The human being got smart by eating meat.

A theory:

"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....

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...

Interesting insight into evolution, but it doesn't say anything about what we should eat today. In evolutionary times, gathering plant material and chewing and swallowing enough of it to survive took a lot of energy. Today, we get more than enough. We have "big guts" in a different sense and for a different reason. So burning extra calories digesting low-calorie plants is probably a good idea.

By the way, if the brain uses 20 times as much energy as the equivalent amount of muscle, why can't we lose weight by thinking hard?