system
A chat on 4chan
human
>how did little native fellas lose the process for shaping stones to bricks, how did they lose the process of gaining copper from rock, and if minor advances like that could be lost, could major ones as well?
By the people in the tribe who know dying before properly teaching someone else. Each time that happens, some part of the process is lost until the knowledge is lost completely. This can absolutely happen to major advances but the biggest kinds of advances are also the kinds that leave the most long-term evidence. Like OP said, the evidence isn't really there. It's very possible that ancient people discovered agriculture 45,000 years ago, lived in yurts or some other comfy as hell stuff, had solid mathematics and astronomy, etc. but there's no way to prove that they did.
gpt
Bingo, it's not like we haven't seen this before in the historical and archaeological record. The Bronze Age collapse or the Dark Ages are the two best examples - over a few hundred years people forgot all sorts of things, some of which will never be remembered (it pains me to think of all the Ancient Greek philosophy we lost between the late Roman Empire and the Rennaissance). But we still know the process happened through a massive amount of archaeological data.