But likewise it doesn't mean that all except for this one mutuation that resulted in us WON'T mutate either.
Some of the population will stay the same, and provided that niche still remains, the original species will as well. The intermediate steps along the chain from apes to humans either interbred or were outcompeted by modern humans. There are arguments for each case.
But wouldn't there still be some members of the ape species that would continue to evolve? Just because these others were "outcompeted" doesn't mean that the tendencies toward mutation and evolution are suddenly "afraid" to come out from that point on -- unless somehow both apes and humans developed genes that miraculously bred these tendencies to mutate out of both species. In other words, given the great lengths of time we're talking about, it follows that there was plenty of time for other mutations and species to evolve after these groups were "outcompeted" by the modern human, would they not?
Also, I am a Christian and I believe that God created life and mapped out the whole thing.
I do too, and it's quite possible that "evolution" is the means God chose for creation. With minor differences, the story in Genesis very closely parallels the sequence of the appearance of the various life forms that is conventional wisdom among biologists. I don't subscribe to the earth being 6,000 years old. After all, God, being the inventor of time itself, is by definition no slave to time and is therefore not in any kind of "hurry".
I do believe there was a Garden of Eden and that it really was perfect, and I hypothesize that the effect of Original Sin was retroactive in time, turning the original peaceful creatures into violent carnivores with flesh-tearing teeth, etc.
First of all, we know from fossil evidence that at least some different species of hominids coexisted for a period of time (in some cases, with very close proximity to one another). The fossil evidence indicates as much at several junctures since the divergence from the great apes (i.e., during the emergence of bipedalism).
Second, we know from multiple examples that it's not too uncommon for just one species to be extant out of a given genus. And, why stop at asking for a 'man-ape'? Why not ask for a "chimp/gorilla" or a "baboon/orangutan" or whatever? The request would be just as ignorant, but more self-evidently silly. Where are the rhino-elephants, and if you had one, would you be asking for a rhinohippophant? Why aren't you just as satisfied with a 'man-monkey' by your reasoning -- i.e., a chimp?
Third, our most recent hominid relatives had the 'misfortune' of occupying precisely the same ecological niche that we did. Our species has a tendency to kill things even if they're not especially bothersome, much less if they're competing with us for the exact same territory and resources. There's an excellent chance we have no hominid companion species because we killed most of the survivors off. Since our species had spread throughout the region possibly inhabited by previous hominids by no later that 80,000 years ago or so, we had plenty of time to finish them off (and we occupied most of the region by tens of thousands of years before that).
Fourth, unless there is geographic separation or some other kind of division, the bulk of a species will evolve in toto via genetic drift. Whatever advantageous traits that emerge will permeate the entire population over the decades or centuries when they haven't drifted far enough apart to inhibit mating. We have a fairly good sense of the degree to which our forebears were (or were not) isolated from one another. As an unsurprising aside, we also find that the most 'extreme' divergences are found where groups of hominids were evidently most distant from the 'core' of the evolutionary line.
Finally, we are obviously better adapted to our environment than would've been our progenitors. As we likely crowded them toward far less hospitable lands, they would've found it much more problematic to survive challenges. There is in fact paleontological evidence of this but I don't have citations on hand so I won't go into that. Combine this with the previous two points and it's not hard at all to accept that the only other surviving primates are those that were in the refuge of largely inaccessible wilderness.
Even if a group of, say, Homo erectus had managed to survive into recorded history, we likely would be reading accounts of how they were driven extinct not long after their discovery...
It amounts to groups of 'less advanced' hominids living in very marginal terrain for no apparent reason other than that 'more advanced' hominids evidently occupied more hospitable nearby territory. I don't have any sources handy as I mentioned before, so take my recollection about that fwiw.