There are alot of very good arguments in favor of vegetarianism, and even veganism, in our modern world - where food is relatively plentiful compared to our species' past. However, despite the existence of very good arguments, I typically hear very poor arguments put forward. I have commented before on how the Caveman Fallacy is often used as a way to justify vegetarian diets (the claim being that humans didn't evolve to eat meat), and recent findings again add to the pile of evidence that this is an argument based on false claims.
We knew that our early hominid ancestors ate meat - the remains of both tools with morphology consistent with meat processing and animals remains exhibiting tell-tale signs of butchery are found in early archaeological sites. This is noteworthy in that our nearest relatives - the chimpanzees - can and do eat meat, but nonetheless typically subsist on vegetable foods. The consumption of meat (and the introduction of cooked food) allowed our early ancestors to absorb more calories and specific rare nutrients, which was vitally important as our lineages brain size increased (growing a brain requires alot of energy and nutrients, and living in a feast-or-famine situation makes every possible source of calories, proteins, and fats vitally important).
What is interesting is that the date at which butchering and meat consumption began appears to be earlier than previously thought. Until recently, the best evidence indicated that this behavior dated to something in the neighborhood of 2.5 million years ago. However, new evidence indicates that it began closer to 3.4 million years ago, and was engaged in by Australopithecus afarensis (you'd know her as Lucy).
I suspect that these early hominids were scavengers or opportunistic hunters at best, and if either modern primates or modern hunter-gatherers are any indication, the majority of the calories consumed by the Australopithicenes probably came from vegetable foods. But this paved the way for what our evolutionary lineage would become, and as such, was an important step in human evolution.