Also, if you're in constant pain you'll try all kinds of random stuff to make the pain go away. If necessity is the mother of invention then desperation is its father.
Tea and beer are pathogen-depleted and so it makes a ton of sense that they were both imbibed 'to excess'.
History of the World in Six Glasses claims that tea and coffee more or less caused the enlightenment. One, it stopped day drinking by the intellectual class. And two, they were foil for socialization.
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.
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.
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.
The cool thing about tea leaves is that the caffeine is stored in crystals that are not particularly soluble. There's an organelle that contains an enzyme that breaks the crystals down into a soluble form.
There are a couple varieties of tea that are actually fermented, but for most teas it's a misnomer. You aren't causing fermentation, just autolysis (latin: self digestion), which is more akin to the malting phase of beer production. Black tea is left to process longer, while green tea is interrupted sooner. The switch is turned on by bruising the leaves, and off by desiccating them.
From a caterpillar's perspective, a tea leaf is booby trapped. Waiting for mandibles to mix the ingredients and create the insecticide.
Also, if you're in constant pain you'll try all kinds of random stuff to make the pain go away. If necessity is the mother of invention then desperation is its father.