WHAT MADE HUMANS different to other primates? Very simple. We started eating meat:

Our earliest ancestors ate their food raw — fruit, leaves, maybe some nuts. When they ventured down onto land, they added things like underground tubers, roots and berries.

It wasn't a very high-calorie diet, so to get the energy you needed, you had to eat a lot and have a big gut to digest it all. But having a big gut has its drawbacks.

"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.

[...] 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.

And then we started cooking, not just the meat but also the veggies:

[E]ven 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.

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

Read it all, especially if you are one of those vegans who not only don't eat anything coming from animals (including dairy products), but they angrily object at anyone eating it in their presence. Over the years, I've found myself in meals with those intolerants. I generally, for courtesy, avoid eating meat, but for some the mere joking ("don't worry, I won't ask for a T-bone") is enough. One, the founder of a quite popular internet startup (now based in the US) wrote me an email first thing after he went back to his hotel after our lunch meeting. There it was, sitting in my inbox when I arrived to my office, berating me for not respecting his sensibilities when I mentioned in a lighthearted way that I wasn't going to eat meat. "What would you say if some aliens came to Earth and saw us inferior animals with a much lower intelligence than themselves, like we see animals, and ate us?" To which I replied, "And what would you think if another much more intelligent group of aliens came to Earth and saw us much much lower intelligence than themselves, like we see plants, and ate us. Would that mean it's also wrong to eat plants? Is it wrong to eat anything?"

The back and forth continued a bit, and it was disconcerting, to put it mildly, to see so much fanaticism over, well, science.