On an evolutionary or naturalized account of morality, the natural selection process endowed us with a set of strong behavioral preferences. That is, morality evolved. We are built to be moral beings by evolution. These behavioral tendencies and preferences have been divided up many ways, but Pinker’s list is a good one:
The idea is that there were selection pressures in place long before we were recognizably human that would have selected for some types of behavioral tendencies over others. And rather than the common misconception that evolution selects for utter selfishness, a growing body of empirical research is showing that cooperative, constructive social tendencies have been built into us down to the genes.
See Franz De Waal, "Morality and the Social Instincts: Continuity with the Other Primates." (source for the picture.)
As I have pointed out, this is all highly speculative, and I’ve glossed over a lot of very complicated issues. But the suggestion is that there could be something about the development of big brains like ours that is inextricably tied to particular categories of behavioral norms. And if that’s right, then the bite has been taken out of the moral relativism charge coming from God believers. Only certain kinds of moral norms can evolve—the do unto your neighbor as you would have him do unto you kind.