The question of the evolution of ethics requires a theory of human evolution, and that we don’t have. We don’t even understand which creature actually evolved. As we examine homo sapiens in his outcome phases, i.e. historical man, we see a core ‘moral nexus’ which varies with respect to culture, but which often shows a series of near invariants. The whole question is related to the parallel evolution of language, art, creativity, ‘soul’, very difficult questions indeed. We must ask if a ‘theory’ will ever work since the behavior of man is not purely causal: he has ‘will’ or free agency, in some sense, and this must lie at the core of this ethical behavior. Conventional science is incapable of arriving at any such set of assumptions, or any sense of how these characteristics could have evolved.
A study of the Axial Age and its larger context in WHEE: history and evolution.com can give a few hints: a mysterious macro process can be suggested as related to the evolution of ethics, although the details are unknown.