How did they hit upon this method? Was it an accidental discovery hundreds of years ago or was ot carefully arrived at through thinking how to make this kind of food?
Almost certainly it was an incremental discovery process, first cooking and/or fumigating tuna for other consumption, then extending its shelf life by fermentation and/or sun-drying, discovering that evening out the exterior with paste helps extend it, then solving the problem that extended drying leads to hard consistency by shaving off very thin slices, stuff like that. They didn’t set out with “oh we need very thin dried tuna slices for dashi and seasoning” and then invented a whole process for that.
I’m sure someone over smoked a fish and someone desperate tried to eat it. Then it was gradually improved.
Meatballs are ubiquitous, every culture has their version. My theory it was invented when we walked out of Africa 60,000 years ago and it went to all the ends of the world. Or maybe we stole it from those who were there before us ...