There are too many people interested in marketing their version of Japanese history that common Japanese knowledge gets lost in the propaganda. One high profile case is sushi since its well knew around the world, people are trying to link Sushi as originating from their country. So here is what I know:
Sushi actually has two origins. One is in rice balls and the other is in fermented fish. First, Japanese rice is sticky and rolling them into balls to make them handy to eat and carry as lunch has been going on for centuries. Likewise, it is common practice to put something inside or around these balls of rice like fish or pickles. One such type was sold as fast food at stalls during the samurai times. The rice ball looked very much like modern sushi but was much bigger, about a half bowl of rice per ball and the topping was cooked fish or shellfish, no raw fish was used at this period. This was popular street food and two balls would fill a person up. The other origin of sushi is in a fermented fish dish called funazushi. Salted carp are stuffed with rice and fermented for a year in barrels. The preparation was hard work and the product took a long time to prepare so that it would be expensive and difficult to sell as affordable street food but one person wanted everyone to enjoy funazushi so set about trying to find a way to make a funazushi like a meal that would be inexpensive and simple to prepare. He did many experiments but could not come up with a way to recreate the texture and taste of funazushi. Funazushi is like raw fish in texture like say smoked Salmon but it also has a sourness due to the fermentation. In trying to recreate the sourness, vinegar was added to the fish but this had the effect of acid cooking the fish which changed its texture. Finally, a solution was found by adding vinegar to a rice ball and then placing raw fish on top in the fashion of existing rice ball meals. This was a big hit and many street stalls sold this new sushi especially near fish vendors since fresh fish was conveniently available. This ultimately evolved into the expensive high-class cuisine it is today.