ARI SHAPIRO, HOST:
Anyone with serious cooking chops knows the importance of good knife skills. A study published in the journal Nature today shows just how important they've been for more than 2 million years. As Allison Aubrey reports, our early ancestors' slicing and dicing helped make humans human.
ALLISON AUBREY, BYLINE: If you're like me, you've probably eaten several times today without ever stopping to think about chewing. Most of us take it for granted, but not Dan Lieberman. He's a professor in the department of human evolutionary biology at Harvard.
DAN LIEBERMAN: Every time I go out to dinner, I watch people chew. Sometimes I will actually count how many times they chew (laughter).
AUBREY: Lieberman says, despite all the calories we eat, we spend remarkably little time chomping on food.
LIEBERMAN: Most of the people listening to this, very rarely do we have to put any effort into anything we chew.
AUBREY: But it was not this way for our early human ancestors. They spent a lot of time chewing. About 50 percent of their day was spent chomping away, just to get enough calories. And Lieberman says their faces showed it.
LIEBERMAN: Our ancestors had big faces, huge chewing muscles, much larger teeth.
AUBREY: That was about two-and-a-half million years ago. But then, our ancestors started to eat meat - raw meat - because they hadn't yet figured out how to cook it. But they had figured out how to break sharp edges off stones to make stone blades, the precursor to modern knives.
LIEBERMAN: It's not coincidental that we see, in the archaeological record, the first evidence for meat-eating showing up around the time that stone tools show up.
AUBREY: And in his new study, Lieberman and his colleague, Katherine Zink, did an experiment that showed just how much of a difference being able to cut made to our ability to eat meat.
LIEBERMAN: If I were to give you a piece of raw goat, you'd chew and you'd chew and you'd chew, and you'd keep chewing, and nothing would happen. But if you were to cut that goat up into smallish pieces, your ability to chew that goat into smaller pieces would improve dramatically.
AUBREY: When they had volunteers chew on pieces of raw goat, they were able to calculate that cutting meat reduced the number of chews we take by 2.5 million chews per year.
LIEBERMAN: Two-and-a-half million fewer times per year. That's an enormous difference. And I think it's just amazing to think that the simplest stone tools could have had such a massive effect on how we can chew something that we take for granted, which is a piece of meat.
AUBREY: And all of this set our ancestors on the course to smaller teeth and faces and other major changes, too.
LIEBERMAN: That's when brains start getting bigger and bodies start getting bigger. And I think the real key story here is that that process wouldn't have been possible without the invention of stone tools.
AUBREY: So it seems knife skills helped make us human. Allison Aubrey, NPR News.
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