If we had evolved from a diet high in animal foods, most of our teeth would be pointed and sharp - like the teeth of natural carnivores.
The long, pointed, and sharp teeth of carnivores are used to hunt, to cach a living prey and keep it in the mouth while killing it. The fact that we do not have that kind of teeth do not indicates that we didn't eat meet in our evolutive history, but that we do not hunt our preys with them.
A second point is that our teeth now are the product of evolution after millenia of eating cooked food. In fact, archeologist know whether an ancient human eat coocked or raw food by looking at the teeth and part of the skull where mouth muscles were inserted. The present weakness of our teeth and mouth muscles is not evidence of our past diet, but of our present habits.
It is true that our diet has much more meat now than in the past, which is probably not healthy. But it is also true that, as our primate cuisins, our origins are omnivore, not herbivore, and that's probably the reason why we were able to spread around the globe.
A final point: in my opinion, the best way to spread vegetarianism is not making appeal to natural/heath issues, as "we are naturally vegetarians"/ "meat is bad for health". None of them is completely true and people notice it. The best way is a moral/ethical discourse, "a better society do not kill animals to eat them"/"any life deserves the same respect". This is something many people could agree with, independently of whether it is natural or healthy.
The youtube channel Kurzgesagt has a beautiful recent video about carnivorism/vegetarianism.
(Sorry about the long comment)