But yeah we have traces of modern culture strewn about history, likely centuries if not millennia before modernism proper. I tend of think of modernism proper gaining steam with Luther, the printing press, Galileo, becoming philosophically canonized by the likes of Descartes, etc.
And then over the following centuries, this kind of thinking became normalized, IE lost its novelty as contradistinguished against Catholic cosmological dogma. This normalization of secular values and epistemologies resets the stage for post-modern culture to gain momentum, although again there are still cases to be made for early instances of postmodern thought, IE artists and thinkers who personally "passed through" these phases philosophically, following some sublime avant-garde instinct, afforded by the ability to distance oneself from what is normalized around them, and contextualize it all within a larger development, in order to probe beyond it.