesn't make a whole lot of sense."
The scientists found that humans carry extra copies of a gene, called AMY1, which is essential for making the salivary enzyme amylase that digests starch. When they studied groups of humans with differing diets, they discovered that those with high-starch diets tended to have more copies of AMY1 than individuals from populations with low-starch diets.
For example, the Yakut of the Arctic, whose traditional diet revolves around fish, had fewer copies than the related Japanese, whose diet includes starchy foods like rice.
These researchers believe that early human ancestors began searching for new food sources other than the ripe fruits primates eat.These were starches, stored by plants in the form of underground tubers and bulbs - wild versions of modern-day foods like carrots, potatoes, and onions.
Coincidentally, in earlier work this year, the team found that animals eating tubers and bulbs produced body tissues with a chemical structure akin to what has been measured in early fossilized humans.
Dominy opines that when early humans mastered fire, cooking starchy vegetables would have made them even easier to eat. It would also have made extra amylase gene copies an even more valuable trait, he says.
"When you cook, you can afford to eat less overall, because the food is easier to digest. Marginal food resources can become part of the staple diet.
"Now you can have population growth and expand into new territories", Dominy postulates.
Yet, Professor John Duprť, a professor of philosophy of science at Exeter University in the UK, urges caution when interpreting the findings.
According to Duprť, it is impossible to conclude that the introduction of starchy foods into the diet lies behind the emergence of larger brains in humans.
"Lots of things differ between ourselves and our closest relatives and apart from the difficulty of establishing the relative places in the evolutionary sequence of any of these, the assumption that there is any one fundamental to such change is dubious.
"The results on amylase genes are quite interesting, and a good indication of something we are beginning to appreciate more widely - the functional plasticity of the genome", he concedes.