The possibilities of soms "magic mushrooms" are known to Man since a long time. "Back in the day" when we didnt have skyscrapers and cars and all that fancy shit, and still foraged for berries, carrots and shot at deer, mushrooms were on the menu, known as edible or poisoinous or whatever. And of some they knew that they got you "very different". In fact, i believe there are accounts of Celtic warriors somtimes consuming them before battle, to gain "courage".
But really, it's not that hard to imagine humans know what plants "do".