It is crucial, I think, to recognize that the relevant question here is not: Are the lives of humans more valuable than the lives of (other) animals? The objection to killing animals need not presuppose that animals' lives and humans' lives are of equal value. Most defenders of animal rights would not, I think, hold such a view. Their claim, rather, is that animals' lives are of sufficiently great value that they ought not to be killed. Note that saying that animals ought not to be killed does not imply that it is never morally permissible to kill an animal. Humans ought not to be killed, but most people would hold that it is sometimes morally permissible to kill human beings, for example, in self-defense. If (say) cows lives are of less value than are the lives of humans, then there may be circumstances in which it is permissible to kill a cow but in which it would not be permissible to kill a human being. But it does not follow from that fact that it is permissible to kill a cow just because you feel like it, or because you would like a leather jacket, or because you would like some filet mignon. Maybe it is permissible to kill a cow for such reasons and maybe it is not, but it does not follow that it is if cows' lives and humans' lives are not of equal value.
My own view, though, is that talk of "value" is not really appropriate here. I think Nicholas is right to suggest that the real question here is: What kind of life does a creature have to have in order that it should be impermissible to kill it (or to harm it in certain other ways)? The question will then be whether there are animals other than human beings that meet the relevant conditions, whatever they might be.