Mickiel, Hey! Nice to meet you!
I have also been intrigued with this subject.
I believe that pre-adamites can indeed fit into the biblical scheme, verified with science and history. Here are some ideas to consider:
"it is not good that man should be alone: I will make him a helper comparable to him." Gen 2:18
That fact that God had to make Eve as a helper "comparable" to Adam implies that there were no women "comparable" to Adam, not that there were no women living at the time. The order of "kind after kind" had to be maintained. Possibly Adam could not "breed" ( I know, a crude term, but true) with another humanoid woman. A man and woman of the same kind or race are comparable.
"there was no man to till the ground" Gen 2:5. The literal translation to this is "there was no adam to till the ground". There were none of the adamic "kind" on earth before Adam was created,, but there were other humanoid "kinds".
Eve is called the "mother of all living" Gen 3:20 and Adam "the first man was of the earth". I Cor 15:45 & 47.
It is inferred that the verse which says Eve is called the mother of all living means she is the mother of all races. Then if we were to take this literally, then why can't we say that she is the mother of all things that breathe, i.e. all animals? All of these are "living" things. I believe it should mean nothing more than that Eve was the mother of all the Adamic race and not the animal humanoids that pre-existed.
The first man, Adam, is of the earth, earthy, and the second man, Christ, is the Lord. Now, if the "first" is meaning universally, then so is the word "second" which means Christ was the second human being on earth. We both know that that certainly isn't so! Those Corinthian verses are comparing Adam and Christ spiritually and not to imply that there were no other prior humanoids (pick your own term - caveman or whatever).
When God cursed and cast out Cain, God placed a "mark" on Cain, "lest any finding him should kill him." I believe Gods statement indicates other people existing at the time of Adam, Eve and Cain.
I have heard of other defenses arguing that no others could have possibly existed before Adam & Eve and then coming to this scripture, their arguments then seem to me distorted. One theory being that because of the longevity of Adam & Eve they bore many other sons and daughters and that sons married sisters, cousins etc. down the line and that it wasn't wrong or sinful because the incest law hadn't been established as of yet by God through Moses. And not only that, the purity of the blood at the beginning was free of diseases so that the issue of abnormalities we would normally be faced with today through interbreeding (like the Hatfields and McCoys that is being discussed on another thread) does not come into play. Those could be possible solutions to those in disagreement, but I think they are bogus. There is too much speculation to those kinds of views.
So, therefore, I see these and other verses do make sense in light of other people and races existing before Adam when proper interpretation and common sense is applied.
doug