Slicing up meat and pounding vegetables before eating them provided early humans the push they needed to evolve into the species we are today, Harvard scientists have revealed. By looking at changes to diets two to three million years ago, researchers have found processing food before eating it allowed our ancestors to get more energy from food with less effort – a move that would lead to the development of larger brains, smaller faces and teeth and bigger bodies.

Published in the journal Nature, the team was looking at the paradox of how Homo erectus came to have smaller teeth, faces and guts, yet managed to get enough energy from food to feed their larger bodies and bigger brains – and all before cooking was invented 500,000 years ago.

The team looked at energy consumption and expenditure of these early humans by getting people to chew up raw meat and vegetables (like our ancestors would have), measuring how much effort was involved. Food was prepared in different ways, including raw, sliced, pounded and cooked. When they had chewed to the point of swallowing, the participants spat it out, so it could be analysed by the scientists.

"What we found was that humans cannot eat raw meat effectively with their low-crested teeth. When you give people raw goat, they chew and chew and chew, and most of the goat is still one big clump – it's like chewing gum," Daniel Lieberman, one of the study authors said.

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Meat is high energy, but it is very hard for us to chew without processing it. Taryn/Flickr

They found that by slicing meat and pounding vegetables before eating it, the number of chews required was reduced by almost 20%. This saved early humans as much as 2.5 million chews per year.

Katie Zink, the first author on the study, said: "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." By eating higher-quality food (meat) and spending less time chewing, early humans opened up the possibility of hunting and gathering, which would have a profound impact on mankind.

"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 snout-less faces," Lieberman said. "Those changes, and others, allowed for selection for speech and other shifts in the head, like bigger brains. Underlying that, to some extent, is the simplest technology of all: slicing meat into smaller pieces, and pounding vegetables before you chew them."