When our genus, Homo, diverged from other hominins at least 2.8 million years ago, human skeletons began to change in significant and puzzling ways. Homo erectus, for example, was much taller and had a much larger brain case than the Australopith ancestors.
But Homo erectus also had significantly smaller teeth, jaws, and faces than the ancestral hominins. How could humans eat enough food to support massive bodies, and presumably massive brains, with such small mouths?
Some particularly compelling force must be responsible for this evolutionary change, scientists thought. They proposed explanations such as a change in diet or the use of simple food processing techniques like chopping, pounding, grinding and cooking.
It turns out that a meat-heavy diet and the use of tools to break foods into smaller pieces could have allowed these early humans to derive enough energy from their meals with minimal chewing, according to a paper published Wednesday in the journal Nature.
Because meat is an energy-rich food, if an early human diet was one-third meat, our ancestors would have needed to chew 13 percent less compared to masticating plant matter.
As for tools, slicing meat into smaller pieces and pounding tuberous vegetables would have further decreased the amount humans would have needed to chew their food.
All told, a diet that was one-third meat and processed with simple tools would have made it so humans could chew 17 percent less frequently and 26 percent less forcefully according to this new research.
"These really small, small changes can have some drastic cumulative effects," the first author on the study, Katherine Zink, tells The Christian Science Monitor in an interview.
To better understand how eating sliced up meat and pounded vegetables could affect our ancestors' chewing apparatuses, Dr. Zink and her team measured the effort and number of times subjects chewed variously prepared food.
The subjects had electrodes attached to their faces to measure the force and amount they chewed, and then they were served goat meat with carrots, beets, and jewel yams. Some of the meat was sliced, some wasn't. And some of the vegetables were pounded and some weren't.
The researchers found that cutting meat and tenderizing vegetables meant the subjects had to chew less before swallowing (or spitting it out, in the case of the raw meat). The tools effectively "pre-chewed" the food some.
By contrast, participants struggled to chew the meat that hadn't been broken down in any way. This suggests that knives, even stone ones, helped our ancestors consume more meat more easily.
A time saver
"What allowed us to not have to spend so much time, so much of our day eating, began with these food processing endeavors and with eating more meat in our diet," Zink says.
"Chimpanzees spend about 40 to 50 percent of their day, their daily waking hours, chewing. If we assume the last common ancestors of chimps and humans were rather chimp-like, ... our ancestors were chewing 40 to 50 percent of their day, actively chewing," she says.
Today, most humans spend less than an hour each day chewing.
Cooking up smaller mouths?
Other researchers have proposed that cooking drove the evolution of our faces, as roasting or another form of cooking would have made foods softer and therefore more easily digestible. This new study also examined the role of roasting. Although it helped, anthropologists are still debating whether cooking became widespread early enough to explain these morphological changes.
Meat consumption increased around 2.6 million years ago and stone tools were being made some 3.3 million years ago.
In this study, "I think they make a strong case that cooking was not critical for explaining the reductions in cranial/dental features with H. erectus," said William Leonard, chair of the anthropology department at Northwestern University who was not part of this study, via email.
But "clearly the use of fire and cooking takes us another step beyond that," Dr. Leonard says in a follow-up phone interview.
Evolving for efficiency
How do we get smaller jaws and teeth from less frequent, more efficient chewing?
"Modern humans have ridiculously small teeth" compared to Australopiths, Bernard Wood, a paleoanthropologist at George Washington University who was not part of this study, tells the Monitor in a phone interview.
Maybe humans didn't need such big mouths once they had food processing tools that could enable more meat eating, he explains. "There is a sort of efficiency argument in evolution that you don't want to develop anything that's larger than you really need."
That shift to smaller faces, jaws, and teeth may have allowed for further evolutionary changes, making us who we are today.
"What we're seeing here is the early stages of the fundamental transition towards a distinctly human nutritional ecology, nutritional strategy," Leonard says. This change may have made it possible for humans to be more mobile, he says, something that lines up with the archaeological evidence that the first early human migrations out of Africa occurred around that time.