Well speaking from a biological standpoint, Dan, humans evolved to be vegetarians, not omnivores. There are two main factors that are accounted for in this belief\standpoint.
The most obvious piece of evidence is the way in which we metabolize our food, not just in how we break it down but also in our internal organs. Human beings have long digestive tracks, just like deer (not as long but a deer is a good example) which is clearly not even a partial meat eater. The reason for this is that plants can be broken down over a long time and cause no ill to the body, the longer it is digested the more nutrients are gained from it. In a carnivore, or even an omnivore, the small intestines is very small and rather short as raw meat is not exactly very healthy thing for anything eating it if it spoils while inside.
The second thing to look at, which is not so readily apparent, is the nutrient need of the human body. While it is true that protein and B3 is needed, the quantity needed is very small. Beans and nuts are more than suffecient to fulfill this miniscule amount, whereas meat is far oversaturated with it.