This is a sentiment many people have lived by. Historically, and even today, we have treated other animals abysmally. This is one of my favourite quotations; it comes from the Reverend W. R. Inge:

"We have enslaved the rest of the animal creation, and have treated our distant cousins in fur and feathers so badly that beyond doubt, if they were able to formulate a religion, they would depict the Devil in human form."

A number of commentators have gone so far as to liken our treatment of the animals to the Nazi Holocaust. Here's another quotation, this one from the author Isaac Bashevis Singer:

"They have convinced themselves that man, the worst transgressor of all the species, is the crown of creation. All other creatures were created merely to provide him with food, pelts, to be tormented, exterminated. In relation to them, all people are Nazis; for the animals it is an eternal Treblinka [a Nazi extermination camp]."

Let's now look at what an evolutionary perspective contributes to the debate. First and foremost, evolutionary theory challenges the doctrine of human dignity. As we've seen, Darwin's theory undermines the idea that we were made in the image of God, and it undermines the idea that we are distinguished in any morally significant way from the other animals by our possession of the faculty of reason. Moreover, it undermines the brute distinction between human beings and all other life, a central element of the doctrine. It does this by stressing our common origin and our kinship with the animals. Chimpanzees, dolphins, frogs - Darwin taught us that these are literally our distant relatives. Certainly, it is easy enough in practice to draw a human/animal distinction, and to import this into our moral reasoning. But evolutionary theory shows that this distinction has none of the significance it was once assumed to have. As such, its application in the moral sphere - that is, our habit of extending our moral concern only as far as the outskirts of our own species - suddenly starts to look arbitrary and unjustified. Why should our moral circle be limited to our species rather than to, say, our taxonomic class (i.e., mammals)? Why, for that matter, should it be limited to our species rather than to the racial group we belong to?

Evolutionary theory also undercuts a number of other arguments aimed at justifying the exploitation of animals. This includes the argument that God put the animals here for our sake. Before scientists pieced together a picture of the history of life on earth, this might have seemed like a reasonable claim. It's not reasonable anymore. We know now that the vast majority of animals finished their sojourn on this planet long before we evolved. We also know that we and the other animals came about through the same natural process, and that our so-called ‘creator' (i.e., natural selection) had no special affection for us. In light of these facts, the suggestion that animals are here for our use seems self-centred, quaint, and - to be clear about this - patently false. As the Pulitzer Prize-winning author Alice Walker wrote, nonhuman animals ‘were not made for humans any more than black people were made for whites or women for men'.

Another traditional justification for the exploitation of other animals comes from the Cartesian view that nonhuman animals are merely nonconscious automata, and thus that we need not worry that the way we treat them could cause them any suffering. But an evolutionary perspective drastically lowers our confidence in this view; after all, we are conscious beings (conscious automata perhaps), and we came about through the same process as every other animal. This being the case, it seems unreasonable to deny that any species other than our own is conscious or has the capacity to suffer, especially in the case of those animals that have very similar brains to our own.

And this brings us to a crucial point. If we decide - and this is our decision; it's not imposed on us from above - if we decide that reducing the amount of suffering in the world is a good ethical principle to live by, then it seems unwarranted and ultimately arbitrary to extend this principle to human beings but not also to extend it to other animals capable of suffering. Why should the suffering of nonhumans be less important than that of humans? Surely a universe with less suffering is better than one with more, regardless of whether the locus of suffering is a human being or not, a rational being or not, a member of the moral community or not. Suffering is suffering, and these other variables are morally irrelevant.

Of course, this is not to imply that all animals should be treated equally; no one would accept that the life of an ant is as important as the life of a human or a chimpanzee, or that swatting a fly should be considered an act of murder. But if we take the reduction of suffering as the foundation stone of our moral system, we may find a principled solution to this conundrum. The solution is to accord animals moral status in proportion to their degree of sentience or their capacity to suffer. Think about it. Why do we consider it morally repugnant to torture human beings? It's not because they're capable of language or rational thought, or because they're embedded in a social network of reciprocal rights and duties, or because they're members of our own species. It's because it causes them pain and trauma. If it didn't, we wouldn't worry about it. Of course, there is no ultimate justifaction for adopting the reduction of suffering as the basis of our morality. It comes down to a choice. I don't know about you, though, but I'd prefer to live in a world with less suffering than with more. I'd also prefer to live in a world where the ascendent moral principle is "reduce unnecessary suffering" than one in which it is "only be nice to creatures who can return the favour, or who can talk, or who happen to have the same kind of genome as you".

With the reduction of suffering as our guiding principle, we now have a solid and sensible rationale for many of our core ethical intuitions. Humans have a far greater capacity to suffer than flies; therefore, it is far worse to harm a human than to harm a fly. Similarly, humans presumably have a greater capacity to suffer than chimpanzees - we are locked into tighter emotional bonds, we grieve for longer - and therefore it is somewhat worse to harm a human than a chimp.