Vowel Harmony as evidence of historical change

As a process of assimilation which is common in human languages, pretonic vowel harmony, such as happens in menina ~ minina, coruja ~ curuja, has been found in European Portuguese since its infancy, developing itself with the language over time half way through the eighteenth century, declining since then until its disappearance in European Portuguese in the nineteenth century, continuing, however, in Brazilian Portuguese. It can be seen as a watershed ever since: Brazilian Portuguese preserving the pretonic harmony on the one hand, and European Portuguese neglecting it, on the other hand; to dialects of the same language.