Says who? If humans start eating only plants and it works out for us, then by definition we were.
In a word; No.
The reason we exist as we are today is because we started eating meat. See these toofs? These are omnivore toofs. Anyone who thinks that Humans evolved to be vegitarian needs to look at the third tooth on either side of the upper jaw. See those teeth? They're kinda pointy aren't they, know what they are? Those are Canines, know what they are evolved for? Eating meat. That's what they do; they serve no purpose to a herbivore.
Yes, our ability to eat a variety of food, including meat, has been very important to our survival over the millenia. So of course our teeth are the sort that would be up to the task.
I'm not talking about what we did eat, or what was vital for our survival then. I'm talking about what is or isn't a nutritionally sound diet now, which is a different matter.
Certainly, someone three thousand years ago eating no sort of animal product whatsoever would likely have nutritional deficiencies, especially when it comes to the rare things you can't get except from animal sources, although B12 is the only thing that comes to mind. These days, we have ways around that.
So again, the issue isn't whether or not our bodies are designed to be able to eat meat. Obviously, they are, and it was extremely important for our survival as a species to be able to eat basically
anything we came across. That's why humans have great livers, for instance; we have an extremely varied diet, historically.
The issue is whether or not a vegetarian diet is actually workable now, and I have never seen any factual information indicating that it is not. If we only want to speak vegetarian and not vegan, then it's easier to get certain types of protein, as well as B12, from things like dairy and eggs. Vegans have slightly more difficulty because of the B12 thing, but there are supplements for that which aren't animal-sourced. It's also fairly easy these days to make sure that you can get all the amino acids you need from vegetable sources, as well as whichever other nutrients. There's just nothing in particular I can think of that you cannot get from a vegetarian diet.
Again, I'm not doubting that it's a useful thing that humans can eat meat, or saying that we never were meant to in the first place. It's been invaluable to us. I'm just saying that, at least when people can afford to eat a healthy, modern vegetarian diet, it may not actually be necessary.
The argument I'm seeing here is "we need to eat meat because eating meat has been vital to our survival for a long time". This is dodging the question of
why it was vital to our survival, which factors cause that, and whether or not those factors still exist and, when they do, whether or not there are other means of satisfying them.