Australopithecus, an ape-like creature and ancestor of humans, was originally a herbivore. Between 2.5 and 2 million B.C., forced by drought it started eating some meat to supplement its diet, probably by picking up small crawling animals and scavenging (so not necessarily by hunting initially), and it is thought the extra nutritional value of (and/or certain substances in) the meat allowed Australopithecus' brain, and therefore intelligence, to grow over generations, resulting in the first human, Homo erectus, around 1.8 million B.C.
More precisely, in this period of drought Australopithecus split up into two variants: One is called "robust", and followed the strategy of eating thick dry leaves to survive in dry conditions. The other is called "gracile", and it is this second variant that ate meat, and as a result underwent an unprecedented growth of its brain, allowing more intelligent forms of behaviour, including the making of stone tools, at which point they became of the genus Homo. These first tool-makers are called Homo habilis, and the successor thereof Homo ergaster. Then followed Homo erectus, with a cranial capacity in the bottom range of current humans.
Important is that brain cells require very much energy compared to other body cells, and therefore a large brain as typical for the genus Homo could never have evolved or been sustained on a herbivorous diet, if only because vegetable food did not contain enough energy (note there was no bread, pasta, cornflakes or the like in those days to provide energy).
This implies eating meat is what caused human intelligence, and thus humans, to come into existence, and that even the earliest real humans (Homo erectus) ate meat.
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 confirms that human teeth were originally adapted to eating meat, and therefore that humans were meat-eaters of origin.
Some vegetarians 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. Meat eating and having a big brain with human intelligence go together (and in that causal direction), and have done so from the start on.
The largest brains in human evolution were those of the Cro-Magnons, who were European ice-age hunter-gatherers. Meat is the primary source of energy in the hunter-gatherer diet. The second largest brains, incidentally, were those of the Neanderthals - also hunter-gatherers - whose diet consisted mainly of meat. Current Western Europeans, according to recent D.N.A. studies, are largely descended from European ice-age hunter-gatherers, however their brains are about 300 cc smaller. This reduction in brain volume must have taken place after the transition from a hunter-gatherer diet to an agricultural diet; in the latter, carbohydrates from grain products form an additional major source of energy which did not exist before the invention of agriculture, thus reducing the need to eat meat.
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 may be dangerous to not eat meat. Being vegetarian or vegan does not seem to have too many bad effects, and nowadays with agricultural and industrial technology we can make non-meat foods that contain any nutrients we need (except that we have no idea which nutrients we need, and except that some vegetarians are inclined to a "natural" life style and thus confronted with the paradox that a healthy vegetarian diet is only possible thanks to the use of modern technology). 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 or worsened by their diet, but we can not exclude that possibility entirely yet.
It has been claimed that not the eating of meat, but rather the cooking (heating) of food has caused our brains to grow, as it enabled more nutrients to be absorbed. While the latter is in itself entirely correct, it in no way contradicts the role of meat-eating as explained above. The two processes took place in entirely different eras, with a million years between them. The brain growth related to meat took place 2.5 to 2 million years ago, the effect of heating food took place after the mastery of the fire by Homo erectus, which happened in many places independently, about a million to half a million years ago.
In recent years the following arguments have been proposed in favour of vegetarianism, or at least in favour of eating less meat and more vegetable food:
To 1., the following responses need to be given:
To 2., the following response is needed: