We still don't really know what kind of psychedelic plants were used as Soma in India and during the Eleusis rituals in Ancient Greek. I've read that they discovered some kind of psychedelic algae in the sea somewhere. But the biggest mystery is how the tribes from Amazonia discovered how to prepare Ayahuasca - a brew that uses two plants which grow in completely different places and has to be prepare very strictly to work - and they do that for thousands of years. If asked, they answer that plants told them how to do it :)