Doesn't come from Animal Liberation at all. If I recall correctly from a philosophy course long ago, its a logical fallacy that occurs whenever you try to derive a statement of "should" from a statement of "is." Otherwise put, normative conclusions cannot be drawn from statements of fact. Therefore, what Joey was saying is that whether your (somewhat melodramatic) cyncicism about human nature is 'true' or not, has no bearing on whether humans have an ethical obligation to extend rights to animals, etc. I consider it a trump card because whether you are right about human nature or not, you cannot justify your conclusions about vegetarianism based on your assumptions about human nature, without committing the naturalist fallacy.

Don't feel too bad though, I have noticed that the naturalist fallacy is abundant in conversations about animal rights and vegetarianism. For instance, over in the original thread there are still several people wasting each other's time arguing about the significance of vestigial human canines, which of course has no necessary logical implications for whether human beings ought to eat meat or not in the future...

The only connection to Animal Liberation, Peter Singer, etc. comes from the fact that Singer's approach to animal rights (and ethics in general) is one that explicitly attempts to and (in my opinion) successfuly bypasses the naturalist fallacy.