While reading though Haruo Shirane’s Classical Japanese: A Grammar, I came across the following passage:
が started as an attributive case particle, became a subject particle, and then turned into a conjunctive particle. In the ancient period, the subject case particle did not exist. The typical structure was subject + predicate, as in 花咲く, or subject + adjective, as in 山高し. It was only beginning in the Muromachi period that the pattern 花が咲く and 山が高し, with が marking the subject, became standard.
While this makes sense looking at actual textual examples, I’m finding it difficult to understand the progression. Would anyone be able to explain what is the semantic basis for an attributive particle (or genitive case) taking on the role of subject particle (or nominative case)?
(As an aside, the same section in the same describes the uses of の as a case particle, which all correspond to the uses of が: subject marker, attributive marker, implied nominal, apposition. Is there some explanation as to why there would be two case particles which seem to serve largely the same purpose? And is this the underlying mechanism for the が/の alternation in modern Japanese attributive clauses?)