While human civilization developed in many places around the world, it first emerged thousands of years ago in the ancient Middle East.

“We see the first cities, the first writing and first technologies originating in Mesopotamia,” says Kelly-Anne Diamond, a visiting assistant history professor at Villanova University, whose expertise includes ancient Near Eastern history and archaeology.

Mesopotamia’s name comes from the ancient Greek word for “the land between the rivers.” That’s a reference to the Tigris and Euphrates rivers, the twin sources of water for a region that lies mostly within the borders of modern-day Iraq, but also included parts of Syria, Turkey and Iran.

The presence of those rivers had a lot to do with why Mesopotamia developed complex societies and innovations such as writing, elaborate architecture and government bureaucracies. The regular flooding along the Tigris and the Euphrates made the land around them especially fertile and ideal for growing crops for food. That made it a prime spot for the Neolithic Revolution, also called the Agricultural Revolution, that began to take place almost 12,000 years ago.

That revolution “transformed human life across the planet, but it was in Mesopotamia where this process began,” Diamond explains.

With people cultivating plants and domesticating animals, they were able to stay in one place and form permanent villages. Eventually, those small settlements grew into early cities, where a lot of the characteristics of civilization—such as concentrations of population, monumental architecture, communication, division of labor, and different social and economic classes—developed.

But the emergence and evolution of civilization in Mesopotamia also was influenced by other factors—in particular, changes in climate and the natural environment, which compelled the region’s inhabitants to become more organized in order to cope.

Civilization didn’t develop in exactly the same way throughout the region, according to Hervé Reculeau, an associate professor of Assyriology at the University of Chicago and an expert in the history of ancient Mesopotamia. As he explains, urban societies developed independently in Lower Mesopotamia, an area in what is now southern Iraq where the early civilization of Sumer was located, and Upper Mesopotamia, which includes Northern Iraq and part of present-day western Syria.

One factor that helped civilization to develop in both places was the climate of Mesopotamia, which 6,000 to 7,000 years ago, was wetter than that part of the Middle East is today.

“The earliest cities of southern Mesopotamia developed on the margins of a great marsh that provided an abundance of natural resources for construction (reed) and food (wild game and fish), with water easily accessible for small-scale irrigation that could be organized at a local level and did not require the supervision of large-scale state structures,” Reculeau writes. Additionally, he notes, the marsh provided a connection to sea routes on the Persian Gulf, which made it possible for people who lived in the south to eventually develop long-distance trade with other places.

In Upper Mesopotamia, the rainfall was reliable enough that farmers didn’t have to do much irrigation, according to Reculeau. They also had access to mountains and forests, where they could hunt for game and cut down trees for wood. Their areas also had land routes to places to the north beyond the mountains, where they could obtain materials such as obsidian, a type of rock that can be used in jewelry or for making cutting tools.

According to the British Museum, early Mesopotamian farmers’ main crops were barley and wheat. But they also created gardens shaded by date palms, where they cultivated a wide variety of crops including beans, peas, lentils, cucumbers, leeks, lettuce and garlic, as well as fruit such as grapes, apples, melons and figs. They also milked sheep, goats and cows to make butter, and slaughtered them for meat.

Eventually, the agricultural revolution in Mesopotamia led to what Diamond describes as the next big step in progress, the Urban Revolution.

Roughly 5,000 to 6,000 years ago in Sumer, villages evolved into cities. One of the earliest and most prominent was Uruk, a walled community with 40,000 to 50,000 inhabitants. Others included Eridu, Bad-tibira, Sippar, and Shuruppak, according to the Ancient History Encyclopedia.

The Sumerians developed what may have been the earliest system of writing as well as sophisticated art, architecture, and complex government bureaucracies to supervise agriculture, commerce and religious activity. Sumer also became a hotbed of innovation, as the Sumerians took inventions that other ancient peoples developed, from pottery to textile weaving, and figured out how to do them on an industrial scale.

Meanwhile, Upper Mesopotamia developed its own urban areas such as Tepe Gawra, where researchers have discovered brick temples with intricate recesses and pilasters, and found other evidence of a sophisticated culture.

According to Reculeau, climate shifts may have played a role in the development of Mesopotamian civilization. Roughly around 4,000 B.C., “the climates slowly became drier and the rivers more unpredictable,” he explains. “The marsh retreated from Lower Mesopotamia, leaving behind settlements now surrounded by lands that needed to be irrigated, requiring added work, and possibly greater coordination.”

Because they had to work harder and in a more organized fashion to survive, Mesopotamians gradually developed a more elaborate system of government. As Reculeau explains: “The bureaucratic apparatus that appeared first to manage the goods and people of the temples in the marshland cities increasingly became the tools of a royal power [that] found its justification in the support of the gods, but also in its ability to get things done.”

That all led to the development of a social structure in which the elites either coerced workers or obtained their labor by providing meals and wages.

“In a sense, the famed Sumerian agrarian system, its city-states and the associated control of land, resources and people were in part the result of people adapting to more adverse conditions, because the riches of the marshes had started to become more scarce,” Reculeau says.

In Upper Mesopotamia, by contrast, people coped with a drier climate by going in the opposite direction socially. That area saw “the devolution to a less complex social organization, relying on villages and their small-scale solidarity,” Reculeau explains.

Mesopotamia eventually saw the rise of empires such as Akkad and Babylonia, whose capital city of Babylon became one of the largest and most advanced in the ancient world.