"How in the world did indigenous peoples in the Upper Amazon come up with the idea of combining DMT with an MAO inhibitor? Many mestizo shamans will claim, of course, that the plants themselves taught humans how to do this. Other commentators point to some mysterious ecological wisdom found only in indigenous peoples. I think the answer is simpler. I think people were looking for a better way to vomit."
Now if I only read this passage for the first time then I may ask myself, what is Beyer's talking about? Well shamans often don't use plants for only the hallucinogenic effects, and most the time that ayahuasca is being used its not for seeing pretty objects or traveling to worlds (something that mestizo shamans don't do in the first place) but for the purge effects. Often ayahuasca is called the purge and for good reason. Purging has some great cleansing properties on the body and the end result is often less toxic substances or sickness. The shamans often relate this to cleaning the bad spirits out of the body as well.
"In the companion plants have many emetic properties of their own, it is plausible to hypothesize that ayahuasca vine and its companion plants were first combined in order to energizer or modulate their emetic and purgative effects, with the serendipitous result of creating and effective delivery form for DMT."