Chimpanzees, humans' closest relatives, spend 50 percent of their waking life chewing their food. Humans spend much less time and devote much less energy chewing.

Researchers at Harvard believe those energy savings were key to the development of early man's cognitive abilities.

Between 2 and 3 million years ago, humans began eating slightly less meat -- meat made up about 33 percent of their diet -- and processing both meat and vegetables prior to consumption. According to a new study published this week in the journal Nature, these behavioral adaptations enabled humans to absorb caloric energy with less effort.

As a result, researchers say, humans evolved a smaller jaw, smaller teeth, smaller intestines and rerouted their surplus of energy and evolutionary capital to the growth of the brain.

"Eating meat and using stone tools to process food apparently made possible key reductions in the jaws, teeth and chewing muscles that occurred during human evolution," study author Katie Zink, an evolutionary biologist at Harvard, said in a news release.

Chewing is a technique employed by all mammals. But only humans have perfected the art. Animals like cows and pandas that rely on limited and low quality diets have to spend much of their time chewing.

Humans are different. To see how different, Zink had volunteers come to the lab and chew various foods, including raw meat and whole vegetables, as well as processed and cooked foods.

After measuring ergonomic forces and energy expenditures, Zink and her research partners determined humans with processed food -- empowered by tools and fire -- were able to reduce the amount of chewing and energy expended while chewing by 20 percent.

"What we showed is that ... by processing food, especially meat, before eating it, humans not only decrease the effort needed to chew it, but also chew it much more effectively," Zink said.

"Those changes, and others, allowed for selection for speech and other shifts in the head, like bigger brains," added co-author Daniel Lieberman, a professor of biological sciences at Harvard. "Underlying that, to some extent, is the simplest technology of all: slicing meat into smaller pieces, and pounding vegetables before you chew them."