Esoteric Source

"You can't have a large brain and big guts at the same time," explains Leslie Aiello, 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, that is, we discovered meat.

"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," Aiello says.

That period is when cut marks on animal bones appeared — not a predator's tooth marks, but incisions that could have been made only by a sharp tool. That's one sign of our carnivorous conversion. But Aiello's favorite clue is somewhat ickier — it's a tapeworm. "The closest relative of human tapeworms are tapeworms that affect African hyenas and wild dogs," she says.

So sometime in our evolutionary history, she explains, "we actually shared saliva with wild dogs and hyenas." That would have happened if, say, we were scavenging on the same carcass that hyenas were.

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."

Carving Up The Diet

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, says Aiello.

"If you look in your dog's mouth and cat's mouth, and open up your own mouth, our teeth are quite different," she says. "What allows us to do what a cat or dog can do are tools."

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 says, they were like "external" teeth.

"Your teeth are really for processing food, of course, but if you do all the food processing out here," she says, gesturing with her hands, "if you are grinding things, then there is less pressure for your teeth to pick up the slack."

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

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