What are the origins of the imperial line?

Just as it is unclear where the Japanese people came from and how Japan as a nation emerged, the origins of the imperial line are also unknown.
There are many hypotheses: Himiko of the Yamatai polity is the direct ancestor of the emperor; the emperor is descended from Yamatai's rival polity, Kunakoku; or the emperor was descended from a powerful clan from the Yamato region.
Yet, despite the different theories, what seems to be true is that the imperial line descended from a regime that would eventually lay claim to the Yamato Court following a power struggle among a number of polities.