I wonder what the evidence is that the Proto-Indo-Europeans created nondual religion. The evidence of their culture is only indirect and has to be reconstructed from the later cultures, but the reconstructions point to polytheism, no?

https://en.wikipedia.org/wiki/Proto-Indo-European_mythology

I agree, though, that psychoactive drug use and other ways of inducing peak states of consciousness are likely the causes of religious experience and thus of radical rethinking of religious ideas. The Axial Age in the second millennium BCE is one origin of nondualism. The radical ideas could have come in waves, I suppose.

You might be interested in these articles of mine:

https://therabbitisin.com/the-psychedelic-memory-of-our-divinity-9f69a7cb22ee

https://medium.com/@benjamincain8/the-mythic-jesus-and-the-fragmentation-of-shamanism-e4f43ba187e3?source=friends_link&sk=7bb8df4030543a29d5a2c7d61a904f4d