In what has to be one of the weirdest thought-experiemnts, Jeff McMahan asks:

Suppose that we could arrange the gradual extinction of carnivorous species, replacing them with new herbivorous ones.  Or suppose that we could intervene genetically, so that currently carnivorous species would gradually evolve into herbivorous ones, thereby fulfilling Isaiah’s prophecy.  If we could bring about the end of predation by one or the other of these means at little cost to ourselves, ought we to do it?

How about starting with ourselves?