Once we use up all of our natural resources (including beef and poultry for meat) we will inevitably end up resorting to cannibalism for our main source of protein. Of course we're far too much of a sophisticated culture to randomly go out slaughtering each other (little joke, there) that we will end up setting up farms of the braindead and condemned to death row. I know this may strike some of us as morbid, but no one can possibly negate the possibility of this option if we keep overpopulating the way we do.