The first version of sushi, called narezushi (“matured sushi"), was originally a method of preserving large amounts of fish caught at the same time. It was made by spreading a layer of boiled rice on the bottom of a wooden cask, then covering it with a layer of salted fish, more layers of rice and fish, an inner lid, a stone weight and water to the top of the container. After about six months, the rice would ferment and lend a tangy flavor to the fish, which was eventually removed from the rice remains, sliced and eaten raw. You might call it the cheese of the fish world. (Or you might just call it gross. And really, I can’t blame you.) Narezushi, versions of which exist all over Southeast Asia, has been made in Japan for the past 1,000 years and is still being made in the Lake Biwa area, though no information exists as to why.
Around the fifteenth century, namanare-zushi (“raw-mature sushi”) appeared. It was the fast-food version of narezushi, ready to eat after fermenting for several days to a month. In that amount of time, the rice would ferment enough to take on an acidic taste, but still remain whole, and was eaten along with the fish. Unlike narezushi, it was only made in small quantities, usually for festivals and other special events.
In the late seventeeth century, someone thought, “Wait—this is still kind of gross. Why don’t we just add some vinegar to the rice instead of packing it in a box for a month?” and thus hayazushi (“quick sushi”) was born. It was later popularized in Edo as a snack food called nigiri-zushi, which is the sushi we eat today.
holy crap, i'm WAY down with the history section.