Humans did not start off as vegetarians, in fact there is credible evidence that meat-eating drove the early growth of our brains. From Homo habilis to Homo erectus, our brains doubled in size, and this was likely facilitated by a switch to a richer meat based diet.

Our own species - Homo sapiens- has been eating meat for as long as it has existed. We have archeological evidence of butchery and cooking meat since the dawn of our species.

Of course, archeology can't provide negative evidence that this individual or that tribe never touched meat. For that, you need history.

The earliest historical record of vegetarianism as a lifestyle is from India. Meat was commonly eaten during the early Vedic period, but the later Vedas and Vedanta begin talking about non-violence, even against animals. This trend is much strengthened in the Puranas, by which time Ayurveda begins adding health reasons to ethical reasons against eating meat. The arrival of shramanic religions like Buddhism and especially Jainism around 500 BC sealed the deal, and from then on we see absolute proscriptions against carnivory on religious grounds.

The earliest defense of vegetarianism in the Vedas and Puranas is on ethical grounds, because they see killing animals for food to be cruel. Medical justifications crop up about 500 years later, talking about the health benefits of vegetarianism. Finally, religion puts its foot down for an outright prohibition. But since India had a caste system, the prohibition was only on Brahmins. Other castes may eat meat, though they still oppose eating beef.