Why Evolve a Narcotic Effect?

[Zinjanthropos]

Why would hemp/marijuana/pot evolve to provide a narcotic effect? Is it a defence mechanism against being eaten, as in incapacitate its enemies? How poisonous is the hemp plant to other animals that eat it? Are animals prone to avoiding the plant in its natural environment or do the narcotic effects affect all animals that eat it such that they help disperse the plant's seeds by coming back for more. IOW animals find its intoxicating effects enjoyable too?

I suppose we have affected the plant's evolutionary path through selective cultivation and genetic engineering. By doing so have we benefitted the plant or weakened it with respect to its natural environment? 

Tried googling but there are so many pages dealing with humans and marijuana. I mean this plant must have a natural history too.

[C C]

Aside from how humans have selectively modified the original wild plants in the course of domesticating them, some substances humans use for drugs are the result of intense processing and refinement. In their feral, natural states these secondary metabolites that the plants use can be toxic in various degrees to herbivores and insects, or yield an unfavorable taste to them. When the caffeine from fallen coffee leaves contaminates the ground, it makes it difficult for other plants to grow there (driving out the competition).

There is a recognized paradox in these views, however. Consumers of the plants may have later adapted to the latter's defensive punishment.

https://anthro.vancouver.wsu.edu/people/...ug-reward/

Current theory of drug origins, in contrast, views almost all major drugs of abuse, including nicotine, cocaine and opiates, as potent plant neurotoxins that evolved to punish and deter herbivores. According to this latter view, plants should not have evolved compounds that reward or reinforce plant consumption. Mammals, in turn, should not have evolved reinforcement mechanisms easily triggered by toxic substances. Situated in an ecological context, therefore, drug reward is a paradox. We are investigating several potential resolutions of this paradox. For example, humans, like other animals, might have evolved to co-opt plant neurotoxins. Specifically, a propensity to seek out and consume toxic plants might have evolved as a means to fight pathogens.

[Secular Sanity]

Why Does Cannabis Produce THC?