I've seen a lot of people lately making the "you’re made out of animal parts, so eat animals" argument, which is reasonable on its face. But there's a pretty serious problem with it: those animals get everything they need from plants. And now we're in very murky territory, because now you have to prove—not just hypothesize, but prove—that the human animal is different from other animals in some very specific ways, or you have to weaken the initial claim to the point where it doesn't really say anything at all.