You are going to open a whole can of worms if you bring Evolution into this conversation my man! hahaha be it biological or the evolution of the mind.
As far as I know, humans are not capable of living off non meat products due to the nutrional value that we gain from being carnivores. One of the most important being iron and the 26+ types of amino acids required for growth etc etc.
Therefore I believe that the vegetarians that we see now adays (the true vegetarians) need to take some sort of dietry supplement to compensate for the loss and these supplements are man made.. also not available at the times you have refered to in your earlier post.
You can argue that a vegetables like beet root is high in iron or that there are sources of protien, but then you will have to go into 'types' of protien etc and I am not qualified to talk about that!
I think there will be a time, perhaps not in our lifetime, when we do realise that eradicating life from this planet to sustain life is not the way to go. we have already depleted the earths resources to the brink. Lets hope we change