Greyfields, if the following does not fall sufficiently well within your 'no arguing with each other' guidelines then feel free to remove 
 I am NOT in fact trying to argue, here, just wanted to make an observation:
On the face of it, it sounds pretty natural and reasonable to say that eating meat is perfectly ok because, hey, humans are descended from a line of omnivorous critters that eat meat when they can, and we have teeth that are well-adapted for biting off pieces of meat, and people have eaten meat throughout human history.
However, what happens when you substitute other, equivalent terms in there:
"Killing people is perfectly ok, because hey, humans are descended from a line of critters that will kill each other when they feel like it, and we have hands that are well-adapted for swinging a heavy club or stabbing with a knife, and certainly people have been killing each other throughout human history."
Hm? 
I think it's awfully hard to successfully argue that just because we CAN do something, and just because there is historical or evolutionary precedent for it, then of course it must be totally ok. I think it becomes clear from this exercise (unless you actually buy my second statement up there, and if you do,
 please do not live anywhere near me 
), I think it becomes clear that there are other things that have to be taken into consideration.
Just sayin',
Pat