Olives! All olives are far too bitter to eat right off the tree, due to a chemical called oleuropein. Some clever person figured they were much better when soaked in water for many weeks. Eventually they learned that a salt brine solution did an even better job leaching the bitterness out. Treating the olives with oil and herbs enhanced the flavor.
Commercially, olives are usually treated with lye, which makes the process about a day long instead of a month or more.
I enjoy olives of all types, so I’m happy some ancient (Greek?) figured out how to make them edible.
Edit: As a few commenters point out, there is evidence that olives may have been used as food, at least to some limited extent, for thousands of years. Some have mentioned that olives can possibly ripen on the tree, or on the ground (if not gobbled by wildlife first), and that some olives may have ended up in brine or water naturally and led some early civilizations down the path of treating them before eating. Certainly using olives for oil has an important documented history of many thousands of years!