Australopithecus, an ape-like creature and ancestor of humans (but not human itself), was originally a herbivore. Between 2.5 and 2 million BC it started eating some meat to supplement its diet, probably by picking up small crawling animals and scavenging (so not by hunting), and it is thought substances in the meat caused Australopithecus' brain (and therefore intelligence) to grow over generations, resulting in the first human, Homo erectus, around 1.8 million BC.
This implies eating meat is what caused human intelligence, and therefore humans, to come into existence, and that even the earliest humans (Homo erectus) ate meat. Also it is logical that if we all stopped eating meat, our brains would get smaller again and we would degenerate back into apes in say half a million years.
Relevant too in this respect are archaeological findings showing that caries (tooth decay) occurred much in groups of humans whose diet contained grains, and very little in those who mainly ate meat. This shows that human teeth were originally adapted to eating meat, and therefore that humans are meat-eaters of origin.
Vegetarians, vegans and the like say humans were originally herbivores, based on a number of physical features we share with other herbivorous animals. They are only right in the sense that our ape ancestors - the genus Australopithecus - were herbivores. Actual humans - the genus Homo - never were, although they still have the herbivorous digestive system they inherited from Australopithecus. Meat eating and having a big brain with human intelligence go together, and have done so from the start on.
This does not mean it is wrong to stop eating meat in individual cases (as an adult, that is; For children and pregnant women it is dangerous to not eat meat). As long as not too many do that, we will likely keep our large brains. Being vegetarian or vegan does not seem to have too many bad effects. Although one could wonder about the warped beliefs, violence and terrorism of some animal rights activists, who of course do not eat meat. It is probably not caused by their diet, but we cannot exclude that possibility entirely yet.