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How Chewing Gave Humans Flat Faces, Little Teeth and Wimpy Jaws

It doesn’t seem like juice diets and Soylent are going to improve matters.

Our soft, processed foods have played a significant role in the development of our jaw bones (Photo: Joshua Rappeneker/CC BY-SA 2.0)

For a couple years as a Harvard graduate student, Katherine Zink spent a lot of time watching people chew. She would patiently wait while people chomped on crisp vegetables and gummy raw goat meat. Right before they would normally swallow, each person would spit out the food into test tubes for Zink to meticulously splay out for scanning and measuring.

Picking through regurgitated food particles would be difficult for anyone with a sensitive stomach, but queasiness was magnified for Zink who had been six months pregnant at the time.

“Nothing says, ‘Let’s get this experiment over with,’ than I’m about to give birth to twins,” recalls Zink, who is now a professor and functional morphologist at Harvard. But her days of nausea paid off: The study, published in the journal Nature this past March, illuminated key information about how chewing foods that have been just simply pounded and sliced with rudimentary tools could have led to the evolution of our current jaws. 

It turns out that chewing has had a huge impact on the way we look. In order to understand the structure of the skull, scientists must study how food is chewed and processed as well as the types of food eaten over evolutionary time. And compared to our ancestors, we are pretty lame chewers.

“If you look at the earliest Homo sapiens, we are kind of weaklings from a chewing standpoint,” says Zink.

While this illustration of the human skull shows flat jaw bones, our ancestors had zygomatic arches that were tall and flared because the chewing muscle underneath was so large. (Photo: Public Domain)

Zink’s study sought to find out just how much cooked and processed foods affect how we chew, and thus the morphology of our jaws. She had subjects chew on carrots, red beetroots, yams, and goat meat prepared three different ways: raw, pounded, sliced, and cooked. Electrodes were attached to the face and jaw muscles so she could collect data on how much force was generated with each chew. After the subjects spit the food out, she placed everything on trays on top of a scanner, dispersed all the particles of food in each sample so they weren’t touching, took pictures, and measured each particle digitally.

She found that with just slicing and pounding meat, early humans would have needed to chew 17 percent less often and 26 percent less forcefully—saving as many as 2.5 million chews per year.

“Including meat in the diet and just using mechanical processing results in a large difference in regards to chewing effort,” says Zink. “To me the surprise was more the magnitude of the benefit rather than the fact that there was a benefit.”  

Even after more than 40 chews of the raw goat meat, particles were still predominately in one large clump.

“When you give people raw goat, they chew and chew and chew, and most of the goat is still one big clump—it’s like chewing gum,” Daniel Lieberman, head of Harvard’s Skeletal Biology Lab and supervisor of Zink’s study, told the Harvard Gazette. ”But once you start processing it mechanically, even just slicing it, the effects on chewing performance are dramatic.”

The study’s results supply more evidence to the idea that less chewing with the advent of food processing and cooking has resulted in major changes in skull and bone development. The bones involved in chewing reveal a lot about our evolutionary history, anthropologists often analyzing the feeding apparatus of fossilized remains. 

Around four million years ago, a group of our ancestors, the small bodied pre-human australopithecines, had thick, strong muscles attached to massive jaw bones, explains David Strait, an anthropologist at Washington University in St. Louis. The large muscles allowed australopithecines to bite with incredible force, which in turn was resisted by bony pillars and buttresses in their facial skeletons. Their cheek teeth—molars and premolars—were also large, round, and covered with an especially thick, hard outer coating of protective enamel.

Australopithecus sediba skull

A fossilized skull of Australopithecus sediba, a clue to the change in chewing behavior. (Photo: Brett Eloff/Lee R. Berger/University of the Witwatersrand, Johannesburg/CC BY-SA 4.0)

Natural selection probably favored these adaptations to allow australopithecines to process tough or hard foods, says Strait, but just over two million years ago, that trend reversed itself. The earliest members of the genus Homo, the group to which we belong to, evolved to have an much smaller chewing apparatus, leading to humans’ very weak chewing muscles, flat delicate faces, and small teeth.

This is “very correlated, in some general sense, with the evolution of food processing, like tool use to slice up food,” says Strait.

The energy needed to chew unprocessed food is used for other developmental growth and function. Zink and Lieberman suggest that our flatter faces and teeth have contributed to speech production, locomotion, thermoregulation, and maybe even larger brain sizes. But, some scientists hypothesize that because our jaws are not being worked enough, the smaller overall size has caused dental issues, such as dental crowning, impacted molars, and malocclusions, which is when the upper and lower teeth do not sit together properly.  

“There wasn’t oral surgery millions of years ago, but now it’s incredibly common to have to take teeth out,” says Strait.

Zink wonders how modern-day, sophisticated food processing techniques will influence the evolution of our jaw morphology. From observation and crude data, she says we spend very little time eating, let alone chewing. Today, it’s even possible to consume juiced and pulverized meals that require little to no chewing at all. Next, Zink wants to find out if there are links between the types of foods that hominins may have eaten and chewing performance. She is also working with an orthodontist in France to see if chewing exercises affect jaw growth in children. However, in her next experiments, Zink will happily pass on the regurgitated food sifting duties to someone else.

“We got amazing data, but it’s probably something I would never do again.”

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