As I am of European descent, I speak of Pagan European civilization mostly. Ancient Europeans were hunter gatherers. Those who say that civilization began with farming, or that it began with the invention of the plow are completely wrong. Christian civilization did, as it developed in the "cradle of civilization", however, there were many other very civilized cultures around the world, such as Europeans and Native Americans. Christian civilization just refuses to accept this and that lie is taught by schools around the world. The farming civilizations became reliant on farming and they would already be unfit for an apocalyptic situation. However, hunter gatherers would.
Back to the main topic. Humans were not designed to eat such a simple diet based off of so-called "staples". We were never meant to eat just grain or vegetables. That is not healthy. Meat generally provides more protein and fat than vegetables, and those two are essential nutrients.
From an ethical point of view, I also see it the same way. Killing animals for food is not "morally wrong". Our ancestors have been doing it for thousands of years, and animals do it. That should be enough to justify eating meat. The guilt anyone my feel from eating meat is just from being raised to see from a certain moral point of view.