In diachronic comparison of languages, say PIE to Latin to Romance, it is a classic recognition that the later languages strictly lose some of the morphologically marked categories. PIE had 8 noun cases (nominative, accusative, genitive, etc), Latin 5, Romance 2 or even 1. Pick a morphological category and pretty much always the complexity is reduced: past participles in English are more likely to become weak rather than strong, the subjunctive is disappearing, there's no grammatical gender at all.
Presumably those forms (that were later lost) came from somewhere, those categories and phonological markers were created. I can imagine a cycle of inflected to isolating (a period of loss) and then back to inflectional where the grammatical markers get phonologically assimilated (fused onto the root), but i have no data to support this.
I feel like I heard a long time ago that Finnish/Hungarian/Turkish might be gaining distinctions or that the word initial inflections in verbs in Irish came from phonological interaction between a pronoun and the following verb, but those are just vague intimations. I am looking for more substantively presented examples.
Is there any definitive data of a language moving from isolating to inflected? Present day examples are best, but attested versions (not theoretical) from the past would be good too.