Pun Salad

If you take a reductionist/determinist view, morality is impossible. It's all (imagine me here waving my hands at the universe) just atoms and fields bumping into each other according to relatively well-understood physical laws. You can't argue that a particular grouping of atoms is acting "badly", compared to another group of atoms that's being "virtuous".

In order to talk morality, you need to assume certain bedrock conceptions of life/consciousness/free will. We don't know where those things come from, at least not yet. But I take them to be (literally) self-evident; if they didn't exist, we wouldn't even be thinking about them.

Given that, add in "human nature": basic facts about our biology, psychology, and how we act in groups. That determines the envelope of our possibilities and desires. Gradually, we learn how best to satisfy our needs, working within our families and tribes.

From there, it's... well, probably see "The Abolition of Man" by C. S. Lewis, especially the stuff at the end where he describes cultures widely separated in time and space coming up with remarkably similar moral codes. (I know Lewis was a Christian, but TAoM isn't particularly theistic.)