Darwin Catholic is also musing about original sin, and has jumped to several points I was going to speculate further down the track. But he has done it so much better than I would have, so I’m just going to quote him. Do read his full post, though, to see how he gets there.
it seems that we have several possible ways we can consider the story of Adam and Eve as presented in Genesis in light of what we currently believe we understand about the history of the human species due to the discoveries of modern science.
1) We could hold that at some point in the distant past, God chose a single pair of humans and made them like himself by infusing them with immortal, rational and moral souls. These first parents fell in some way which was best described to God’s chosen people through the Genesis story. Their children all had souls, and at some points interbred with the rest of the early human population, with the result that at some point in the still quite distant past all humans had “Adam and Eve” as one of their ancestors and possessed a soul.
2) We could hold that at some point in time God infused the entire population of humans with souls, and one couple from among them were tested and fell. Given this eventuality, one could hold either that 2.a) as a result of the sin of the representative couple, all humans were stained with original sin at once or 2.b) only the descendants of this fallen couple had original sin, but through some sequence of events, all of us are direct descendants of them. (It strikes me that this leaves the most room for a really fascinating set of fantasy novel plots.)
3) One could hold that they may well never have been a single couple who went through a fall, but that this mythological story was inspired or guided in development by the Holy Spirit in order to describe to the Israelites (and eventually to us) the fallen nature of humanity and the nature of humanities relationship with God. In this way of thinking, we pretty much have to admit that while there was clearly some “fall” at some point after humans came to have souls and be able to have a moral consciousness and awareness of God, and after which humanity possessed original sin and all that that entails, we really have no idea what it was that happened other than that God chose to describe it and its results to us via the story of Adam and Eve.