Just a thought Faith. We don't need an absolute answer for everything. There is plenty of mystery and ambiguity in the Christian faith.
I understand the concept of original sin this way. Everyone of us is born without knowledge of right and wrong. All we are born with is an instinct for survival.
That instinct for survival that we have at birth is a necessary part of survival. We cry when we need food or attention etc. It is all about us. So essentially we are born selfish.
As we grow and learn to distinguish right from wrong, selfish from unselfish, we are forced to make choices. With these choices we establish a pattern for our choices. We can continue to think selfishly or we can alter that basic instinct and start to think unselfishly.
Over an extended period of time our selfish or unselfish thoughts become habit and become part of who we are. As a Christian I believe that it is Gods Spirit in us that nudges us in the direction of unselfishness but we have the free will to ignore those nudges and maintain our selfish nature.
He has told you, O man, what is good ; And what does the LORD require of you But to do justice, to love kindness, And to walk humbly with your God.
Micah 6:8