Many theories, such as a theory stating that the appearance of large keyhole-shaped tomb mounds seen in and after middle 3rd century suggests the establishment of unified government in the Japanese Islands, and the dynasty established at that time were the ancestors of the Imperial Family, a theory regarding the lineage of Himiko of the Yamatai Kingdom, if it existed in the Kinki region of the Yayoi period, as the ancestors of the Imperial Family, and a theory stating that the dynasty of the ancestors of the Imperial Family was established in 4th century, has been presented, and has not been settled.
3世紀中葉以降に見られる前方後円墳の登場は日本列島における統一的な政権の成立を示唆しており、このときに成立した王朝が天皇家の祖先だとする説や、弥生時代の近畿地方にあった場合の邪馬台国の卑弥呼の系統を天皇家の祖先とする説、天皇家祖先の王朝は4世紀に成立したとする説、など多くの説が提出されており定まっていない。