Over a period of thousands of years, the symbols originally used for keeping track of goods evolved into the first writing system, Sumerian cuneiform. A. Three-dimensional tokens used to keep track of debts of grain and livestock eventually gave way to two-dimensional symbols on clay tablets. B. Writing was probably developed by farmers and artisans, since the symbols were first used to keep track of agricultural products and items produced by artisans. C. Two separate sets of symbols were used to keep track of each accounting of goods to avoid mistakes in the accounting when only one set of tokens or marks was used. D. Early tokens representing three-dimensional geometric shapes show that ancient Mesopotamians invented geometry as well as writing. E. The inventors of numerals were probably lower-ranking accountants because they were assigned the job of developing new methods of accounting for large inventories. F. Symbols were first used in a direct one-to-one correspondence with the commodity being counted, but eventually true numerals were developed.