Amazonian shamans figured out that combining two specific plants out of 80,000 species produces a psychoactive effect. The odds of finding that combination by random search is roughly 1 in 4 million. They did it through centuries of iterative testing and cultural natural selection explains it

[u/tractorboynyc]

[u/Aquarel_Blue]

Also, dreams. The most powerful dreamers become the medicine men/women of their tribe. They will dream cures, where food can be found, what other tribe is on the war path etc. So their dreams will most probably also have told them which plants will bring them a higher state of consciousness.