Vegetarianism has been a part of human culture since the dawn of civilization, when Greek philosophers debated the morality of killing animals for their flesh. It was Pythagoras, the father of mathematics, who said, "As long as man continues to be the ruthless destroyer of lower-living beings, he will never know health or peace. For as long as men massacre animals, they will kill each other." (For what it's worth, that is not necessarily true.)
More than 2,000 years later, we're still having that debate. Last week, Intelligence Squared hosted a panel on the rights and wrongs of meat-eating, entitled "Don't Eat Anything With a Face."