I love the sniping, divisive comments, and personal crusading going on in what is essentially a thread about nutrition. It's interesting to see the offensiveness of both sides of the isle and really demonstrates the identity politics at play in what we as a society and as individuals consume, and the degree to which we moralize our choices and are threatened by the moralizing of others. I guess it's true what they say -- we are what we eat.
As far as what the human body has evolved to eat; your body is a complex piece of machinery that's evolved to consume and process whatever it is that will promote the greatest chance of survival and spreading your genes. Part of the reason for the immense proliferation of humanity is our innate adaptability, and as such you should eat whatever it is you feel is going to promote your longevity and health. There are plenty of available studies that extol the many health virtues of a vegetarian diet, just as there are many studies that demonstrate value for a number of diets, lifestyles, and exercise modalities. There are even studies coming out now about paleolithic (or heavily animal based) diets succeeding at promoting good health and body composition.
We like to think in terms of singular factors, that there's a lone variable that can be plugged into the equation of good health and unlock it. The truth is that the solution is multifaceted and good health is a measured result of many considerations. In terms of nutrition alone a dietary choice is successful when it reduces the consumption of unhealthy calories (typically processed) and increases the consumption of whole foods. If you feel that a vegetarian diet will provide you with the framework for making healthy eating choices, then you should do it. Just remember to balance it with adequate sleep, water, and exercise.