The aforementioned Professor Denise Schmandt-Besserat teaches art history at UT Austin. Her book details her theory regarding the origin of Cuneiform (sp?), the earliest known form of writing. To summarize (very loosely: I was a poor student); about five or six thousand years ago people (in Mesopotamia) were begining to make the switch from nomadic hunter-gatherers to permantly settled cultivators. The first manmade "permanent" structures were not human shelters, but rather storage units for things like grain and oil. In excavating these earliest settlements, archeologists contiually found small, simple, clay artifacts called, for lack of any idea of there origin or use, tokens. These artifacts came in numerous different shapes: balls, cones, rods, et al. They are roughly formed and bear the signs of being fired in an open fire rather then a kiln. Prof. DSB's theory is that these items were used to signify business transactions; the comunal nature of the storage facility demanded an accounting system. In theoretical practice, one would show up at a storage facility, deposit one's stuff, and recieve in exchange clay tokens which signified one's stuff and could be used at a later time to redeem one's stuff. At some point a clever person in need of a means of insuring that a transaction conducted by proxy could not be tampered with came up with the idea of sealing clay tokens up in a clay ball, to be busted open upon delivery (these have been recovered from sites also). The only drawback; you can't tell what's inside until it's broke. In response to this some other clever person came up the idea of incising marks on the outside of the clay ball to signify what was within. Obviously, this rendered the original tokens inside redundant and they quickly fell to disuse.
Like I said, this is a very rough paraphrase of DSB's theory, for the real deal, look for her book in UT library system.