Some of the earliest signs of habitation in the area of Osaka were found at the Morinomiya remains , with its shell mounds, including sea oysters and buried human skeletons from the 5th–6th centuries BC. It is believed that what is today the Uehonmachi area consisted of a peninsular land, with an inland sea in the east. During the Yayoi period, permanent habitation on the plains grew as rice farming became popular.
By the Kofun period, Osaka developed into a hub port connecting the region to the western part of Japan. The large numbers of increasingly larger tomb mounds found in the plains of Osaka are seen as evidence of political-power concentration, leading to the formation of a state.