Firing up the grill can invoke feelings of kinship with our early human ancestors. But while a growing appetite for meat probably did shape our evolution, some of the biggest changes may have happened when our ancient relatives developed a taste for tartar.

After measuring chewing and biting in modern humans, scientists found that a diet that includes one-third raw meat requires far less chewing and bite force exertion than meals of tubers alone. The researchers suggest that with the advent of stone tools, ancient human relatives were able to tenderize their food and make it far easier to chew and digest.

“An important step was just using a simple stone tool to cut our meat and bash our vegetables,” says Harvard University evolutionary biologist Daniel Lieberman.

“Chewing is something we take for granted—we don't do it all that often and we don't think about it much," he says. "But if you're a chimpanzee, you spend half the day chewing. If you're an australopith ancestor of Homo, you probably spend half the day chewing. And later we went through some amazing transitions in our evolutionary history where we now chew so little that we barely think about it at all.”

By the time Homo erectus appeared on our family tree some 2 million years ago, humans were boasting bigger brains than their ancestors, as well as bigger bodies that required a more substantial caloric payload. But H. erectus also had smaller teeth, weaker chewing muscles and more feeble bite force than earlier humans—less than half that of australopiths—and a smaller gut to boot.

These developments seem to be at odds. A diet of more calorie-rich meat could help explain things, but consuming it regularly would have presented some challenges.

“If I gave you a piece of raw goat, you would just chew and chew it, like a piece of bubble gum,” Lieberman explains. “Human teeth don't have the kind of shearing ability that, say, dogs' teeth have, and that is necessary to break down meat. With human chewing it just stays in a clump, and studies have shown how that makes digestion far less efficient.”

Cooking makes it easier to chew meat, but evidence suggests that the regular use of fire for cooking didn't pop up until perhaps half a million years ago—far later than the changes to H. erectus. Also, evidence from archaeological and paleontological research points to a spike in human meat consumption by at least 2.6 million years ago.

However, we have plenty of evidence that hominins had begun making stone tools some 3.3 million years ago. Those tools could have been used as pounders to tenderize foods, a practice seen in modern chimps. Flaked tools can also slice foods into easily chewable pieces or remove skin, cartilage and other bits that are harder to chew.

“It's not a coincidence that the oldest evidence for eating meat shows up around the same time as tools,” Lieberman says. “We know that the evolution of meat-eating basically required stone tools. And that had a huge effect on our biology.”

Using experimental biomechanics (and iron-stomached volunteers), Lieberman and co-author Katherine Zink set out to see just how mashing up food might have influenced our evolution.

They fed a total of 34 adults samples of raw goat meat, as well as starch-rich tubers including jewel yams, carrots and beetroots. As each person chowed down, the scientists measured how much muscular effort they exerted when chewing and how well each chewer broke up the food before they wanted to swallow—but then had the volunteers spit out their masticated meals.

The team looked at a ratio of one-third meat to veggies based on the diets of modern African foraging peoples. They found that even with unprocessed meat, chewing was reduced by 13 percent compared to a diet of just root vegetables. Also, chewers required 15 percent less bite force to nosh on a mixed diet compared to a vegetarian one.

Tool use made those gains even larger. When the meat was sliced and the plants pounded by stone tools, participants had to chew 17 percent less than with unprocessed food items and could exert 26 percent less bite force. People's ability to chew meat into smaller particles was 41 percent greater.

The scientists were “sort of astonished at how dramatic the results were,” Lieberman notes.

“When I see a study like this that tends to corroborate an aspect of evolution, in this case biological, and combine it with some of the behavioral aspects of meat eating, like pretty direct evidence of cut marks on bones from 3.5 million years ago, it kind of fills in a gap,” says paleoanthropologist Henry Bunn at the University of Wisconsin-Madison, who was unaffiliated with the study

“For years people have said, well, there's a package of biological adaptations that relate to a marked change in diet. Larger brains, larger body size, smaller teeth and a smaller gut all point in the same direction—more meat and better means to obtain it,” Bunn adds.

“Nobody is arguing that hominins were eating all meat and nothing else—it's not that sort of extreme. It's more a matter of a long view of the last 5 million years of hominin evolution," he adds. "Some apes kept being apes, and some evolved into us. When you ask what changed, one of the pretty clear answers is an interest in meat and the invention of the tools to butcher it.”

The authors suggest that gains in chewing efficiency may have allowed for subsequent selections over the course of human evolution, like smaller teeth, jaws and faces. In turn, they speculate, less emphasis on robust teeth and jaws could have helped enable boosts in other functions that shaped modern humans, like speech production or even brain size.

“All else being equal, anything that actually saves an organism calories, permitting them to gain more than they spend, is something natural selection should be intensely interested in,” says biological anthropologist Katharine Milton at the University of California, Berkeley.

But she cautions that “while it's fun to speculate, I am not sure that quantifying the energetics of chewing beet root versus goat meat in itself sheds too much light on the energetics of evolving humans.”

Milton notes that the study wasn't able to address many factors in the equation of ancient diet. For example, while consuming unprocessed meat may require less effort than tubers, the ancient menu likely wasn't limited to those items.

“The data we have from recent or extant foraging peoples suggest they often use wild plant foods such as mongongos [a type of calorie-rich nut], bush tomatoes, grass seeds, palm fruits, piñon nuts and other richer food sources—not tubers—as their primary source of daily calories,” she says.

“So in a way, this paper might support the view that tubers as a class may only have become of key importance in the diet of evolving humans after they could routinely depend on meat for meeting their protein needs.”

Either way, Lieberman argues that the study builds a case for the way a change in eating and chewing behavior affected our shift from hunting and gathering to haute cuisine.

“Until at most 600 generations ago, everybody's ancestor was a hunter and gatherer. Part of that system is hunting, part of it is foraging and digging up tubers, and part of it is cooperation and sharing among individuals. But it also can't work without food processing," he says.

"That whole constellation of behaviors shows up around 2.5 million years ago, and it's important to note that food processing is a fundamental part of helping our ancestors become who we are today. It's one of the many things that helped to make us human.”