Necessity is the mother of invention. One day somebody was just that hungry, a cassava plant was available, experimentation ensues, bam staple crop. It’s not that huge of a leap though. Most societies have some kind of root or tuber food, and once you’ve got the idea that roots and tubers can be food it’s not a huge stretch to go looking for others. Pretty much all of them have to be cooked at least to be edible and palatable.