small shaped pieces of stone that are clearly playing pieces go back much older than that, all over the world and found by archaeologists. Go has a rich prehistory of various kinds of games played with markers and pits or boards
Just likely to be race games or mancalas as types of Go, wouldn’t you think?
Or a game like hasami shogi / ludus latrunculorum. I wouldn’t be surprised if “hasami shogi” (which isn’t a type of shogi whatsoever) had been a precursor of Go, seeing as it uses custodial (two-stone) capture.
Not a difficult jump, one would think, to shift from a game of undifferentiated moving pieces capturing custodially to one of undifferentiated non-moving pieces capturing in the “double custodial” manner of Go.
I know of two detailed speculations on the origins of the game, Claire’s thread and Shotwell’s book, both of which I can link if you’d like. Claire idea is especially interesting, that Go is a product of sedentary culture and represents disputes over irrigation.
On when “modern Go” emerged, I remember hearing it claimed that early references to Go speak only vaguely of achieving a certain goal, like perhaps making a certain shape as in gomoku – gomoku and other connect-N games are another good explanation for early stones – rather than specifically of surrounding territory. But I don’t know how accurate that claim is.