Early Homo evolved bigger brains and larger bodies than our more ancient ancestors, yet members of our genus also had smaller jaws and a weaker bite force. According to a new Nature study, using basic stone tools to slice raw meat allowed our Homo ancestors to develop smaller features related to chewing. These shorter faces and smaller jaws may have led to further changes, such as improved speech.
By about 2 million years ago, Homo erectus were sporting big brains and big bodies that, along with larger foraging ranges, increased their daily energetic requirements. But in addition to their smaller, more characteristically human facial features, they also had smaller guts. This seemingly paradoxical combination of increased energy demands with decreased chewing and digestive capacities may have been made possible by introducing more processed meat into the diet – but cooking wasn’t common until just 500,000 years ago.
To study the chewing performance of modern-day humans, Harvard’s Katherine Zink and Daniel Lieberman fed dozens of adult subjects goat meat and starch-rich underground storage organs like jewel yams, carrots, and red beetroots. They measured the muscular effort required for chewing by using instruments attached to the jaw, and they also examined how well the food was broken up before it was swallowed. "What Katie did was creative but sometimes, frankly, a little stomach-churning," Lieberman says in a statement. "Not only did she have people come into the lab, chew raw meat and other foods, and spit them out, but then she had to analyze the stuff."
By consuming a diet that’s one-third meat, and by using stone tools to slice up raw meat and pound plant material before eating, early Homo would have needed to chew food 17 percent less often and 26 percent less forcefully. This would have saved them 2 million chews a year.
"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," Lieberman adds. "But once you start processing it mechanically, even just slicing it, the effects on chewing performance are dramatic."
Not to mention, a diet that’s one-third meat would have enabled early Homo to grow their costly big brains and large bodies. Unlike most other animals that swallow their food whole, mammals chew their food into smaller pieces that allow digestive enzymes to break them down efficiently. But most mammals survive on relatively low-quality diets (of grass, for example), so they have to chew all day to extract enough energy. Even our closest relatives, the chimps, spend half their day chewing – and forcefully too.
Over evolutionary time, the smaller chewing-related bones and muscles of our ancestors may have made it possible for other functions related to speech production, locomotion, thermoregulation, or even changes in the size and shape of the brain.