Off the top of my head it could be said that the first organism in our lineage that rose to a comparable level of cognitive functioning to modern humans could then have brought about the whole Original Sin mess. I haven't quite thought it through so I'm unsure if it has any holes in it.
Honestly in my experience Christians don't seem to give much thought to what exactly it is they believe in, they just attend the service, say the words when required, go get that delicious icr-cream cone Eucharist and call themselves Christian. I have met very few Christians for example that have read the bible from back to front. I reckon if a few of them actually sat down and gave serious thought to it, tried to make all their various beliefs piece together into one nice coherent picture, they would be unable to do it.