There were human creatures, other than Adam's descendants that inhabited the earth. What is my proof ? The Bible. The whole Genesis narrative makes absolutely no sense whatsoever, unless there were non-Adamic people at the time Caine was cursed and banished by God. Who were the people of Nod where Caine found a wife ? Where did they come from ?
In order to make their speculative theory work, some Bible scholars erroneously suggest they must also have been descendants of Adam, perhaps later descendants. That assertion however doesn't make any sense from a biblical standpoint. Caine was cursed by God, so it logically follows any of Adams descendants, no matter how distant, or removed would have had to recognize Caine, and would have abided by God's decision to cast him out. There's no way Cain could have remained with them or taken a wife. That clearly wasn't the case.
In addition, this theory reverses the narrative in Genesis. The Bible clearly states Eve did not conceive another son to replace Able until after Caine went to the land of Nod and got a wife. The chronology of events described in Genesis do not support the conjecture and speculation of scholars who claim otherwise. I also find the idea that Adam's descendants procreated with angels totally absurd. The Bible says, the "sons of God found the daughters of man fair and took of themselves wives of all that they chose" (Genesis 6:2).
Nowhere in the Bible that I'm aware of, are angels described as the "sons of God", and who were these daughters of man ? Were they the descendants of Caine and the people of Nod ? I think that's a far more likely translation than baseless conjecture about angels and humans having sex, don't you ?
What about the story of Noah ? Noah sent a dove out from the ark, and it returned with a olive twig plucked off a tree. Where did the olive twig come from, if the entire surface of the world had been covered by water ? Noah would have immediately and intuitively recognized the flood was not a global, but a regional event. He also would have realized somewhere out there the primordial non-Adamic people Caine found refuge with had probably survived the flood. In Noah's mind, the flood and all its' associated death was completely pointless, because one of his sons ventured out into that world, just as Caine did, and the whole process of corruption was just going to repeat itself over again.
Hence, Noah plants a vineyard and becomes a falling down drunk. What other explanation is there ? Was Noah merely suffering form PTSD ? I don't think so. He faithfully did everything God had instructed him to do. He had no reason other than what I've described to have suffered such a debilitating mental breakdown. Does this fall into the category of conjecture ? Sure it does. Is it any less speculative or unbiblical than Caine marrying one of the descendants of Adam, or Adam's descendants having relations with angels ? Duuh ?!