
The same way we know a lot about other things in the chemistry domain - somebody tasted it, just to see what happens. "Mendeleyev's Dream" is a great book that contains many fascinating anecdotes of people going "let me just lick this, just to try". (Or, if you want to go more modern, the history of hallucinogens is pretty fun too)
Humans seem to have a strong tendency to stuff unknown things into their mouth, just because they can.
