Where do relative clauses come from?

Relative clauses (of the type ... le livre qui est sur la table..., la lettre que tu as ecrite...) appeared in Latin by a shift from the correlative structure quas litteras scripsisti, eae... 'quelle lettre tu as ecrite, elle...' to litterae quas scripsisti... 'la lettre que tu as ecrite...': the qu-word quas, whose meaning was to express a variable (quas litteras = a letter x), and which was initially a determiner of N, became pronomina(2)lized and anaphoric of the N that became its antecedent. This structure underwent a great development in Latin and passed onto French, in spite of the morphosyntactic transformations which occurred in late Latin: the relative pronouns (having an antecedent N) separated from the other qu- words to constitute a heterogeneous paradigm, borrowing from several logical principles: casual opposition qui subject / que direct regime, reinforcement by the adverbs ou 'where' and dont 'from where', Roman invention of lequel, late (17th c.) and partial return of the +/-H distinction after preposition. This heterogeneity does not prevent the extensive use of relative clauses in contemporary French.