Discussions are dangerous and essential things, I think, because they can tempt one to cling blindly to an ideal in the face of opposition--or they can illuminate one's understanding of his convictions. Sometimes they can do both simultaneously, and that to me seems like a great paradox. I know it's true, though, because I've felt it.
In any case, I was in a discussion earlier about nature and, moreover, about ethical responsibilities our humanity might entail. My friend, a very serious human and animal rights activist, insists that suffering is bad and that causing suffering may be equated to evil or unjust action. I offered that some animals may not be thought capable of suffering: suffering, after all, is a complex state that can occur only when a being's neural anatomy is advanced to a certain point. I can't claim to know the exact physiology behind it, but it makes sense: in order to suffer, we need a highly developed nervous system and, through whatever means it takes, a sense of awareness that can process signals as "pleasure" or "pain." He agreed that some animals don't have the pieces in place to suffer and even offered that shrimp are one such example. Do they, then, have any less a claim to life than a vertebrate that happens to be born with more complicated neural connections?
My gut instinct is to say "no." I want to say that we as humans, based on our own characteristics--namely, our way of experiencing the world and our aversion to pain--seek to judge "right" and "wrong" completely by standards inherent to our sensory experience as a species. It's not speciesism as the term is generally used, but I feel the term can be thought to apply nonetheless: when we are restricting the rights of every species in accordance with a standard based on our limited conscious experience (as members of the human race), we are engaging in a prejudice no different in principle from any other. Whatever a shrimp's ways and whatever its fashion of experiencing life, we as animals ourselves have no valid way of objectively judging its right to exist.
At the same time, we as humans are naturally sentimental creatures, and I think many of us have a large level of compassion. When we suffer and develop an aversion to suffering, we (some of us, anyway: particularly those of us who don't murder our brethren for sport and hang their heads on our walls) learn to empathize with other animals who, structurally very similar to ourselves, exhibit similar ways of internalizing pain. And that's touching, isn't it? Forget cross-cultural similarities: here we are recognizing similarities across species. As someone who finds it deeply touching that people across the world can listen to the same song and, despite their drastically different backgrounds, "understand" it in the same way, this is huge. But it's very subjective. For that matter, I think it tempts us to further attribute things to animals: there is a distinct possibility that we may go so far as to associate all kinds of human characteristics with an animal who feels few of them, and all because of our sentimental and compassionate nature. Who knows? That same nature may blur our eyes to the truth of even human experience: emotion and its resultant conviction can do that.
Then what about those of us who eat meat all the time? I know I have chicken rather often and I enjoy it! If it's wrong for me to kill, is it wrong for a lion to kill? I can't catch the difference; some say we should "know better," but I think what it amounts to is that we simply know differently. Where lions, I'd suspect, are in a constant state of living in the moment, we are different. We wonder, worry, evaluate, categorize, and judge. We look at our own nature and try to extrapolate. It has served us well, at least from a Darwinian perspective, as we have outlasted all our humanoid cousins and even established a global network of contact and trade. Does this give us an edge? As Douglas Adams comments, "Man had always assumed that he was more intelligent than dolphins because he had achieved so much... the wheel, New York, wars, and so on, whilst all the dolphins had ever done was muck about in the water having a good time. But conversely the dolphins believed themselves to be more intelligent than man for precisely the same reasons." The man has a great sense of humor, but he may have a point.
The point is not that we're arrogant or selfish, incidentally. I think we're just animals, making do with our lot in life and adapting to our environment as best we can. We're animals who build skyscrapers and drive intimidating metal capsules around on cold stretches of concrete, but we're still very much a part of nature. It's all we can ever be.
So what's right? Can there be a "right"? I don't know. I know how I feel when I see innocent suffering, and I know what I think when I try to make sense of this all through more strictly logical means. I think we're all animals on this same earth with our own means of experiencing life and killing and dying, and I think any notion that I can see through or beyond that is probably a bit naive.