If Genesis is a metaphor, where did humans acquire original sin?

[True Federalist (진정한 연방 주의자)]

That assumes one accepts the point of view that the reason a substitutional sacrifice was needed was because God required it. I don't. It was needed because some people required it as proof that the Divine loves them despite the human frailties they perceive as making them unlovable. Our original sin was not eating some random piece of fruit, it was doubting God's love.

[True Federalist (진정한 연방 주의자)]

What exactly would original sin be a metaphor for?

The fact that there is something inherently and inescapably wicked about human nature, that we all experience from our earliest moments of consciousness, and that we can try to combat but can never entirely eradicate (at least, in a religious perspective, not without God's grace).

Interesting. I've always thought the concept to be one of my least favourite in theology, because it makes every single human essentially irredeemable without relying on an external agent. Even if I did think there was  a great maliciousness hidden within every human (i dont) I've always been of the belief that we ourselves can overcome it, not the machinations of a deity.

Oh, I'm very much an optimist myself regarding the future of humanity (I don't think I could be able to live a fulfilling life if I didn't have the hope, as generic and vague as it might be, that we are headed to a brighter future - cynicism just isn't for me). But that doesn't resolve the fact that evil exists in this world, more or less everywhere, and it's something we have to acknowledge. I know Freud is rightfully vilified for a lot of his writings, but I think he got one fundamental intuition very right about the existence of a "thanatos" (a drive toward violence, aggressiveness, oppression) in all of us.

I think we can tame and control this part of ourselves, especially through the development of a culture and a civilization that encourages the noblest traits of humanity. I think this can get us very far toward a society without war, crime and exploitation. But do you really think we could, by ourselves, fully extinguish something that is so deeply ingrained in our psyche? If there is a way to extinguish it (which I doubt), following the guidance of a being infinitely greater, better and purer than ourselves (which doesn't have to imply the absence of human agency, although I guess in DC's Calvinist view it does) strikes me as a sensible one.

I have a quite different conception of the problem of evil.  Our lack of omniscience guarantees that we will undertake evil actions.  Worse, we will do so in the belief that we will are acting in the name of good and with the knowledge that others will do evil unto us in the name of good. That knowledge of good and evil is what the Genesis tale is referring to.