For quite a while now, I've had a question about historical linguistics that has driven me nuts. In many languages, you can see a gradual but inevitable move from case-based grammar to a more rigid sentence structure with fewer case endings, which have a tendency to drop off over time.
So if most languages are moving away from a declensional system, where did it originally come from? Why does it seem like no languages are developing a case system? I mean, it's not like people got together thousands of years ago and said, "Okay, let's create this thing called the ablative case, and it'll have this paradigm. . . ." That's just silly. But it wasn't until just last week that I finally got a reasonable answer.
Many sets of words have a tendency to slowly change from lexical words (words with a full meaning) into function words (words that do something in the sentence but don't mean much by themselves). Modal verbs (like
can and
will come from real verbs. Definite articles come from demonstrative pronouns. Relative pronouns often come from other kinds of pronouns (like demonstrative or interrogative pronouns).
And sometimes, as these words change meaning, they get shortened, too, because there is less stress put on them. For example,
will often becomes
'll and gets tacked on to the end of another word.
So let's hypothesize that you've got a language that puts a verb after nouns to indicate what function those nouns serve. According to
Wikipedia, in Chinese you would form the sentence "I travel from Shanghai to Beijing by airplane" as "I sit airplane originate Shanghai arrive Beijing travel." The verbs give the same sense of directionality that we get from prepositions. After time, these verbs may become function words, losing their status as full verbs and becoming postpositions (like prepositions, but they come after the noun). If they continue to erode, they'll become enclitics (words that cannot stand by themselves) and then eventually affixes.
Bingo! You've got case endings. Then those case endings eventually erode away, and you have to develop another system to convey the same information: prepositions and word order. If for some reason those prepositions erode away, too, then you've got to come up with another system again.
Of course, this still doesn't explain where language come from, but it's a start.