What might the final sin look like, the un-original sin? This is pure speculation on my part, but there seems to be some hint in Scripture...
The very first command given by God to man is to "Be fruitful and multiply..." (Gen. 1:28 RSV) With rare exceptions, this command has been obeyed throughout human history, even by pagans and heathens. Indeed, fruitfulness has been considered a blessing, and sterility a shame in virtually every culture - until recent times.
Jesus, carrying his cross, spoke his final prophecy, "For behold, the days are coming when they will say, 'Blessed are the barren, and the wombs that never bore, and the breasts that never gave suck!'" (Luke 23:29 RSV) Those words must have seemed incredible to his hearers. Yet here we are.
The spurning of the first command, and the fulfillment of the final prophecy. Sounds rather apocalyptic.