An excerpt from post 16.
Emphasis in bold is mine.
At my first reading of post 16, it appeared to be full of interesting speculations.

However, I wondered if the answer to – “what exactly is this parable of which you speak?” asked in post 20 – was in post 16.
So, I now have post 16 in hand. Up front it says: “As for “all their descendants”, the concept of “original sin” came from the philosophy of St. Augustine.” As a speculation, this “Augustine idea” is a day late and a dollar short when it comes to understanding the one and only Original Sin, named original because it was committed by the one and only original human.
Whoever composed the first three chapters of Genesis knew that Original Sin was at the dawn of history. That is why creation of the pinnacle of creation (humans) follows in order from Genesis 1:1. The creation ladder begins with the earth, a formless wasteland, and tops out at the crown of creation, you and me. After all the great works of the Creator in whatever manner was proper to the material/physical universe, it is only when the humans Adam and Eve appear that
human history could begin. And since the whole human race is in Adam as one body of one man (CCC 404), humans had to have a unified beginning and not just popping up here and there.
Catholics do not have to wait until St. Augustine in order to learn about the Original Sin concept.
Original Sin is right there in the first three chapters of Genesis. In addition, St. Paul, inspired by the promised Holy Spirit, clearly recognized the importance of the Original Sin in relationship to Jesus Christ. The truth of Original Sin is found in Romans 5: 12-21 which is a tad earlier than St. Augustine.