When and where did atheism first start ?

[Angrboda]

Atheism predates religion. Humans learned believe in the nonsense that is religion.

As an interesting aside, this brings up the question if disbelief can predate the proposal that said thing exists. If it can, that would mean that you categorize nonbelief as a kind of "default." But doing this creates all kinds of problems.

It would mean that you "disbelieve" every future scientific theory that will be proposed. Who would assess their attitude concerning future scientific theories and come to this conclusion? Not me. I think that some future scientific theories will be correct, others will be incorrect. I'd have to hear/read about the theory first before I determine a potential belief status. Taking this way of thinking about it into mind, the "first" atheist would be the first caveman who disbelieved a "God story" after having heard it.

Perhaps, but I certainly lack belief in those future theories. In the sense that many if not most atheists apply it, atheism predates any claims or beliefs in religion by definition.