Cooking before eating may have helped humans evolve

Pre-processing meat before eating helped our ancestors 2 to 3 million years ago spend far less effort chewing their food, which helped humans to evolve to have larger brains and smaller jaws, according to a new Harvard study.

The researchers estimate that such a diet would have saved early humans as many as 2.5 million chews per year, and made possible further changes that helped make us human.

One of the biggest puzzles in human evolution is how species such as Homo erectus evolved smaller teeth, smaller faces, and smaller guts, and yet managed to get more energy from food to pay for their bigger brains and bodies before cooking was invented.

"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" said first author Katie Zink, a lecturer at Harvard University in US.

By changing their diets to include just 33 per cent meat, and processing their food - slicing meat and pounding vegetables - before eating, researchers found that the muscular effort required per chew and the number of chews required per day was reduced by almost 20 per cent.

They also found that by simply slicing meat with the sorts of simple tools available more than 2 million years ago, humans were able to swallow smaller, more easily digestible pieces than would have been possible without using tools.

"What we found was that humans cannot eat raw meat effectively with their low-crested teeth," said Daniel Lieberman, professor at Harvard. "But once you start processing it mechanically, even just slicing it, the effects on chewing performance are dramatic," said Lieberman.

"The evolution of the ability to chew food into smaller particles gave mammals a big boost of extra energy because smaller particles have a higher surface area to volume ratio, allowing digestive enzymes to then break food down more efficiently," Lieberman said.

"Most mammals, however, eat a relatively low-quality diet - think of cows eating grass and hay - that they need to spend most of the day chewing," he said.

Humans' closest ape relatives, with a diet that consists mainly of fruit, must spend nearly half their day chewing to extract enough energy from their food, Lieberman said.

Pre-processing, and the reductions in chewing effort that came with it may have opened the door to one of the most important lifestyle changes in human evolution - the emergence of hunting and gathering, researchers said.

"With the origin of the genus Homo, we went from having snouts and big teeth and large chewing muscles to having smaller teeth, smaller chewing muscles, and snoutless faces," Lieberman said.

"Those changes, and others, allowed for selection for speech and other shifts in the head, like bigger brains," he said. The study was published in the journal Nature.