Egami Namio (1906-2002), was a famous Japanese archaeologist who stunned the Japanese academic world in 1948 when he propounded his audacious "Horserider Invasion Theory", suggesting that the emergence of a unified, centralized state in 5th century Japan could best be explained by a sudden invasion by a horse-riding race of people from continental northeast Asia.
In Egami's view, this Horserider theory was the best way to explain what he viewed as the sudden emergence of a centralized state in Japan, as well as what he understood to be the fairly sudden appearance of images of horses and horse-related implements such as bridles and saddles in the archaeological record where previously they had been absent. Egami also tied the appearance in Japan of massive earthen tombs, or kofun, to the arrival of these supposed "horseriders."
What made Horserider theory so shocking to the Japanese establishment was that in 1948 most Japanese still believed strongly in the pre-war dogma that the Japanese were a unique race that lived in the Japanese isles since time immemorial. The idea that a group of marauding horseriders conquered Japan in the 4th century and established the Yamato state was also a full frontal assault on the cherished myth of an unbroken line of Japanese emperors going back to the god-emperor Jimmu in 660 BC.
Nevertheless, Egami seemed to have lined up a large amount of evidence behind his theory, even if that evidence was all circumstantial, and his theory came to be widely accepted among Japanese archaeologists and leftists, even as it was roundly and angrily rejected out of hand by the conservatives on the right.
Over the course of the 1960s and 70s, however, as gaps in the archaeological record were increasingly filled in, it became increasingly clear that there had never been a sudden invasion of Japan by a race of horseriders, as the introduction horse-riding technology to Japan and the evolution of the Japanese state were increasingly and convincingly shown to have been gradual events which occurred over hundreds of years. By the 1980s, horserider theory had come to be almost universally rejected by all credible scholars, in a turn that was energetically celebrated by the Japanese right, who felt vindicated.
It had also become increasingly clear, however, that even if there had been no sudden invasion, the formation of the Yamato state was intricately tied to gradual but increasing interchanges with continental Asia, particularly Korea, and that indeed the Japanese Imperial family is in all likelihood of Korean ancestry. This interpretation is now widely accepted, and thus, even though he got the timescale wrong, Egami was essentially right about the continental origins of the early Japanese state, and the myth of the unbroken line of Emperors did not survive "Horserider Theory" after all.