Although widely associated with Japan, the origins of sushi are actually believed to emanate from ancient China, where the first historical reference to sushi was made in the year 200 A.D. It is understood to have been introduced to Japan only some 500 years later (although the exact date is not clear). Sushi represented the art of preserving fish by a process of fermentation using salt and rice. The process involved pressing fresh raw fish, which had been cleaned, between layers of salt and rice, all weighed down with a stone. As it fermented, the rice produced lactic acid which effectively pickled the fish, preserving it to be eaten after a few months. In these early days, the rice was discarded and only the fish was consumed.
Sushi as we know it today probably evolved sometime during the 1700's in Japan, where the fermentation process was dropped, due to the idea of adding vinegar to the sushi rice. Two sushi styles emerged - Kansai from Osaka (more ornamental) and Edo from Tokyo.
In the early 19th Century, the idea of the sliced raw fresh seafood served with fingers of vinegar rice was conceived, again in Japan. It became an instant success and modern day sushi was born.
Today sushi comprises sushi rice, which is a white, short-grained rice mixed with a dressing of rice vinegar, sugar and salt, kombu (edible seaweed) and sometimes sake. The filling generally consists of fresh raw ocean fish such as tuna, snapper, conger eel, mackerel, squid, octopus, shrimp or salmon. The fish and the rice are bound together using nori - or seaweed, traditionally cultivated in the harbours of Japan. Vegetables, often pickled, can be radishes or soy beans. Essential condiments are soy sauce, wasabi (the very hot green stuff) and gari (pickled ginger).
Part of the West's obsession with sushi is no doubt due to its highly nutritious features, being generally low in fat, high in protein (from the fish), carbohydrates (from the rice) , vitamins and minerals (from the gari and nori).