Hi Michael
Quite right that Augustine and many others added to St. Paul's concept of 'original sin' – but I think it did have it's origin with  Paul and his desire to 'mythologize' Christ as the 'bringer of Life' to the world, and so made a mis-connection to a 'first man who brought death' through disobedience.'

It is Paul who first twists the myth - first by not recognizing it as a myth – and then making the disobedience as 'bringing' or causing death, when in the myth itself it is clear that 'Adam' was created  with death expected, and would have had to eat of that magic 'tree of life' in order to prevent it.

When the myth story has God saying "When you eat of the forbidden tree, you shall 'surely' die, the emphasis should be on the word 'surely'. Before that, it wasn't 'sure' yet, since that 'tree of life' was still accessible.

Mary being 'conceived without original sin' turns out to be quite factual, since there was no Adam and Eve, and so no sin. 'Original sin' is the myth.