Philosophers and theologians in the Christian tradition have regarded human beings as distinguished from the other animals by the presence within them of a divine spark. This inner source of illumination, the soul, can never be grasped from outside, and is in some way detached from the natural order, maybe taking wing for some supernatural place when the body collapses and dies.
Recent advances in genetics, neuroscience and evolutionary psychology have all but killed off that idea. But they have raised the question of what to put in its place. For quite clearly, although we are animals, bound in the web of causality that joins us to the zoosphere, we are not just animals.
There is something in the human condition that suggests the need for special treatment. Almost all people believe that it is a crime to kill an innocent human, but not to kill an innocent tapeworm. And almost all people regard tapeworms as incapable of innocence in any case — not because they are always guilty, but because the distinction between innocent and guilty does not apply to them. They are the wrong kind of thing.