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You know, you might call this next story food for thought. In our series "The Human Edge," NPR's Science Desk has been examining the evolutionary changes that make us uniquely human. Among them is our diet - or at least, the diet of our ancestors. NPR science correspondent Christopher Joyce has found there really is such a thing as brain food.

CHRISTOPHER JOYCE: Our ancestors ate their food raw...

(Soundbite of chewing)

JOYCE: ...fruit, leaves, maybe some nuts. And when they ventured down onto land, they added things like underground tubers...

(Soundbite of chewing)

JOYCE: ...roots and berries. It wasn't a very high-calorie diet. To get the energy you needed, you had to eat a lot and have a big gut to digest it all. And let me tell you, having a big gut has its drawbacks.

Ms. LESLIE AIELLO (Anthropologist, Director, Wenner-Gren Foundation): You can't have a large brain and big guts at the same time.

JOYCE: Leslie Aiello is an anthropologist and director of the Wenner-Gren Foundation in New York City, which funds research on evolution. Digestion, she says, was the energy hog of our primate ancestor's body. The brain was the poor stepsister who got the leftovers - until we discovered meat.

Ms. AIELLO: What we think is that this dietary change, around 2.3 million years ago, was one of the major, significant factors in the evolution of our own species.

JOYCE: That period is when cut marks on animal bones appeared not a predator's tooth marks, but incisions that could only have been made by a sharp tool. That's one sign of our carnivorous conversion. Aiello's favorite clue is ickier it's a tapeworm.

Ms. AIELLO: The closest relative of human tapeworms are tapeworms that affect African hyenas and wild dogs.

JOYCE: So sometime in our evolutionary history...

Ms. AIELLO: We actually shared saliva with wild dogs and hyenas.

JOYCE: That would have happened if, say, we were scavenging on the same carcass that hyenas were.

Ms. AIELLO: Exactly.

JOYCE: But dining with dogs was worth it. Meat is packed with lots of calories and fat. Our brain, which uses about 20 times as much energy as the equivalent amount of muscle, piped up and said, please, sir, I want some more.

As we got more, our guts shrank because we didn't need a giant vegetable processor any more. Our bodies could spend more energy on other things, like building a bigger brain. Sorry, vegetarians, but eating meat apparently made our ancestors smarter smart enough to make better tools, which in turn, led to other changes.

Ms. AIELLO: If you look in your dog's mouth or your cat's mouth, and then you open up your own mouth and look at that, our teeth are quite different. And what allows us to do what a cat or dog can do are tools.

JOYCE: Tools meant we didn't need big, sharp teeth like other predators. Tools even made vegetable matter easier to deal with. As anthropologist Shara Bailey at New York University puts it, they were like external teeth.

Ms. SHARA BAILEY (Anthropologist, New York University): You know, your teeth are really for processing food, of course. But if you do all the food processing out here - you know, if you're boiling things, if you're grinding things - then there is less pressure on your teeth to kind of pick up the slack.

JOYCE: Our teeth, jaws and mouth changed, as well as our gut.

But adding raw meat to our diet doesn't tell the whole food story, according to anthropologist Richard Wrangham. Wrangham invited me to his apartment at Harvard University, to explain what he believes is the real secret to being human. All I had to do was to bring the groceries.

Mr. RICHARD WRANGHAM (Anthropologist): You're a hero, my goodness. Getting all this - hauling all this stuff up.

JOYCE: Let me take these out and show you what I brought. Got a bit of steak, which I hope can fill in for kudu or antelope, or something like that.

Mr. WRANGHAM: Excellent. OK, that's great.

JOYCE: And a turnip.

Mr. WRANGHAM: Oh, one - excellent. Right, really good.

JOYCE: Well, since you're English, I thought that the idea would be to get some tubers and boil the hell out of them.

Mr. WRANGHAM: Absolutely.

JOYCE: Plus a mango, some peanuts and potatoes.

(Soundbite of slicing)

JOYCE: Wrangham says that even after we started eating meat, raw food just didn't pack the energy to build the big-brained, small-toothed, modern human. He cites research that showed that people on a raw food diet, including meat and oil, lost a lot of weight. Many said they felt better, but also experienced chronic energy deficiency. And half the women in the experiment stopped menstruating.

Now, it's not as if raw food isn't nutritious; it's just harder for the body to get at the nutrition.

Mr. WRANGHAM: Are you going to try some of this raw turnip?

JOYCE: You're going to make me do it?

Mr. WRANGHAM: Well, I'll try some, too. They've got a tremendous amount of caloric energy in them. The problem is that it's in the form of starch - which, unless you cook it, does not give you very much.

JOYCE: Then there's all the chewing that raw food requires. Chimps, for example, sometimes chew for six hours a day. That actually consumes a lot of energy.

Mr. WRANGHAM: Plato said that if we were regular animals, we wouldn't have time to sit around and write poetry. You know, he was right.

JOYCE: One solution might have been to pound food, especially meat like this steak.

Mr. WRANGHAM: If our ancestors had used stones to mash the meat like this, then it would have reduced the difficulty that they would have had in digesting it. Are you going to try it?

JOYCE: I'll have at it. And that's quite easy to chew. It's not bad at all.

But not as good as cooking that steak.

Mr. WRANGHAM: Onto the fire. Whoa.

(Soundbite of sizzling)

JOYCE: That's what, Wrangham says, was the really big change that created our modern body. Someone discovered fire no one knows exactly when and then someone got around to putting steak and veggies on the barbie, like this one in Wrangham's kitchen. People said, hey, let's do that again. Besides better taste, cooked food had other benefits.

For one thing, cooking breaks up the long protein chains in meat. That makes them easier for stomach enzymes to digest.

Mr. WRANGHAM: The second thing is very clear, and that is that the muscle, which is made of protein, is wrapped up like a sausage in a skin, and the skin is collagen, connective tissue. And that collagen is very difficult to digest. But if you heat it, it turns to jelly.

JOYCE: As for the starchy foods, like our turnip, cooking gelatinizes the tough starch granules and makes them easier to digest as well. Even just softening food which cooking does makes it more digestible. In the end, you get more energy out of the food.

Yes, cooking can damage some good things in raw food, like vitamins. But Wrangham argues that what's gained by cooking far outweighs the losses.

(Soundbite of cutting steak)

JOYCE: So, I'm going to have some of this steak sitting under my nose.

Mr. WRANGHAM: All right. OK.

(Soundbite of cutting steak)

JOYCE: Wrangham likes to think that cooking also led to some of the finer elements of human behavior. It encourages people to share labor...

(Soundbite of cork popping)

JOYCE: ...it brings families and communities together at the end of the day and encourages conversation and storytelling...

(Soundbite of pouring liquid)

JOYCE: ... all very human activities.

Mr. WRANGHAM: Ultimately, of course, what makes us intellectually human is our brain. And I think that comes from having the highest quality of food in the animal kingdom. And that's because we cook.

JOYCE: So as the Neanderthals liked to say around the campfire: Bon appetit.

Cheers.

Mr. WRANGHAM: So cheers.

JOYCE: Christopher Joyce, NPR News.

INSKEEP: Who knew the Neanderthals spoke French? You can find all the stories in our "Human Edge" series at NPR.org/Science.

(Soundbite of music)

INSKEEP: This is NPR News.

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