Modern humans don't spend much time chewing. We don't really have the jaw muscles or teeth for it, so instead we cook food to make it tender. The funny thing is, we've had small teeth and jaws since Homo erectus first stood upright 2,000,000 years ago—at least 1,000,000 years before we invented cooking. It's a puzzle, but new experiments suggest a solution: Perhaps Homo erectus ate more meat and developed some of the earliest food processors, which saved time and energy, and may have led to other important evolutionary advantages.
"We studied a topic that of course is of great importance to everybody on the planet but something we often don't think about, which is how important chewing is in terms of how we live as human beings," co-author and Harvard University professor of biological sciences Daniel Lieberman told reporters at a press conference last week. Apes spend roughly half their days just chewing, he says, while humans spend at most around five percent of their time chewing, and usually much less.
Apes spend roughly half their days just chewing; humans spend at most around five percent of their time chewing.
Cooking explains some of that shift, but the thing is, Homo erectus didn't cook, at least not at first. "This raises an interesting paradox," Lieberman says, because, although they weren't cooking, their bodies and brains were growing larger while their teeth and jaws shrank. If our ancestors needed more food but were ill-equipped to eat it, how did they survive?
Two other developments offer clues. There's evidence H. erectus ate meat, and of course they had stone tools, suggesting they could have cut meat and vegetables into more manageable bites in order to save time and energy that would've otherwise been spent chewing.
How much energy does that actually save? To find out, Lieberman and Katherine Zink, a Harvard lecturer in biological sciences, brought 34 people into their laboratory, wired their faces up with electrodes to measure chewing force, and had them chew on beets, carrots, yams, and goat (cooked, but tough), either in large pieces, small cuts, or pounded into smaller pits.
Humans, those experiments revealed, basically can't eat large pieces of meat—chewing will soften it, but not break it up enough to swallow. "For meat, slicing is super important," Zink says. Raw vegetables are similarly tough to eat in large pieces, and slicing doesn't help, but pounding "six times with a rock" made it possible to consume them, Zink says.
Finally, Zink and Lieberman used the chewing-force data to estimate how much energy Homo erectus could have saved. They estimate that a diet comprising one-third sliced meat and two-thirds pounded vegetables would require 20 percent less effort and 2.5 million fewer chews per year, compared to a diet of unprocessed vegetables alone.
But this isn't just about chewing, Lieberman says. The change in our jaws and faces could have helped lead to the ability to speak more clearly and move our heads more easily—two major benefits for the emerging human race.
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