I want to respectfully say that I disagree
The thing with morality is, you either intuitively see it or you don't.
There are endles theories that try to prove how what we call morality today has evolved. Like this theory, self-interest.
But it isn't about that. One must simply ask himself: Do humans have intrinsic value or worth? If the answer is yes, than evolution is in trouble. For how can something evolve to be intrinsically worthy? It doesn't make any sense.
If thew answer is no, than morality is a illusion, or a subjective thing, a thing that is tied to feelings or instincts, or whatever-the-theory-propose, be it self-interest or fear-of-gods.
But saying that morality isn't actually so-much-of-a-morality, and has evolved, its dangerous. How we can make laws and use power to protect society? If morality is a illusion, than whosoever has the power, controls everything. So for the one who has the power, slavery isn't something bad (because it really isn't, if there is no morality). It can be just self-interest.