It's one we need to answer alright. Kinda like what happened to Neanthernal man and the dinosaurs.
If there is a human civilization predating Adam then throw out the bible because it lied telling us he was the first man and the one through whom death through sin entered the world.
Wait, if death entered through Adam then this earlier people should still be alive not having the sin nature. 'Hold it boys', bring back the bible and chuck this other malarky.