Southeast Asia and China:
Very similar and sushi-like (Nare-zushi type) foods are still found in Southeast Asian countries (Thailand, Laos, Myanmar, Malaysia, etc.) now, and the plains of the north of Thailand and Myanmar, where people make their living by both rice cultivation and fishery - fishing in rivers and rice paddy fields, could be considered as the birth place of the 'original sushi' from well before Christ, and it disseminated to the south part of China, and Japan consequently.
The original concept of sushi is, by the aid of starchy rice, to preserve those of protein-rich foods, fresh water fish and flesh meat, which were not always obtainable through the dry and rainy seasons. This kind of preservation method could not be existed without the development of rice cultivation.
Natural fermentation is taken place when fish is kept long with millet or rice, starchy grains, and the generated lactic acid prevents from rotting. But in a long storage time, the rice part gets too soppy to eat and it was abandoned as a waste after all.
By getting well-off, people did not need a long storage period for preservation and also utilized the 'precious' rice part, then sushi became a dish to eat both the rice part and fish, in still semi-raw, together (Nama-nare type.)
In those countries, sushi has not much changed or improved its basic style till now, further it had completely disappeared in China by ca. 1800 eventually as the fact.