The alluvial plains of southern Mesopotamia in the later half of the fourth millennium B.C. witnessed a immense expansion in the number of populated sites. Scholars still debate the reasons for this population increase, which seems to be too large to be explained simply by normal growth. One site, the city of Uruk, surpassed all others as an urban center surrounded by a group of secondary settlements. It covered approximately 250 hectares, or .96 square miles, and has been called "the first city in world history." The site was dominated by large temple estates whose need for accounting and disbursing of revenues led to the recording of economic data on clay tablets. The city was ruled by a man depicted in art with many religious functions. He is often called a "priest-king." Underneath this office was a stratified society in which certain professions were held in high esteem. One of the earliest written texts from Uruk provides a list of 120 officials including the leader of the city, leader of the law, leader of the plow, and leader of the lambs, as well as specialist terms for priests, metalworkers, potters, and others. Many other urban sites existed in southern Mesopotamia in close proximity to Uruk. To the east of southern Mesopotamia lay a region located below the Zagros Mountains called by modern scholars Susiana. The name reflects the civilization centered around the site of Susa. There temples were built and clay tablets, dating to about 100 years after the earliest tablets from Uruk, were inscribed with numerals and word-signs. Examples of Uruk-type pottery are found in Susiana as well as in other sites in the Zagros mountain region and in northern and central Iran, attesting to the important influence of Uruk upon writing and material culture. Uruk culture also spread into Syria and southern Turkey, where Uruk-style buildings were constructed in urban settlements.
Recent archaeological research indicates that the origin and spread of writing may be more complex than previously thought. Complex state systems with proto-cuneiform writing on clay and wood may have existed in Syria and Turkey as early as the mid-fourth millennium B.C. If further excavations in these areas confirm this assumption, then writing on clay tablets found at Uruk would constitute only a single phase of the early development of writing. The Uruk archives may reflect a later period when writing "took off" as the need for more permanent accounting practices became evident with the rapid growth of large cities with mixed populations at the end of the fourth millennium B.C. Clay became the preferred medium for recording bureaucratic items as it was abundant, cheap, and durable in comparison to other mediums. Initially, a reed or stick was used to draw pictographs and abstract signs into moistened clay. Some of the earliest pictographs are easily recognizable and decipherable but most are of an abstract nature and cannot be identified with any known object. Over time, pictographic representation was replaced with wedge-shaped signs, formed by impressing the tip of a reed or wood stylus into the surface of a clay tablet. Modern (nineteenth-century) scholars called this type of writing cuneiform after the Latin term for wedge, cuneus.
Today, about 6,000 proto-cuneiform tablets, with more than 38,000 lines of text, are now known from areas associated with the Uruk culture, while only a few earlier examples are extant. The most popular but not universally accepted theory identifies the Uruk tablets with the Sumerians, a population group that spoke an agglutinative language related to no known linguistic group.
Some of the earliest signs inscribed on the tablets picture rations that needed to be counted, such as grain, fish, and various types of animals. These pictographs could be read in any number of languages much as international road signs can easily be interpreted by drivers from many nations. Personal names, titles of officials, verbal elements, and abstract ideas were difficult to interpret when written with pictorial or abstract signs. A major advance was made when a sign no longer just represented its intended meaning, but also a sound or group of sounds. To use a modern example, a picture of an "eye" could represent both an "eye" and the pronoun "I." An image of a tin can indicates both an object and the concept "can," that is, the ability to accomplish a goal. A drawing of a reed can represent both a plant and the verbal element "read." When taken together, the statement "I can read" can be indicated by picture writing in which each picture represents a sound or another word different from an object with the same or similar sound.
This new way of interpreting signs is called the rebus principle. Only a few examples of its use exist in the earliest stages of cuneiform from between 3200 and 3000 B.C. The consistent use of this type of phonetic writing only becomes apparent after 2600 B.C. It constitutes the beginning of a true writing system characterized by a complex combination of word-signs and phonogramssigns for vowels and syllablesthat allowed the scribe to express ideas. By the middle of the third millennium B.C., cuneiform primarily written on clay tablets was used for a vast array of economic, religious, political, literary, and scholarly documents.