We have three common foundations for moral behavior: command, self-interest, and self-pride. The command foundation is provided by religion: be good because god says so. A second foundation is also included in many religions: be good because it's in your own best interest. If you are good, you'll go to heaven; if not, you'll go to hell. However, this foundation does not require religious belief: even atheists can argue that a moral life will, in the long run, yield greater happiness. "What goes around, comes around"; "Bad karma". And there's always the prison system to convince the more skeptical members of society. These first two foundations are common in the West; the third foundation is more often encountered in Eastern philosophy. Be good because you soil yourself whenever you sin; evil is a poison on the soul.
I'll not attempt to evaluate the relative merits of any of these foundations for moral behavior; lord knows, we need all the help we can get these days. Instead, I'd like to offer a new foundation for moral behavior, arising from genetic considerations.
I begin with the observed phenomenon of altruism in animal behavior. Some animals, under some conditions, will sacrifice their well-being for the benefit of their kin. At first, this puzzled biologists as an apparent denial of genetic imperatives. The explanation, it turned out, was that the genetic imperative is not to enhance one's bodily survival, but the survival of one's genes. Since kin share many genes with the individual, there are some cases where the sacrifice of the individual could nevertheless advance the position of that individual's genes. For example, a bee drone will attack an hive-invader by stinging it, whereupon the stinger is torn loose from the drone's tush and he dies. This truly heroic behavior advances the survival of the hive, and hence the genes that the drone carries. "Tis a far, far better thing I do than I have ever done before."
The utility of altruistic behavior can be calculated based on genetic proportions. For example, each of my siblings carries about half the same genes that I carry, therefore, it's a clear win for my genes if I lay down my life for the survival of three siblings. Cousins carry, on average, one-quarter of my genes, so I can sacrifice myself advantageously for five cousins. The process continues ad infinitum; there are situations in which it remains to my genetic benefit to die to save, say, a thousand distant cousins. Indeed, we know that there's a considerable amount of common heritage in humanity, so there's some large number of people that it would be appropriate for me to lay down my life for.
This genetic altruism forms a foundation for morality. The Golden Rule asserts an identity between me and others -- an identity that has a basis in scientific fact, if exagerrated. I may not be my brother's keeper, but I am half his keeper.
What fascinates me is the complete blurring of hard moral boundaries arising from this moral foundation. Moral obligation extends as far as the eye can see, merely shading off with distance rather than going to zero. And a kind of Olbers' paradox applies here: while my highest individual obligations go to my siblings, there are only a few of them, where there are millions of twentieth-cousins. My net obligation to all those twentieth-cousins could well exceed my net obligations to my siblings.
There's no reason to draw a line at the species-boundary. After all, a chimpanzee shares 97% of my genes -- surely I have some moral obligations to that chimp. I can't say how much of an obligation; perhaps it is only an obligation to refrain from casual killing, or perhaps it's an obligation to respect the privacy and territorial integrity of its home. I don't know -- what I do know is that there exists some moral obligation arising from this genetic foundation for morality.
That obligation extends down to the lowliest life form. There's a little worm that roams the soil, much used in research. It shares 40% of my genes. Under this line of thinking, I owe some moral obligation to that worm. It might be vanishingly small, but it's definitely nonzero.
I am part of the brotherhood of life, and I care for my brothers in the best proportions I can.