I think there’s a good way to look at this which harmonizes catholic dogma (monogenism/original sin) and modern scientific evidence. Saint John Paul II emphasized that it is acceptable to believe that the human body evolved by natural processes, but that we must believe that the human soul is a direct creation of God.
I tend to think that the first two humans, biologically speaking, evolved like other life by evolutionary processes. There may even have been a large pool of anatomically modern homo-sapiens who lacked souls, making them not actually human, supposing a soul is necessary to our humanity, which according to catholic doctrine is indeed the case. Perhaps out of this pool of homo-sapiens God elected two individuals, a male and a female, and created and infused into them immortal souls and entered into covenant with them.
The story of the fall, as I believe the catechism explains, depicts actual historical events in figurative language. I doubt the fall concerned literal trees, a snake, and fruit. These images convey the underlying message that our first parents defied God and chose to go their own way, thereby forfeiting the gifts of integrity and sanctifying grace which they had from God.
Since the fall we can imagine that the descendants of these two persons survived while the descendants of the non-ensouled homo-sapiens died out. Hence we are all descended from an original pair and yet this does not conflict with the evidence of population genetics which points to origins from an original pool of perhaps a few thousand.
We can also imagine that Noah’s flood took place a very long time ago, that it was a cataclysmic local flood either in north Africa or the Mediterranean Basin, which wiped out the early and localized human population except for Noah and his family. The purpose of the Ark with the animals would have been to preserve the biological diversity of the environment. It wouldn’t be contrary to Catholic exegetical principles to suppose that the account of the flood employed a good deal of hyperbole to describe an historical cataclysm, enabling us to accept a local event being described in universal terms.
To sum things up, it makes good sense to understand much of the early chapters of Genesis as describing real historical events in figurative language. The creation and the fall especially read this way, and the story of the flood seems to be filled with hyperbole. None of this negates the innerancy or infallibility of sacred scripture. And as Pope Leo XIII said, paraphrasing, we stick to the literal interpretation unless sound reason demands otherwise. With the discoveries of science over the past two centuries I think we are definitely at the point where reason requires us to understand certain components of Genesis as figurative historical accounts, rather than strictly literal.
Of course this is just one interpretation and Catholics are free to believe in a strictly literal interpretation of Genesis as well.
