Figure 4. Early Mesopotamian representation of a plow from a cylinder seal (drawn from the photo of a cylinder seal rolling in Dominique Collon, First Impressions: Cylinder Seals in the Ancient Near East [Chicago: University of Chicago Press, 1987], 146, fig. 616).

      This narrow strip of exceptionally fertile land produced bumper crops. But the surpluses depended upon building, maintaining, and operating the network of canals that watered the fields. Keeping the system going required both technical expertise and considerable organizational control, spawning the inseparable twins of bureaucracy and government. By about 5000 BC people with a relatively common culture in which a religious elite oversaw food production and distribution populated nearly all of Mesopotamia—the land between two rivers.

      All the good, fertile land in Mesopotamia was under cultivation by 4500 BC. There was nowhere else to expand once agriculture reached the coast. Running out of new land only intensified efforts to increase food production and keep pace with the growing population. About the time the whole floodplain came under cultivation, the plow appeared on the Sumerian plains near the Persian Gulf: it allowed greater food production from land already farmed.

      Towns began to coalesce into cities. The town of Uruk (Erech) absorbed the surrounding villages and grew to about 50,000 people by 3000 BC. Construction of huge temples attests to the ability of religious leaders to marshal labor. In this initial burst of urbanization, eight major cities dominated the southern Mesopotamian region of Sumer. The population crowding into the irrigated floodplain was now a sizable proportion of humanity. Whereas hunting and gathering groups generally regarded resources as owned by and available to all, the new agricultural era permitted an unequal ownership of land and food. The first nonfarmers had appeared.

      Class distinctions began to develop once everyone no longer had to work the fields in order to eat. The emergence of religious and political classes that oversaw the distribution of food and resources led to development of administrative systems to collect food from farmers and redistribute it to other segments of society. Increasing specialization following the emergence of social classes eventually led to the development of states and governments. With surplus food, a society could feed priests, soldiers, and administrators, and eventually artists, musicians, and scholars. To this day, the amount of surplus food available to nonfarmers sets the level to which other segments of society can develop.

      The earliest known writing, cuneiform indentations baked into clay tablets, comes from Uruk. Dating from about 3000 BC, thousands of such tablets refer to agricultural matters and food allocation; many deal with food rationing. Writing helped a diversifying society manage food production and distribution, as population kept pace with food production right from the start of the agricultural era.

      Rivalries between cities grew along with their populations. The organization of militias reflects the concentration of wealth that militarized Mesopotamian society. Huge walls with defensive towers sprang up around cities. A six-mile-long wall circling Uruk spread fifteen feet thick. Wars between Sumerian city-states gave rise to secular military rulers who crowned themselves as the governing authority. As the new rulers appropriated land from the temples and large estates became concentrated in the hands of influential families and hereditary rulers, the concept of private property was born.

      The few million acres of land between the Tigris and Euphrates rivers fed a succession of civilizations as the rich valley turned one conquering horde after another into farmers. Empires changed hands time and again, but unlike soils on the mountain slopes where agriculture began, the rich floodplain soil did not wash away when cleared and planted. Coalescence of Sumerian cities into the Babylonian Empire about 1800 BC represented the pinnacle of Mesopotamian organizational development and power. This merger solidified a hierarchical civilization with formalized distinctions recognizing legal classes of nobility, priests, peasants, and slaves.

      But the irrigation that nourished Mesopotamian fields carried a hidden risk. Groundwater in semiarid regions usually contains a lot of dissolved salt. Where the water table is near the ground surface, as it is in river valleys and deltas, capillary action moves groundwater up into the soil to evaporate, leaving the salt behind in the ground. When evaporation rates are high, sustained irrigation can generate enough salt to eventually poison crops. While irrigation dramatically increases agricultural output, turning sun-baked floodplains into lush fields can sacrifice long-term crop yields for short-term harvests.

      Preventing the buildup of salt in semiarid soils requires either irrigating in moderation, or periodically leaving fields fallow. In Mesopotamia, centuries of high productivity from irrigated land led to increased population density that fueled demand for more intensive irrigation. Eventually, enough salt crystallized in the soil that further increases in agricultural production were not enough to feed the growing population.

      The key problem for Sumerian agriculture was that the timing of river runoff did not coincide with the growing season for crops. Flow in the Tigris and Euphrates peaked in the spring when the rivers filled with snow melt from the mountains to the north. Discharge was lowest in the late summer and early fall when new crops needed water the most. Intensive agriculture required storing water through soaring summer temperatures. A lot of the water applied to the fields simply evaporated, pushing that much more salt into the soil.

      Salinization was not the only hazard facing early agricultural societies. Keeping the irrigation ditches from silting up became a chief concern as extensive erosion from upland farming in the Armenian hills poured dirt into the Tigris and Euphrates. Conquered peoples like the Israelites were put to work pulling mud from the all-important ditches. Sacked and rebuilt repeatedly, Babylon was finally abandoned only when its fields became too difficult to water. Thousands of years later piles of silt more than thirty feet high still line ancient irrigation ditches. On average, silt pouring out of the rivers into the Persian Gulf has created over a hundred feet of new land a year since Sumerian time. Once a thriving seaport, the ruins of Abraham's hometown of Ur now stand a hundred and fifty miles inland.

      As Sumer prospered, fields lay fallow for shorter periods due to the growing demand for food. By one estimate almost two-thirds of the thirty-five thousand square miles of arable land in Mesopotamia were irrigated when the population peaked at around twenty million. The combination of a high load of dissolved salt in irrigation water, high temperatures during the irrigation season, and increasingly intensive cultivation pumped ever more salt into the soil.

      Temple records from the Sumerian city-states inadvertently recorded agricultural deterioration as salt gradually poisoned the ground. Wheat, one of the major Sumerian crops, is quite sensitive to the concentration of salt in the soil. The earliest harvest records, dating from about 3000 BC, report equal amounts of wheat and barley in the region. Over time the proportion of wheat recorded in Sumerian harvests fell and the proportion of barley rose. Around 2500 BC wheat accounted for less than a fifth of the harvest. After another five hundred years wheat no longer grew in southern Mesopotamia.

      Wheat production ended not long after all the region's arable land came under production. Previously, Sumerians irrigated new land to offset shrinking harvests from salty fields. Once there was no new land to cultivate, Sumerian crop yields fell precipitously because increasing salinization meant that each year fewer crops could be grown on the shrinking amount of land that remained in production. By 2000 BC crop yields were down by half. Clay tablets tell of the earth turning white in places as the rising layer of salt reached the surface.

      The decline of Sumerian civilization tracked the steady erosion of its agriculture. Falling crop yields made it difficult to feed the army and maintain the bureaucracy that allocated surplus food. As their armies deteriorated, the independent city-states were assimilated by the younger Akkadian empire from northern Mesopotamia at the time of the first serious decline in crop yields around 2300 BC. During the next five hundred years the region fell to a succession of conquerors. By 1800 BC crop yields were down to a third of the initial yields and southern Mesopotamia declined into an impoverished