Humans spend about 35 minutes a day chewing. That’s more than a full week each year. But it’s nothing compared to our “cousins”: chimpanzees chew for 4.5 hours a day, and orangutans 6.6 hours.
The differences between our chewing habits and those of our closest relatives offer insights into human evolution. A study published Wednesday in the journal Science Advances explores how much energy people use when chewing and how that may have guided our gradual transformation into modern humans.
In addition to preventing choking, chewing makes energy and nutrients from food accessible to the digestive system. But the very act of chewing requires an expenditure of energy. Adaptations to teeth, jaws and muscles play a role in how efficiently humans chew.
Adam van Casteren, lead author of the new study and a researcher at the University of Manchester in England, said that until now scientists have not delved into the energy costs of chewing in part because, when compared to other things we do, like walking or running, chewing is consuming. just a thin slice of the energy use pie. But even comparatively small advantages can play a big role in evolution, and Casteren wanted to find out if that might be the case for chewing.
To measure the energy expended on chewing, Casteren and his colleagues fitted study participants with plastic hoods, which he said resembled an astronaut’s helmet. The hoods were connected to tubes to measure the oxygen and carbon dioxide of the breath. Because metabolic processes are fueled by oxygen and produce carbon dioxide, gas exchange can be a useful measure of how much energy is spent on an activity. And then the researchers gave the participants gum.
But not sweet gum: the gum bases they mask had no flavor or aroma. Digestive systems react to tastes and odors; the researchers wanted to be sure they were measuring only the energy associated with chewing, not the energy of a stomach preparing to receive a tasty meal.
Participants mask two pieces of gum, one hard and one soft, for 15 minutes each. The results surprised the scientists. The softer gum raised the participants’ metabolic rate 10% higher than when they were at rest; the harder gum pushed the index up 15%.
“I didn’t think there would be such a big difference,” Casteren said. “Very small changes in the material properties of the object you are chewing can cause substantial increases in energy expenditure, and that opens up a whole universe of questions.”
Since chewing harder foods — in the case of the study, harder gum — uses significantly more energy, these findings suggest that the metabolic cost of chewing may have played an important role in our evolution.
When food became easier for humans to process, by being cooked or mashed with tools, in addition to growing plants optimized for food, it may have reduced the pressure on humans to be super-chewers. The evolution of our chewing needs may have even shaped the appearance of the human face.
“One thing we still can’t really understand is why the human skull looks so bizarre,” commented bioanthropologist Justin Ledogar of East Tennessee State University, who did not contribute to the study. Compared to our closest relatives, our facial skeleton is delicate, with relatively small jaws, teeth and masticatory muscles. “All of this reflects less reliance on strong chewing,” he said.
But Ledogar added that our flatter face and shorter jaws allow us to bite more efficiently.
“This reduces the metabolic cost of the whole feeding process,” he explained. Humans have evolved ways to chew more efficiently, but not harder. Casteren, who hopes to continue his study using real foods, says he’s excited at the prospect of finding out more about how humans evolved.
“For me, knowing about the environmental, social and food causes that led us to get here is infinitely interesting,” he said, because it allows humanity “to try to unravel what the murky path ahead will look like.”
