Popular writers prefer to simplify things by describing the lives of chimpanzees either in Hobbesian terms, as nasty and brutish, or by stressing their friendly side, but in fact it's never one or the other. It's always both. If people ask how chimpanzees can possibly be called empathic, knowing that they sometimes kill one another, my return question is always whether by the same token we shouldn't abandon the whole notion of human empathy as well.In order for our complex moral rules to have evolved, we had to have already been both good and bad in our potential interactions.
This duality is crucial. Morality would be superfluous if we were universally nice. What would there be to worry about if all that humans ever did was show sympathy for one another, and never steal, never stab someone in the back, never covet another's wife? This is clearly not how we are, and it explains the need for moral rules. On the other hand, we could design a zillion rules to promote respect and care for others, but they'd come to naught if we didn't already lean in that direction. They would be like seeds dropped onto a glass plate: without a chance of taking root. What permits us to tell right from wrong is our ability to be both good and bad. (Frans de Waal, The Bonobo and the Atheist, 27)
We would also expect our morals to have emerged from the bottom-up:
The view of morality as a set of immutable principles, or laws, that are ours to discover ultimately comes from religion. It doesn't really matter whether it is God, human, reason, or science that formulates these laws. All of these approaches share a top-down orientation, their chief premise being that humans don't know how to behave and that someone must tell them. But what if morality is created in day-to-day social interaction, not at some abstract mental level? (de Waal, 23)Hume argues that our morals emerge from our sentiments. I and de Waal agree. From this instinctual foundation, our moral systems are further developed in our daily social interactions with others. It is only after the fact, after these interactions have given rise to our moral orders and the institutions of those orders, that moral philosophers come along and theorize morality into higher levels of abstraction. This then works as eminent criticism of the moral order -- it thus has an effect, but it's not what truly drives the emergence of our complex moral rules.
Finally (for this post, at least), one would expect there to be a power law distribution of moral rules. Some moral rules are more important than others. We could probably place first degree murder at the very top as being the one worst thing one could do to another person. At the bottom, so to speak, would be all the various rules of etiquette -- there are many, and the worst reaction you will get from an incursion against them is a disapproving look from whoever noticed. I'm not going to proceed to give a power law ranking of all the other moral rules, as they are going to vary to a certain degree from place to place and from time to time. But we can probably get a general idea of the general rankings by considering all of the moral incursions that could have gotten you the death penalty vs. the fact that in places like the U.S., the only thing that can still get you the death penalty is first degree murder (and often it has to be accompanied by other crimes).
All of these elements suggest morality is a spontaneous order. Clearly more work needs to be done on this, but this seems a good place to start.