The most ancient sushi was very different from the modern sushi we recognize today. Before there was Edomae* nigirizushi, Osaka oshizushi and makizushi, common items in sushi bars, there was a more primitive version whereby fish was first wholly preserved in fermented rice and salt. When eating, the rice is thrown away.
While I am working on making a diagram for it, try to imagine a salted fish, covered in cooked rice and allowed to sit for weeks, months, even years. The process hasn’t completely died out, and has been replicated in Japan, being featured in regional cuisine and after looking at a few contemporary examples of funazushi, the first thing that comes to mind is “Mummy Sushi”, fish embalmed in rice and salt.
Edomaezush*, though made popular towards the end of the similarly named Edo period of Japanese history (the Tokugawa Shogunate era), is named so because the fish and seaweed used in preparation is taken right from shores of Edo or modern day Tokyo Bay. Translated into English, Edomae literally means “in front of Edo”.