> "why would the brain evolve to benefit from a chemical it doesn't normally have?"
That's the thing, that's not how it works
The brain didn't evolve to be affected by substances unknown to it, it's plants that evolved substances that have CNS effects. (Though the body did evolve to depend on a lot of external substances - be that calorie and protein sources or vitamins)
Nicotine is literally a pesticide (and yes, people do use it as such in some occasions). Not sure how other substances evolved though (I think caffeine made the coffee seeds travel further)
That's the thing, that's not how it works
The brain didn't evolve to be affected by substances unknown to it, it's plants that evolved substances that have CNS effects. (Though the body did evolve to depend on a lot of external substances - be that calorie and protein sources or vitamins)
Nicotine is literally a pesticide (and yes, people do use it as such in some occasions). Not sure how other substances evolved though (I think caffeine made the coffee seeds travel further)