Shinto's recorded history is typically dated to the Kojiki and Nihon Shoki, but most sources suggest that oral traditions go back much further than this. What evidence is there to support this - asides from simply saying 'if it was recorded it must have had an oral tradition'?
The earliest origins of Shinto are lost to history, but it seems to have been established by the late Jomon period. Most likely, after the arrival of the earliest ancestors of today's Japanese, each tribe and area had its own collection of gods and rituals with no formal relationship between each of the areas. Following the ascendency of the ancestors of today's Imperial family to a position of power among the other groups, their ancestral deities were given prominence over the deities of other groups, though different systems continued to coexist.
The introductions of writing in the 5th century and Buddhism in the 6th century had a profound impact on the development of a unified system of Shinto beliefs. In a brief period of time, the Kojiki (The Record of Ancient Things, 712) and the Nihonshoki (The Chronicles of Japan, 720) were written by compiling existing myths and legends into a unified account.