I know it’s a rhetorical question—but I’m sure that someone discovered animal glues because they were cooking, left the animal products to cook in water, and then let it dry out.

You cook it because it’s supposed to be food, but sometimes you’re left with this thick, sticky substance. Voilà, glue.

I spend an inordinate amount of time wondering how people discovered amadou, https://en.wikipedia.org/wiki/Amadou as well as glue.

manioc is the one that really makes me wonder, it needs some pretty complex and time consuming processing before it's not poisonous.

Right- reminiscent of nixtamalization.