Once upon a time there was no such thing as sushi. This was very inconvenient for anyone who had a craving for a really nice California roll. Of course, California hadn’t been invented yet either, so a lot of people wandered around not really knowing what they were craving.
Sometime in the 8th century (I wasn’t there, so I can’t be more exact), the Japanese figured out that fish would last if it was preserved and rather than mummifying it (which was not a very yummy flavor) they figured out how to preserve the fish in fermented rice.
Then people figured out you could eat both the rice and the fish and – ta-da! – sushi was born. California rolls would still have to wait a while.