The first cities date back to around 4000-3000BC, which is around five to six thousand years ago. Although this includes the entire period of written history, it is very short when compared either with the 300,000 to 400,000 years of human evolution or with the 40,000 years that Homo sapiens existed. By this we can conclude that the 6,000 years of urban existence represent only 1-2 percent of human existence on earth.

Although the earliest cities appeared in approximately 3500 in Southern Iraq (called Mesopotamia), but other urban civilizations later developed in such widely separated places as the Nile Valley (currently Egypt), the Indus Valley (Pakistan) and the Yellow River Valley (China). In all four cases, cities developed on rich river-irrigated land. This alerts us to one ecological prerequisite for the appearance of cities, namely, extensive agriculture.

There seems, however, to have been several possible routes to urbanization. While the earliest cities were found near rivers, late

r urban centers arose along major trade routes and at ceremonial sites. And in the case of Central America, whose cities developed even later, location on lakes as well as rivers occurred.

While the "invention" of cities did not require diffusion, the appearance of cities was associated with the development of more sophisticated means of transportation and with the expansion of control over a wide area surrounding the city. Both of thsese trends drew previously isolated societies closer together, permitting them to learn from one another. Although the archaeological record is still far from complete, every year additional evidence is discovered that places urbanization earlier in time, that suggests cities were denser and more numerous in certain regions than was previously thought possible, and that uncovers connections between centers of urban culture formerly believed to have been quite independent.

Thus, precursors to the Egyptian political institution of kingship, an institution that helped create urban developments at Memphis by about 2700BC, have now been found much farther upstream in Nubia and much farther back in time, Circa 3300BC. This suggests that the idea of the city did not diffuse to Egypt from Mesopotamia, but rather, that changes indigenous to the Nile Valley itself were sufficient to account for the rise of cities there. Eventually, however, contacts between the two river valley civilizations did occur, which greatly enhanced the culture of each.

On the other hand, Mesopotamian society apparently helped to diffuse urbanism elsewhere. On the land bridge of Iran archaeologists have excavated centers for long-distance trade that date back to the fourth millennium. The existence of these trade transit towns makes it easier to understand how Mesopotamia could have influenced the third millennium development of such Indus Valley urban centers as Mohenjo-Daro and Harappa, even though evidence of direct contact between the two centers of civilization is still modest. Connections between Mesopotamia and urbanizing regions of the north and west (in Syria and Anatolia, now Turkey) have also been coming to light. Italian archaeologists digging near Alepppo in northern Syria uncovered evidence of Ebla, a previously unknown urban commercial civilization that apparently rivaled Egypt by the second millennium. More and more smaller urban centers are being uncovered on the lands intervening.

Urbanization first developed, then, in a densely settled region that stretched from Anatolia and Egypt on the west to the Indus Valley on the east. Interlaced by long-distance trade routes, small local empires flourished, each supporting a few urban centers and many more small towns and villages. By 2500BC to 1700BC, remarkably advanced cities coexisted in different cultural regions and were in indirect, if not direct, contact with one another.

Toward the end of this period (Circa 1850 BC), a parallel development began in the Yellow River Valley of northeast China. Between 1750BC and 1100BC the Shang dynasty controlled much of northern China. At least one of the major capitals of their empire, the city of Cheng-Chou, has been excavated.

The New World, in contrast to Asia, lagged far behind. Evidence of that region's first towns dates back only to 300BC, when major urban centers appeared along the lakes of the Mexican central plateau. It has been traditional to treat these developments as completely independent from other urban centers, but more than many have argued that migrations from China may have influenced New World urbanization.

Even though cities appeared in a number of places, they were so rare and unusual in human history that their emergence must be explained. Facts

1. The City Emerged relatively recently.
2. Some 2000 years elapsed between its first appearance in Mesopotamia and the point in time when no one could say that this prototype had spread or made multiple appearances along the route between Egypt and China
3. Except for this admittedly large swath of terrain across Asia between the 20th to the 35 parallels, the world had no cities.

How then did cities come into being, and why did they appear where they did? In answering these questions we may come closer to understanding the essential nature of cities and the prerequisites for their development.