There is no scale of how valuable different species’ lives are that has been prepared by an unbiased observer. All the scales we have, despite our pretensions of objectivity, have been created by us. And we have rather conveniently put ourselves on top of these scales of importance.
Not only that, we have even convinced ourselves that our reasons for doing so are more than plain old vanity and self-love. We have made up stories to justify our illusions of how important we are in the order of things, about how much value we bring to the workplace (this planet). In these stories, we tell ourselves that there exist reasons for our being here, that there are conscious higher beings who created us out of compassion and have a clear vision of our future.
We are like Narcissus of Greek myth.
We human beings are one of many species of life on this planet. Our reasons for being here are as random (in the evolutionary sense) as any other species’. A large part of what we are is our ability to coordinate and cooperate. It is why I am sitting inside a room right now, typing this into an electronic device that will, network permitting, push my ideas out and make them accessible to other members of the human species. When others read this note, they will hopefully consider my life as having some value. And even if they don’t read it, they will consider my life valuable because they and I are characters in the same story.
The societies we have built from the ground up, the world we have created, all depend on the assumption that there is sense in valuing human life and that every single human being, as long as he agrees with a set of rules (contained in the general narrative of the aforementioned story, within the context of specific societies) is worth having around. Because we have all believed in the story about our importance, we have continued to exist. And because the story has proved useful, we continue to tell it to ourselves and to each other.
Objectively however, human lives have no more intrinsic value than anything else. It’s just that our belief in the importance of human life keeps us going in a generally useful direction.