The human species started as herbivores then, when they invented tools and weapons, evolved to become omnivores, eating food of both plant and animal origin. Reverting to vegetarianism has to be an evolutionary regressive step.
There's a reason why, in the animal kingdom, quick witted carnivore predators so often feast on slow witted herbivores! That's their respective roles in nature! Hard to increase the gene pool on tofu and bird seed.
