When it comes to ways to burn calories, few people think of chewing. But about 3% of the daily energy we burn comes from grinding gum, gristle, and other goodies, a new study finds—and maybe more if you’re partial to salads and celery stalks. That’s far less than walking or even digesting, but it may have been enough to reshape the faces of our distant ancestors.
The study adds concrete data to the debate over the reasons human jaws are so different from those of our distant ancestors and modern primates, says Callum Ross, an anatomist at the University of Chicago who was not part of the study. “This gives us a number we can start working with.”
Scientists have long suspected our jaw size and tooth shape evolved to make chewing more efficient. As our hominid ancestors shifted their diet to easier to chew foods and developed technologies such as chopping and cooking to reduce the time and effort spent chewing, jaw and tooth shape changed, too, shrinking compared with other primates. But without knowing how much daily energy we expend on chewing, it’s difficult to determine whether saving energy was also a factor in driving these evolutionary changes, says Adam van Casteren, a biological anthropologist at the University of Manchester.
So in the new study, van Casteren and his colleagues put 21 men and women in a bubblelike helmet. The device measured the amount of oxygen they consumed and carbon dioxide (CO2) they exhaled. Then, the scientists gave the participants a flavorless, odorless, calorie-free gum to chew for 15 minutes.
While chewing, CO2 levels in volunteers’ breath rose, indicating their bodies were working harder. (Because the gum had no odor, taste, or calories, it didn’t trigger the digestive system, which also consumes energy.) When the gum was soft, the metabolism of the volunteers rose an average of 10%; stiffer gum required 15% more energy than resting. “It’s not huge, but it’s still significant,” says study co-author Amanda Henry, an archaeologist at Leiden University.
Overall, chewing the gum represented less than 1% of participants’ daily energy budgets, the team concludes today in Science Advances. But chewing gum in a lab was essentially a proof of concept: Before the advent of cooking and tool use, early humans likely spent a lot more time chewing. If ancient people had spent as much time chewing gum as gorillas and orangutans do, the authors estimate they could have used up at least 2.5% of their energy budget chewing. “If you were eating harder foods and chewing longer, you’d end up with a far larger proportion of the total energy expenditure,” Henry says.
The findings came as a surprise. Henry says even some of her collaborators were skeptical that the energy required to chew would be enough to measure in the lab. “I think it’s a great study. It shows there’s a measurable amount of energy used,” Ross says.
The finding supports the idea that more efficient chewing, tailored to diet, might have been an evolutionary advantage, Henry says. “By saving energy in the chewing category, you have more energy to spend on other things, like rest, recovery, and growth.”
Calculating the energy cost of human chewing could give a glimpse into the evolutionary strategies of other hominids, too. For example, Australopithecus—a hominid that lived in Africa between 2 million and 4 million years ago—had teeth with chewing surfaces four times larger than modern humans and massive jaw muscles. They must have spent more energy on chewing, and the new study is a first step to calculating how much. “They were presumably … taking advantage of very energetically costly food,” Henry says. “We have the first piece of evidence to explain that pattern.”
Still, Ross isn’t convinced energetics alone can explain the way jaws and teeth evolved over time. Other factors—such as jaw shape that minimized tooth breakage or wear, for example—might have been more important. “Natural selection probably cares more about not wearing your teeth out than energy efficiency,” he says; an animal with no teeth at all would run out of energy quickly.
Compared with Australopithecus or primates living today, humans are an outlier: Some estimates suggest we spend just 7 minutes a day chewing. By contrast, mountain gorillas can spend up to 90% of their waking time chewing, on par with ruminants such as goats and cows. “Modern humans are the weird ones. We have really soft foods and low chewing times,” van Casteren says. “Reducing the amount of energy you’re spending on chewing is another element to these milestones in human evolution, or in agriculture, where you’re selecting foods that are less fibrous or chewy.”
