> “Going the other way, there's plenty of molecules from plants and fungi that affect the human nervous system.” That is true. On the other hand it sounds like

> “Going the other way, there's plenty of molecules from plants and fungi that affect the human nervous system.” That is true. On the other hand it sounds like you are saying it is because “ Receptors and transmitters are highly conserved across the evolutionary hierarchy”. The most commonly used explanation for poisonous plants and fungi is that they make these chemicals as a defence mechanism. The mamal eats the thing, the mamal has a bad time and learns to avoid eating the thing. The thing thrives, which provides positive selection pressure for the fungi/plant which can make nasty chemicals. So it is not an accident that mamals react to the secreted chemicals, and it is not because “transmitters are conserved” or something like that.