From the earliest impressions on clay and the first scratchings on flat surfaces we've somehow gotten from the idea of a symbol for something to the idea of symbols for sounds.
It's a long and convoluted story and one that fascinates me. Besides the bookshelves of books on language I have a collection of books on how writing developed and changed over time. So it's a topic near and dear to my heart.
Lene passed along a link to a nice overview from ilovetypography. The origins of abc gives a overview of the history of writing. At least the history of writing words. I also found a good overview of the history of numbers in a powerpoint presentation by Tope Omitola and Sam Staton.
We've come a long way from impressions on clay yet it's amazing how similar things are. Turn ideas, words, and sounds into little pictures, pictograms, or symbols. Several thousand years later we've taken that basic idea and hopefully refined it and made it more expressive.