Vegetarian Humans are omnivores, as during their evolution they required the ability to eat a broad diet in order to survive. Carnivores develop in species that are swift and voracious, but only where a continuous and ample supply of prey exist. Imagine a tiger with nothing but grass to eat. The tiger may be willing to eat almost anything when the hunger pangs get strong enough, but his digestive system won't process anything but meat.
Carnivores die when their prey dies out.
Species that are vegetarians also have specialized digestive tracts, designed to break down the tough fiber that is intrinsic to plant life. They have multiple stomachs in many cases, digesting in stages. All species that eat plants consume insects as a matter of course, as insects are scattered throughout the plants they munch on, and thus are always part of the intake. Thus, species that evolve are either carnivores, vegetarian, or omnivores. The omnivore, of which mankind is a member, evolve to meet wildly swinging cycles of food availability.