There actually was a species of early human around that was vegetarian; A. boisei.
This species died out and the human ancestor, Homo Habilis
, took over. This was mainly cuz H. Habilis
was omnivorious and therefore better adapted to survival; also, the habit of eating meat fed the brain and made it bigger.
So, there you have it. If we were all vegetarians back then, human beings would have never existed cuz we would all have died out before modern times.