Man's ability to digest starchy foods like the potato may explain our success on the planet, genetic work suggests.
The average Brit eats 500 medium-sized potatoes each year
Compared with primates, humans have many more copies of a gene essential for breaking down calorie-rich starches, Nature Genetics reports.
And these extra calories may have been crucial for feeding the larger brains of humans, speculate the University of California Santa Cruz authors.
Previously, experts had wondered if meat in the diet was the answer.
However, Dr Nathaniel Dominy and colleagues argue this is improbable.
"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."
They discovered humans carry extra copies of a gene, called AMY1, which is essential for making the salivary enzyme amylase that digests starch.
Next the team studied groups of humans with differing diets and found 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 centres around fish, had fewer copies than the related Japanese, whose diet includes starchy foods like rice.
The researchers believe our earliest human ancestors began searching for new food sources other than the ripe fruits that primates eat.
Man has a larger skull capacity than other primates
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.
In work earlier this year, the team found that animals eating tubers and bulbs produce body tissues with a chemical signature that matches what has been measured in early fossilised humans.
Dr Dominy said that when early humans mastered fire, cooking starchy vegetables would have made them even easier to eat.
At the same time it would have made extra amylase gene copies an even more valuable trait.
"We roast tubers, and we eat French fries and baked potatoes. When you cook, you can afford to eat less overall, because the food is easier to digest."
And marginal food resources can become part of the staple diet.
"Now you can have population growth and expand into new territories."
Professor John Dupré, a professor of philosophy of science at Exeter University in the UK, urged caution when interpreting the findings.
He said it was impossible to conclude that the introduction of starchy foods into the diet lay 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."