Now, as for humans, considering that we have evidence of tool use in the
harvesting of ungulates (proto-horses, proto-bison, mammoths, proto-deer, etc.) as
well as other species like the great sloth, cave bear, etc., for several hundred
thousand years, and we have fossilized spears etc. which are also several hundred
thousand years old, we know that hunting has been a constant practice of human
beings throughout their history. We also know that the late homo erectus preyed
upon the early homo sapiens, as well as many other species, so the evolved
practices go much farther back than just human history.
I think some of the most striking evidence that humans are suppose to be meat eaters, comes from a comparison of the human digestive system with other species.
http://www.panix.com/~paleodiet/comparison.html
One thing we know for sure, is that human beings are not designed to eat grains!