The most tedious part of a chimpanzee’s life is chewing. Our primate cousins spend 6 hours a day gnashing fruits and the occasional monkey carcass—all made possible by the same type of big teeth and large jaws our early ancestors had. So why are our own teeth and jaws so much smaller? A new study credits the advent of simple stone tools to slice meat and pound root vegetables, which could have dramatically reduced the time and force needed to chew, thus allowing our more immediate ancestors to evolve the physical features required for speech.

The reason modern humans are able to spend so little time chewing is that “we eat a much higher quality diet than our ancestors,” says Daniel Lieberman, an evolutionary anthropologist at Harvard University and an author of the new study. Whereas chimpanzees survive mostly on fruit, humans eat foods that pack more nutrients and energy into smaller portion sizes—namely, meat. (Chimps eat some meat, too, mostly in form of hunted monkeys, but it’s not a dietary staple in the way it is for humans, including hunter-gatherers.) Today, cooking helps us make meat easier to eat and digest, but Lieberman thinks our ancestors started eating meat long before they learned how to roast it. There’s evidence that our early ancestors—upright apes called hominins—were regularly eating meat as far back as 2.5 million years ago, but cooking doesn’t seem to become common until 500,000 years ago, Lieberman says. “What did humans do before they regularly had access to cooking?” he wondered.

The answer might have come in the form of another innovation that appeared around the same time hominins adopted meat-eating: stone tools. Lieberman teamed up with fellow Harvard evolutionary anthropologist Katherine Zink to investigate how those tools could have changed the kinds of foods hominins could eat. “We chose the simplest forms of processing that we could think of,” Zink explains. For root vegetables like yams and beets, that meant pounding them into a paste, and for meat, it was slicing. Then Zink glued electrodes all over the faces of a few dozen volunteers and tested the amount of time and force required to chew meat and veggies that were either raw and unprocessed, raw and processed with simple slicing or pounding, or cooked.

The first thing they discovered was that raw meat is almost impossible to eat if you have human, or even chimpanzeelike, teeth. Modern cows are bred to have soft flesh, so Zink and Lieberman fed their volunteers goat, whose tough meat more accurately mimics that of the wild game early hominins would have eaten. “Eating raw goat is not pleasant,” says Lieberman, who tried out the methods himself. “You chew and you chew and you chew and you chew and nothing happens.” Human teeth simply can’t break up the flesh into smaller pieces we can swallow. “It’s almost like chewing gum,” Zink says. Chimps’ teeth are similarly bad at chewing meat. For our early ancestors, with their chimplike teeth and mouths, eating meat was probably a similarly time- and energy-consuming ordeal.

Slicing, whether with a knife or a sharp stone flake, changes all that. Suddenly, hominins could cut up the elastic muscles of a carcass into smaller bits before putting them in their mouths, making them chewable and easier to digest. Pounding has a similar effect on tough, fibrous root vegetables. “What we found is that by simply slicing meat and pounding vegetables, a hominin would be able to reduce the number of chews they use by about 17%,” Zink says. “That equates to 2-and-a-half million fewer chews per year.”

Zink and Lieberman, who publish their findings today in Nature, believe that that reduction was more than enough to allow early members of our genus Homo to evolve smaller teeth and jaws. Once early humans didn’t have to spend so much of their lives chewing, big teeth and long jaws stopped providing an advantage—and natural selection could start favoring other traits instead. For example, a smaller snout freed up space for maneuverable lips, a key component in forming words, and also makes the head easier to balance while running, an important skill for hunting. “Simple food processing technologies had a really enormous benefit for the hominins 2-and-a-half million years ago who invented them,” Lieberman says.

The study challenges a competing hypothesis that it was cooking that drove the changes in tooth and jaw size. Zink and Lieberman cite archaeological evidence showing that cooking first appeared 1 million years ago and was widespread by about 500,000 years ago. But just because archaeologists haven’t found evidence of cooking and fire pits before that doesn’t mean that early hominins weren’t using them, says Richard Wranghman, a biological anthropologist also at Harvard and leading proponent of what’s known as the cooking hypothesis. “I cannot see how the diets that [Zink and Lieberman] are proposing can explain the combination of smaller chewing [features] and, very importantly, the smaller gut” seen in early members of Homo starting about 1.9 million years ago, he says. “For me the big problem is you need a big gut to be able to ferment the raw plant foods that these animals would have been eating.”

Lieberman and Zink say they don’t discount cooking, but instead propose a two-step process: Pounding and slicing provided an initial evolutionary boost to smaller teeth, jaws, and guts; and cooking finished the job later. One thing’s for sure: Modern humans cannot survive on raw goat alone, however finely you slice it.