I don’t think that there is any scientific explanation so far for either the nature or the evolution of morality. Its enigma eludes us, even as we act out its innate aspects, in the same way that we speak language, without understanding it.
I find that Darwinian assumptions strike out completely in the realm of moral evolution: we lack a paradigm for the evolution, first, of consciousness/self-consciousness (a distinction lost on science), let alone the evolution of morality. Darwinism has perpetrated a unique fraud in this respect. It can’t explain anything in this area, and yet is able to enforce the illusion that it has complete answers (e.g. with the altruism nonsense), with junk theories of kin/group selection (and the pointless debates between them). I am often puzzled as to how intelligent scientists could fall for the nonsense peddled here. Perhaps the fear that reductionism will fail is so great that scientists become believers instead of thinkers.