As I drove to work this morning, I had the radio tuned to a discussion of the DEA's decision to hire translators for Ebonics (AKA Black Enlgish). A linguist was on the show explaining that Ebonics, while not a language in and of itself, is an internally consistent dialect of English*, largely derived from the dialects of English spoken by indentured servants who worked alongside slaves in early historic America. This led to one person calling in to the show and attempting to tell the linguist that Ebonics was not a dialect, but was simply "improper English."
But, well, for there to be a "proper language" you need two things that are rather new in human history: 1) Someone with the authority to dictate what is and is not proper (in the case of language, this is usually in the hands of academics or government ministers) which usually comes with the formation of a state (although it could come from other forms of organization), and 2) the ability to codify what is or is not proper, which until the 20th century could only be done with writing, and is still most practically done with writing.
For most of human history, we lived in communities where the exact dialect of a language would gradiate from band-to-band or village-to-village, and two groups who lived quite distant from each other would speak dialects that were similar to each other, but different enough that they could arguably be called different languages (think of Spanish, Italian, and Portuguese in the modern world). And languages change over time as well as space - Elizabethan English (what you would find in one of Shakesepare's plays) is a different dialect than modern English in the very same part of England in which Shakespeare lived and wrote.
Language is a living thing - costantly changing, constantly being altered. We all recognize this, and yet to most of us the idea of a "proper" form for a language seems intuitive. We seem to be able to accept that different nation-states will have their own proper form of a language (Spanish from Spain being different from Spanish from Mexico, and there are differences between British English and American English) - in fact the caller who insisted that Ebonics was not a dialect cited his experience as an infantryman traveling with the Marines, experiencing different dialects in different countries - but we seem to be unwilling to accept that there may be true dialects within each nation state. We accept regional accents and slang, but we regard full dialects as simply "improper" language, when, in fact, they are simply evidence of the normal evolution of language.
Which is not to say that there aren't many advantages to artificially slowing language change, creating an agreed-upon proper language, through both writing and education. I would say that the benefits outweigh any negatives that you could imagine, and it's an absolute necessity in modern nations. But, nonetheless, our adherence to an artificially glaciated language is anomolous. It's a tremendous innovation and a wonderful tool, but it's not the norm, it's a recent invention and the development of dialects such as Ebonics is the norm.
*I know that somebody's going to try to get me to argue about whether or not Ebonics belongs in the classroom. So, I'll just say this - that's beside the point of what I'm trying to discuss here, and I'm disinterested in discussing that matter, so piss off.