I voted "other". Dunno if it is actually a "discovery", but I always figured that the written language was the greatest invention, creation, whatever of mankind. Imagine, being able to pass down knowledge from one generation to the next, and having the next generation build upon that knowledge. If it wasn't for written language, we wouldn't have recipes to brew beer, and would have to figure out fermentation and ect. all over from one generation to the next. But, I know that fermentation works, that it can be used to make beer, and that there are tried and true recipes out there just waiting for me to brew and possibly tweak upon, all thanks to the written language.
Allan