Surprise, surprise, the original form of sushi actually came from SE Asia, eventually making its way to the shores of Japan and accidentally becoming the sushi we know today. During ancient times, sushi was not a kind of food, but a way of preserving fish for fermentation, a verb instead of the noun we know today. This was called nare-zushi.
Fish was preserved in fermented rice and when it was ready for consumption, the rice was discarded and only the fermented fish was eaten. Consuming the fermented rice along with partially raw fish/partially fermented fish became popular during the Muromachi period, called sesei-zushi. This is where sushi began to be known not as a way of preserving food, but an actual dish. It wasn’t until the Edo period, at the beginning of the 19th century, that sushi became what it is today. Because of the popularity of food stalls in Tokyo or Edo, as it was called back then, sushi was made to be eaten easily and informally and this is the sushi we know today, called nigiri-zushi.
As a result of the Great Kanto Earthquake of 1923, many nigiri-zushi chefs lost their jobs, packed up their things, and set out for somewhere else to settle and practice their trade, eventually spreading the popularity of nigiri-zushi from Edo to the rest of Japan.
Interestingly enough, we can still try nare-zushi today, near Lake Biwa in Shiga prefecture, about an hour from Osaka, Japan. Carp from the lake are caught, scaled and gutted, and then packed with salt and stored for a year before being repacked annually in rice for up to four years. That’s a long time people! So if you go to Japan, forget about your goal to try fugu, (poisonous blowfish) and make a resolution to try nare-zushi!