That might be due to the fact that, apparently, your phrasing is kind of sloppy…
Now back to my original question. Hypothetically, IF the Fall did not literally happen, more specifically, if sin wasn’t literally introduced into the world via one person (Adam),
Here’s the thing: if the story of the Fall is allegorical, that does not imply that “sin wasn’t literally introduced into the world via one person.” It simply means that the story that is used as the vehicle for the truth (i.e., the truth that sin was introduced into the world via one person) is not a story that’s based in historical fact. Your confusion seems to be that, if the story isn’t historical, then the allegorical meaning is somehow disproven. That doesn’t hold up. Let me use an example to illustrate what I’m saying:
Some have suggested that Tolkien’s “Lord of the Rings” is an allegory for the experience of World War II. (Tolkien himself replied that it is no such thing, but that’s immaterial for the sake of the argument I’m making here: let’s suppose, for the sake of the argument, that it is an allegory for World War II.) Your question seems to be presupposing that, if someone were to prove that there were never any such things as Sauron, Saruman, Gondor, elves, and hobbits, then that would demonstrate that the meaning behind the allegory – that is, the experience of World War II – is likewise disproven. Can you see that this makes no sense?
However, the question that does make sense – and which I’m hoping you’re trying to ask – deals with the question of a first human. The (somewhat more reasonable) question you might ask is “what if there was never one solitary first human, such that this first human was the source of original sin?” Now that would be an interesting question to ask!
Yet, you seem to be pointing to this question as if it were a natural outgrowth of scientific discovery. And, as it turns out, this isn’t a question that science is able to answer. Don’t get me wrong – science may certainly be able to posit that there was never a scenario in which there was only a single hominid (or a single breeding pair of hominids), but that’s not the question you’re asking, now is it? Since the definition of a ‘human being’ in a theological context is ‘an ensouled human person’, and since there is no way for science to quantify or identify a soul, then there’s no way that science can point to a specific instant in time and say, “see! there are hundreds or thousands of ensouled humans here at this time, not just one or two!” (Science could certainly make that claim about hominids, but not about ensouled humans.)
Might a person posit, then, that among the thousands of hominids in an initial breeding population, God only ensouled two (who, in the story of Genesis, are named ‘Of The Red Earth’ (i.e., ‘Adam’) and ‘Mother of All the Living’ (i.e., ‘Eve’))? Yes, it is possible to posit that! Does that suggestion imply polygenism? Nope… polygenism requires multiple ensouled humans.
Don’t get me wrong – I’m not asserting that this is doctrinal Church teaching; I’m just saying that it’s a possibility, logically speaking. And, if it’s a possibility, then one cannot rule out the (allegorical) truth of the Fall. And therefore, no – it’s not possible that a scientific assertion about a small breeding population of hominids can derail our theological statements about Original Sin, the need for Christ’s death, or substitutionary atonement.
In other words, strictly speaking there’s no need to address your question, since it makes a presumption that isn’t valid. (If you had asked, “if pigs had wings, could they fly to the moon?”, then what I’m saying here is that, since pigs don’t have wings, then it’s not necessary (or even possible!) to discuss the possibility of swine-o-nauts… )