[Diamond]

Not a justification, but a source, as it were

I'm sorry - I meant justification as the source of the moral rule.

It seems to me that to say descriptively that the trait has been selected for millennia does not explain where the requirement came from or why it is incumbent upon me to obey it as a moral rule in the future. You have to presuppose a prior morality before there is any right or wrong, but then how can natural selection account for what came before it?

Cordially,

[Junior]

The requirement came from the need to survive. There would have been all sorts of survival strategies attempted by our ancestors. The ones that worked (cooperation) allowed those who practiced them to survive, while those practicing less successful strategies were eventually weeded out. That all there really is to it.