I decided to become a vegetarian (or pescatarian...don't even know myself at the moment) six months ago for the love of animals. Videos of factory farming lead me to that decision, but then of course there is humane raised meat available. But I go back to one thing. I was at a fair last year and we were walking through. We stopped to see a cow and she was just so nice. So I asked myself a question. Why would I want to eat this sweet girl? And the answer is I wouldn't want to. That is me though and I don't tell people they shouldn't eat meat. All I do want people to do it be aware of what factory farming is.
I don't know if I'll be a vegetarian/pescatarian forever, but I do know that right now, killing an animal, cutting it up, frying it, and eating it is something I want to pass up. But hey, I reserve the right to change my mind.
As far as meat being an important part of the diet, we have no nutritional need for it whatsoever. As long as it is done correctly, some would argue that vegetarians are healthier than omnis.
I have no answer for you about human brains developing from eating meat because I don't believe in evolution.