Sushi has its origins in China in the 4th century as salted fish placed in cooked rice to ferment. It may not have become popular until the 9th century in Japan. Early forms of sushi involved fermenting rice and salted fish together for months. Over time, methods developed where the fermentation period was shortened to a month or just one day. In the 18th century, forms like hako-zushi and maki-zushi that did not require long fermentation were created. By the 1820s, nigiri-zushi was developed using fresh rice, vinegar, and fish on top. Sushi spread to North America in the 1970s, with California introducing rolls