And cannabis evolved to mimic the human body's naturally produced endocannabinoids - or perhaps the other way around.
It evolved to mimic mammalian endocrinal secretions because this is the method plants have always used to produce toxins to mitigate predation.
The plant evolves a chemical similar to a nervous system process in their predators in an effort to KILL OR DISABLE the predator.
I suspect if humans ate an entire marijuana plant (as might a herbivore) they would probably walk off a cliff or sit in a pasture until the wolves got them.
In the dosage humans take, it does little more than mess up their minds for a little while. (so far as we currently know.)
The THC produced by plants is a defense mechanism to make animals leave them alone, and here we stupid humans got to come along and start messing with the poor darlings.
Leave the plants alone! They don't want to get smoked! (or eaten!)