The Church has defined that Adam and Eve were real persons and that all human-kind is descended from them inasmuch as we have all inherited their fallen condition.
The Church has not defined the historicity of the Flood narrative, Neither has she pronounced on it’s NON-historicity, so we are free to believe as we choose either way, both skeptics and fundamentalists notwithstanding.
Therefore, if we presume the historicity Adam and Eve AND of Noah and the Flood, remember that Noah was descended from Adam and Eve. So for all intents and purposes, it works out that we all are descended from that first couple. Actually, that would be true even if one does not accept the historicity of the Flood narrative.
Works for me.