Let’s start with the first three chapters of Genesis. This ancient story is about a person named God and two people named Adam and Eve.
The Catechism of the Catholic Church, Second Edition, paragraph 390 refers to our first parents based on the Council of Trent and subsequent teachings of Pius XII and Paul VI. While CCC 396 can be taken generally, CCC 397-398 is definite that there was one man who sinned. CCC 402 affirms the one man Adam. This goes back to St. Paul who also affirmed that there was one man in Romans 5: 12-21. At this point St.Thomas Aquinas gets in the act with DeMalo 4,1 which not only states one man but also says that the whole human race is in Adam “as one body of one man.” This reference along with the Council of Trent is in CCC 404. In connection with CCC 404, CCC 405 addresses Adam’s descendants. The Protoevangelum ("first Gospel) referred to in Genesis as being addresses to two people is in CCC 410. It is suggested to check the cross-references for CCC 410 which are in the margin.
Going back to CCC 389, there is this telling sentence.
“The Church, which has the mind of Christ, (1 Corinthians 2:16) knows very well that we cannot tamper with the revelation of original sin without undermining the mystery of Christ.”
By the way, as I check the footnotes, St.Paul’s teaching on one Adam (Romans 5-12-21) is also found in CCC 385 & 388 & 397. However, to get a better perspective, I suggest reading CCC 396 - 409. Actually to understand Adam’s human nature which is our fallen human nature, read CCC 355 - 421.