There is a significant amount of research relating to tonogenesis -- the mechanisms by which a toneless parent language develops tone. But what about the genesis of pitch accent?
For instance, the falling tone in Middle Chinese is regularly derived from a reconstructed final *-s in Old Chinese (which is thought to have been toneless).
In historical Japanese linguistics, is pitch accent thought to have been part of the language from its earliest attested stages? Or is pitch accent thought to have arisen later?
Also, I appreciate that the realisation of pitch accent in Japanese varies by dialect. Is the pitch accent part of the "underlying form" -- that is, can the manifestations in the daughter dialects be derived regularly from rules applied to an underlying assignment of pitch accent?