The shamans of South America when asked how they came to discover how to brew ayahuasca(which contains DMT and a MAOI) they replied, "The plants led us to it?". They are stating that the plants spoke to them and told them how to make the psychedelic brew.
I always had the thought that they watched the animals and tried different combinations of what they saw animals consuming. There is no way to no for certain how they came up with the combination to make ayahuasca. There are thousands and thousands of plants in the jungle and they somehow picked two that create these profound psychedelic experiences.
Search for 'history of ayahuasca' if your interested in learning more. There is a plethora of information on the topic on the internet.
"In the universe there is an immeasurable, indescribable force which shamans call intent, and absolutely everything that exists in the entire cosmos is attached to intent by a connecting link." ~Carlos Castaneda