My understanding (being a former RC) is that just what you have to believe if you're RC is a slippery slope.
It used to be that you couldn't believe in polygenism (the idea that humanity originated with several pairings), that you had to believe there was a literal Adam and Eve. Now it seems to be (I'm not clear on this though) that all you have to believe is that there was a literal "first man" as in "first soul" - so while Adam may not have been the first physical homo sapien, he was the first to receive a rational soul.
All of that, of course, is hogwash, imho. I'm really hoping I'm wrong about what the modern RCC requires people to believe, but that seems to be what I've read and been told by people who should know better.
As time goes on, the more willing I find myself to accept a "literalist" understanding of the early chapters of Genesis. The fads of modern scientism come and go, often fueled by quasi-religious ways of thinking amongst scientists themselves (who often have a slavish devotion to philosophical materialism and it's creation myth, "evolution"). I'm becoming more able of accepting that a basic surface reading of Genesis is pretty indicative of what actually happened, though recognizing it not to be "natural history" per se (more concerned with the "why" than the "how".)
The world before the fall existed in a way that we can hardly fathom. Our experience of this life is so conditioned by decay and death, that we cannot even imagine what this universe once was. When the Saints have experienced theoria, we get a glimpse into that pre-fall world; in such a state of grace, they did not suffer corruption, nor did they even need to pay heed to the things otherwise necessary to physical well being (sleep, eating, avoiding the harsh elements, etc.)
Well, such a state used to be the norm, for everything. Obviously, materialistic scientists, expecting the condition of the world to have always basically been like it is now, cannot possibly get an accurate picture of the past - their assumption of uniformity is just that, an assumption. It's arbitrary on a natural level, and according to the Holy Scriptures, it's incorrect (the universe was not always like it is now.)
Seraphim