I went 100% vegan in like 2007. I always got blood work and it was always perfect. My protein was the same as a meat eating person. I ate a lot of legumes and nuts. I ate so many fruits and vegetables. My only flaw was that I didn't know about B12. Had I known that I would have supplemented. But my health was excellent. I was never weak or sickly from my eating. I did get sick later but it had nothing to do with my diet. If a vegan eats the correct foods there is no reason that it should be a harm. But I do agree that humans are omnivores. I am not sure if we were always meat eaters, but I do know that we have lived on meat for a very long time. But not like we do now. Meat used to be used differently than it is now. Now people eat it every day in large amounts. But meat was not eaten like that before.
I also believe that our diets would have been seasonal. Probably meat would have been a winter staple, and fruits and vegetables in the summer and fall, perhaps with eating much less meat during those times. But then when winter came again and the greens were not plentiful, perhaps meat would have been the only thing on the menu besides maybe tubers and roots.
I once read a book by a Zen master, I don't recall what book it was, but he made a statement about how people believe that we cant survive or be healthy without meat, and he said something about how they say that while whipping the cow while he grazes on grass. His point was that a cow for instance is a very large animal, he can grow huge and strong with no meat. But us humans believe we have to have meat, and so we eat the cow.
Anyways it was something like that. I don't hate on meat eaters, because I was born into a meat eating family. Humans have been eating meat for as far back as we can look in history. For me it is just a personal decision. I just really don't want to hurt any animals. I know that not eating meat does not exempt me from harming animals. I am still complicit. But I am just trying to do the least possible harm, even though it is still not good enough. I do eat eggs now but even that bothers me.
I know that there are places in India where lots of people are vegetarians. They don't get things like b12 deficiency, because they don't wash the fruit and by doing so they eat the larvae that are on the skin, at least that is something I read in the past. It was some article about Jainism. I guess they only eat fruit once it has fallen from the tree. It is really different. Here is an article about their diet.
http://www.jainfoodie.com/jain-food-restrictions/ There are a lot of vegetarians in India, only some of them are Jains. I find these things to be interesting there is always something to learn.