Let's start by asking what sort of beast morality is. I come up with something like the following:
A1. A moral system is a social structure that imposes limits on the actions of individuals who are part of a given social group. These limits are enforced by a system of rewards and punishments. Rewards include praise and increased social status, punishments range from shame to shunning to ostracism to death.Thus, morality acted as a legal system, back before laws and punishments were formalized and written down. But morality is more than just a system of rewards and punishments, it is internalized through feelings of pride, guilt, etc.
If we ask what is the origin of such moral systems, the answer seems pretty clear: evolution. Humans, like other primates, are highly social animals. Our survival depends to some extent on our ability to cooperate with each other. Just as we have evolved an innate capacity for language, we have evolved some innate capacity for moral behavior: not just the external rules of the system, but the internal emotions that result when the rules are obeyed or disobeyed. Presumably, this sort of behavior gave a better survival rate, so that groups with stronger moral institutions (and containing individuals with stronger moral feelings) out-competed other groups. Let's summarize this as
A2. Morality evolved as a way of subordinating the interests of the individual to the interests of the group.
Clearly, different cultures have implemented widely varying sorts of moral systems. I take it that what we have evolved is a basic instinct for conforming to the group morality. The specific content of that morality differs from culture to culture, and is learned. Here the language analogy is useful again: we have some innate capacity for language, but the specifics of vocabulary, grammar, etc., are learned.
All this seems rather obvious and straightforward. But it is already enough to answer some of the big questions that moral philosophers ask. In fact, it makes the search for a true account of morality look rather pointless. Why, given A1 and A2, would we expect any one "correct" moral system? That's like asking what's the correct grammar for a language to have, or what's the ideal legal system.
Here the language analogy seems to break down, however. When we hear, "Throw your father down the stairs his hat," we think, "How charming!" but when we hear of a practice like female genital mutilation, we say, "That's just wrong."
But there is a reason our response to other moral systems is different from our response to other languages. That is just what a moral system is: a system of deciding what is right and what is wrong. So we should not be surprised that we have an instinctive - maybe even irrational - response to other moral systems.