Humans spend about 35 minutes each day chewing. This adds up to over an entire week each year. But that’s nothing compared to the time our cousins spend chewing: chimpanzees chew 4.5 hours a day, while orangutans spend 6.6 hours.
The differences between chewing habits and those of our closest relatives provide insights into human evolution. a A study published Wednesday in the journal Science Advances It explores how much energy people use while chewing, and how this may have informed – or informed – our gradual transformation into modern humans.
Chewing, in addition to protecting us from choking, also makes the energy and nutrients in food more accessible to the digestive system. But the act of chewing requires us to expend energy. The adaptations of teeth, jaws, and muscles play a role in how well humans chew.
Adam van Kastern, author of the new study and a research associate at the University of Manchester in England, says scientists haven’t delved much into the active costs of chewing in part because compared to other things we do, like walking or running, it’s a thin slice of the energy pie. But even relatively small advantages can play a big role in evolution, and he wanted to know if that was the case with chewing.
To measure the energy that goes into chewing, Dr. Van Castren and his colleagues outfitted study participants in the Netherlands with plastic wraps that looked like an “astronaut’s helmet,” he said. The caps were attached to tubes to measure oxygen and carbon dioxide from respiration. Since metabolic processes are fueled by oxygen and produce carbon dioxide, gas exchange can be a useful measure of how much energy something takes up. Then the researchers gave the subjects chewing gum.
However, participants did not get the type of glucocorticoid; The chewing gum bases were flavorless and odorless. Digestive organs respond to flavors and aromas, so the researchers wanted to make sure they were only measuring the energy associated with chewing and not the energy of the stomach preparing for a delicious meal.
The test subjects chewed two pieces of gum, one hard and one soft, for 15 minutes each. The results surprised the researchers. Soft chewing gum raised the participants’ metabolic rates 10 percent higher than when they were resting; Hard gum causes a 15 percent increase.
“I thought it wouldn’t be much of a difference,” said Dr. Van Kastern. “Very small changes in the material properties of the item you’re chewing on can cause very large increases in energy expenditure, and that opens up a whole world of questions.”
Since chewing harder food—or in this case, firmer gum—expends more energy, these findings suggest that the metabolic costs of chewing may have played an important role in our evolution. Making food easier to process through cooking, mashing food with tools and cultivating crops improved for eating may have reduced the evolutionary pressure for us to be super chewers. Our evolving chewing needs may have shaped the shape of our faces.
“One of the things we haven’t really been able to figure out is why the human skull is so funny,” said Justin Ledugar, a biological anthropologist at East Tennessee State University who was not involved in the study. Compared to our closest relatives, our facial skeletons are precisely built of relatively small jaws, teeth and masticatory muscles. “All of this reflects a reduction in reliance on vigorous chewing,” he explained.
But, he added, our flat faces and short jaws make us bite more efficiently. “It makes the entire feeding process less costly from a metabolic standpoint,” Dr. Ledugar said. Humans have evolved ways to chew more intelligently and not with greater difficulty. Dr. Van Kastern, who hopes to continue his research using actual foods, says he’s excited about the possibility of learning more about how humans evolved.
“To know the environmental, societal, and nutritional reasons why we got here, it is infinitely interesting to me,” he said, “because it enables humanity to “try to work on the foggy path of the future.”
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