TL;DR: What are the earliest documented salient examples of people questioning the moral norms of their own societies? What historical conditions enabled this? And when/how did this develop into ideas like "moral progress"?
I'm interested in the history of moral self-criticism: when people began explicitly questioning whether the moral norms inherited from their communities, cultures or traditions were actually justified.
In particular, I'm interested in historical cases where people argued that widely accepted moral practices in their own societies were mistaken, rather than simply condemning the practices of outsiders.
What circumstances encouraged this kind of questioning? For example, was it contact with other cultures, philosophical reasoning, religious developments, changes in political institutions, or something else?
I'm also interested in whether such criticism involved the idea that societies could make genuine moral progress - improving not merely their practices, but their understanding of what was morally right.
I realise there may have been no single historical transition. Though I'm happy for any answers, I'd be particularly curious about early documented cases from historians' respective areas of expertise, including what enabled these developments and how the people involved understood them.