I think there's some evolutionary pressure on morality, so that it's a mixture of requirements for behavior which improve the odds of survival for the group, maintain the status of high-status people, and/or are just people making things up because they sound cool/seemed like a good idea at the time/distinguish the group from other groups. People are encouraged to think of all three as the same sort of thing.

My thinking is along similar lines... yet I divide the three modes from a slightly different perspective. I don't put much weight on the 'odds of survival for the group' for example, although there is definitely a sense in which moral claims are meant to be interpreted as declarations of pro-social norms. I had tended to leave off 'just making things up because they sound cool' because I was focusing on the explicit political influences but come to think of it "seemed like a good idea at the time" probably does explain a lot!