Contents › Section 2: Morphology, Syntax, Lexicon › answer 1 of 37
Why are there inflections?
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I'm surprised nobody mentioned the concept of grammaticalization in this context. Asking why in linguistics is almost never a good question. But grammaticalization can certainly help explain how. (See http://en.wikipedia.org/wiki/Grammaticalization).
The process through which words become morphemes is described as the following cline:
content word → grammatical word → clitic → inflectional affix
Of course, there are many questions about when and how grammaticalization can happen - particularly when it comes to encoding complex concepts like dual or evidentiality. Peter Trudgill (http://youtu.be/rjy1CkH1FOE) recently proposed a hypothesis that certain concepts like these can only be grammaticalized in languages of small communities and tend to be lost through development in the context of expansion and contact.
Also, there is no way to trace the origin of a complex morphological system like that of Slavic or Bantu languages through this process. Nor is it possible to easily account for the detail of associated phenomena like vowel harmony in Turkic languages. RMW Dixon's adaptation of the punctuated equilibrium concept can perhaps give some hints of how these things can happen (http://www.amazon.co.uk/Rise-Fall-Languages-R-Dixon-ebook/dp/B004YXVVZM).
© Dominik Lukeš. Licensed under CC BY-SA 3.0. Originally published on Linguistics Stack Exchange, attributed there to Dominik Lukes (profile).