Well, it’s exactly as you say — there’s no clear-cut evidence as for who discovered the non-dual nature of reality. But there are definitely some hints that point at a common origin dating way back before Axial Age.

The key points (just a few off the top of my head and in no particular order) are:

1. Despite the variety of religions, the thread of non-dual “teachings” running through all of them is almost word-for-word identical across all cultures. Sure, it says nothing about their date or place of origin, but…
2. Proto-Indo-Europeans introduced cannabis (also) into Levant.
3. Animistic societies were/are overall egalitarian (at least within their own tribe). So, what was the cause of a clear shift to stratified society 4000–3000 BCE? As far as we know, only “Northwest Coast Indians elaborated a hierarchical form of organization, or chiefdom [which makes them] the only hunter-gatherers to have done so.” and even that is nothing like what is at the foundation of our civilization.
4. Considering oneself a sole manifestation of god on Earth is strongly indicative of “enlightenment” gone wrong, i.e. “undigested” non-duality (thus also the Buddhist saying, “If you meet Buddha on the road, kill him”).
5. Nephilim — the Hebrew word itself literally means “fallen ones”, is usually translated as “giants”, but can also mean “warriors”, “the violent ones”, “the ones falling [upon their enemies]”, “sons of the rulers”, or “sons of the judges”. Well, if you ask me, that’s a pretty good description of “heroes of old, men of renown” (as per Genesis 6:1–4). If we go by known occurrences of large floods in Levant, it puts Nephilim right before 3000 BCE.
6. Nephilim were descendants of Levantine women and “sons of God”. Suspend your disbelief for a moment, and imagine reaction of animistic people in Levant, when a nomadic pastoral tribe of people claiming to be children of god (what to Levantine population translated as Elohim or Anunnaki) descended from Armenian highlands. Would they be considered fallen from the spirit world? Also, if we consider that the women were “taken” (common practice among tribes to this day, even among chimps) those “sons of God” were probably not perceived favorably.
7. The socioeconomic hierarchy is almost identical in all cultures that tried to incorporate non-duality into their original animistic worldview. At the top are priests, healers, teachers, etc. (what were shamans of the old), then come kings and warriors (the strong men protecting the priests), next are merchants and “owners”, followed by working class, and at the bottom are slaves, outcasts, and untouchables (i.e. all who are deemed almost “not human”).

And all these events roughly correlate with arrival of Proto-Indo-Europeans into Levant (and later their migration to India). I don’t think that it is all a mere coincidence.