[neuromanser]

More like: This plant has no insect parasites, it must be special somehow, let's try and use it in different, increasingly "close approach" ways. I mean, humans must have figured quite early that on average, inhaling smoke of a poisonous plant has a fraction of the effect of chewing the same. Someone gets really sick after chewing some leaves, their family burns the rest of the "crop" and they get high.

[nerdponx]

It seems likely to me that any form of smoking was discovered incidentally by burning stuff that smelled nice, repelled bugs, repelled evil spirits, etc. and happening to inhale some of the smoke.

[hinkley]

Most likely.

For mushrooms it was probably desperation, but my head cannon for mushrooms is Trial by Mushroom: You can be expelled from our community for stealing, or you can eat this mushroom and if you survive, you get to stay.