The Church teaching on this is found in
Humani Generis, You may find
this tract from
Catholic Answers helpful. Here is an excerpt:
**"It is equally impermissible to dismiss the story of Adam and Eve and the fall (Gen. 2–3) as a fiction. A question often raised in this context is whether the human race descended from an original pair of two human beings (a teaching known as monogenism) or a pool of early human couples (a teaching known as polygenism).
"In this regard, Pope Pius XII stated: “When, however, there is question of another conjectural opinion, namely polygenism, the children of the Church by no means enjoy such liberty. For the faithful cannot embrace that opinion which maintains either that after Adam there existed on this earth true men who did not take their origin through natural generation from him as from the first parents of all, or that Adam represents a certain number of first parents. Now, it is in no way apparent how such an opinion can be reconciled that which the sources of revealed truth and the documents of the teaching authority of the Church proposed with regard to original sin which proceeds from a sin actually committed by an individual Adam in which through generation is passed onto all and is in everyone as his own” (Humani Generis 37).
“The story of the creation and fall of man is a true one, even if not written entirely according to modern literary techniques. The Catechism states, “The account of the fall in Genesis 3 uses figurative language, but affirms a primeval event, a deed that took place at the beginning of the history of man. Revelation gives us the certainty of faith that the whole of human history is marked by the original fault freely committed by our first parents” (CCC 390).” **
I never liked the Humani Generis teaching, and I simply can’t believe it. It doesn’t actually deal with the logically-necessary corallary – that therefore, when God said to humans, “be fertile and multiply,” He was functionally commanding the first and second and third generations after the original two progenitors to engage in incestuous sex.
I have seen discussions here and elsewhere make up their own “theology” to explain the impossible-to-explain corallary – the first few generations after the two original progenitors “did not need” the incest prohibition" because they were “closer to God” – I THINK meaning, “not yet as scurrilous as the rest of us.” However, the Church never taught, “Original Sin got serious-er and serious-er with each generation.” It seems to clearly teach that the “bite” of Adam’s sin was precisely the same for ALL generations.
Also, Scripture itself seems to indicate that IMMEDIATELY protection of siblings against incest would have been necessary.
In Scripture, what is the FIRST thing Adam and Eve did after the Original Sin? Answer: Cover their personal regions. In Scripture, what is the sin in the SECOND generation described there? Murder of a sibling.
In other words, they weren’t “closer to God.” They were horrible.
They DRASTICALLY needed the incest taboo to protect the sexual security of the female siblings. If the children of the initial progenitors (a) suffered from weakened sexual control mechanisms like their parents, and (b) they could KILL their siblings, they were certainly potential RAPISTS.
I believe that the Adam and Eve story is just a hypothetical showing what mankind WOULD BE like IF grace were not available, I.e., Original Sin is the innate alienation from God whih results when an intellect and a free will is placed in mortal flesh without grace, so that grace generated by the suffering and death of a Messiah becomes necessary.