Things moving in one direction wouldn’t preclude the existence of cycles. For example: a language has noun cases -> phonological changes cause the endings to m

Things moving in one direction wouldn’t preclude the existence of cycles. For example: a language has noun cases -> phonological changes cause the endings to merge together, requiring prepositions to disambiguate -> prepositions become more tightly bound to the words they modify and are eventually reanalyzed as case markings. Now we’re back with cases even if languages can only take each step in the given direction. > Incidentally Chinese did not evolve from Proto-Indo-European. You’re right, but I meant to imply that PIE is the oldest language whose grammar we know a lot about, not that it’s the ancestor of every modern language.