Well, I don’t really know enough about linguistic history to answer the first question. I did see a text of the Oaths of Strassburg (which were sworn in then-contemporary Teutonic and Romance languages of the Frankish domains) once, but it was about a quarter of a century ago, and I don’t recall fine details. I can say, however, that Ciceronian Latin was strictly a literary language and, even in the first century BCE, the spoken language may have been quite a bit simpler. The general progression, IIRC, is that the vocabulary of one language is combined with the grammar of a second to form a “pidgin”, and that, when that pidgin is adopted as a “creole” (is spoken as a “milk tongue”), a “grammar template” that appears to be an unlearned attribute of the brain is used for it. I may well be behind on the current thinking, however.
Latin contains the preposition de, meaning “(down) from”, and which I believe takes the genitive case (most, although not all, other prepositions take the ablative); the evolution of Romance de’s from it seems simple enough. Likewise the evolution of a from ab and ad.
In accordance with the pidgin/creole hypothesis mentioned above, many linguists attribute the radical restructuring of a language, and especially the simplification of its grammar, with “interference”: its intimate interaction with another language (cf. English with High German or Icelandic). The noun structure of Baltic and, less so, Slavic languages is remarkably conservative (although the verb conjugations are much less so); some have interpreted this to mean that Proto-Balto-Slavic had very little interaction with speakers of other languages, and that therefore the Baltic littoral must be the Indo-European homeland. OTOH, it could just mean that Proto-Balto-Slavic speakers killed all of the natives before they could do more than yell, “Stop!” (or its equivalent in the local language).
I’ve often suspected that the interaction of languages was
a factor in the case of English. Though you didn’t mention it, I assume you refer to the devastation of the language that was brought about by the Norman Conquest. I wonder if
sarcastic Norman judges and officers, unable or unwilling to
learn correct English grammar, dropped into a crude hybrid when addressing some of the English landowners they sent packing…as in “Your lands is my land now”, sort of like
cliche examples of Ugly Americans trying to speak to
WRT grammatical complexity in general, have any laws been
proposed to explain why, no matter how complicated a language may seem as perceived by nonnative speakers, all normal children seem to learn to speak their own language
at about the same time in their development?
Not true at all. First of all, it depends on how you mean most. Most as in the ones most commonly used, or the sheer number of prepositions?
The only prepositions that take the accusative are those which indicate motion towards. Otherwise the accusative functions as a direct object, as the subject of indirect discourse, or adverbially. There is rarely any need to tag an accusative with a preposition.
Other languages do or at least did have declensions, German, for example. Most western European languages are Indo-European, and have the same structural roots. Literary languages are, well, literary. Compare Cicero to the “vulgate” Latin. Cicero, Caesar, et al. conciously and delibertly wrote in a formal,educated, and somewhat old fashioned manner - they were concerned with preserving the “Latinitas.” (Although the really educated people such as Caesar probably spoke Greek to their snooty friends). Ordinary people, of whom we are all descendants, did not sound like Cicero. The cosmopolitan nature of Rome, and the extent of the Roman empire, would have spread Latin far - but that does not mean everyone spoke it well - just enough to get by. When I was traveling as a student in Europe, I met some Turkish students who spoke a little German and French, but almost no English. We communicated in sentences that had a mixture of French, German, and Greek that must have sounded atrocious to a purist, but we got along just fine. So who cares? I agree with you Javaman about your example. The strain of communicating with your new, mostly illeterate, barbarian masters after the fall of Rome, and you get what must have been a “me Tarzan - you Jane” mode of Latin for a lot of people, and the necessity of the Romans to learn a little of the tongue of those smelly trouser wearing invaders.
According to “Vulgar Latin” by Jozsef Herman, even in Classical Latin, the simplification of the original system had already started. This goes back to Indo European, but, Classical Latin had fewer cases than Indo-European did. So, it was pretty much a gradual change from the parent language, and probably not much language interference per say (some scholars thought that the substitution of /h/ for /f/ in words like “facere” (hacer in Spanish) was attributed to interference of Euskara (Basque), but there are examples of this happening in places where the basques couldn’t have been).
Herman says of the phonetic changes…
Herman goes on to say that the loss of the declensions was in part due to simple confusions of useage. Herman says that it seems that the accusative seems to have been thought of as a kind of “prepositional case” and it usually resulted in it being used in Vulgar texts after prepositions that normally required nouns in the ablative (such as cum, as in “cum filios tres” (with his three children), a and ab, as in “posita a fratres” (“put up by his brothers”)).
So, even though the declensional system was lost, the use of pronouns made up for it, and allowed the language to express as much as it could when it had the declensions.
Anyway, it’s really quite a complex subject (way more than I can explain here). I suggest checking out “Vulgar Latin” by József Herman to read up more on this (the book is pretty thin, but it’s interesting (I think), and explains this situation in some detail).