History’s first empire rose  out of a hot, dry landscape, without rainfall to nourish crops,  without trees or stones for building. In spite of all this, its inhabitants built the world’s first cities, with monumental architecture  and large populations— and they built them  entirely out of mud. Sumer occupied the southern part  of modern Iraq in the region called Mesopotamia. Mesopotamia means “between two rivers”— the Tigris and the Euphrates. Around 5000 BCE, early Sumerians used irrigation channels, dams, and reservoirs to redirect river water and farm  large areas of previously bone-dry land. Agricultural communities like this  were slowly springing up around the world. But Sumerians were the first to take the next step. Using clay bricks made from river mud, they began to build multi-storied homes and temples. They invented the wheel— a potter’s wheel, for turning mud  into household goods and tools. Those clay bricks gave rise  to the world’s first cities, probably around 4500 BCE. At the top of the city’s social ladder were priests and priestesses, who were considered nobility, then merchants, craftspeople, farmers, and enslaved people. The Sumerian empire  consisted of distinct city-states that operated like small nations. They were loosely linked  by language and spiritual belief but lacked centralized control. The earliest cities were Uruk,  Ur, and Eridu, and eventually there were a dozen cities. Each had a king who served a role  somewhere between a priest and a ruler. Sometimes they fought against  each other to conquer new territories. Each city was dedicated to a patron deity, considered the city’s founder. The largest and most important building  in the city was this patron god’s home: the ziggurat, a temple designed as a stepped pyramid. Around 3200 BCE, Sumerians began  to expand their reach. The potter’s wheel found a new home  on chariots and wagons. They built boats out of reeds  and date palm leaves, with linen sails that carried  them vast distances by river and sea. To supplement scarce resources,  they built a trade network with the rising kingdoms in Egypt,  Anatolia, and Ethiopia, importing gold, silver,  lapis lazuli, and cedar wood. Trade was the unlikely impetus for the invention  of the world’s first writing system. It started as a system of accounting for Sumerian merchants conducting business with traders abroad. After a few hundred years,  the early pictogram system called cuneiform turned into a script. The Sumerians drafted up the first written laws and created the first school system,  designed to teach the craft of writing— and pioneered some less exciting innovations, like bureaucracy and taxes. In the schools, scribes studying  from dawn to dusk, from childhood well into adulthood. They learned accounting, mathematics, and copied works of literature— hymns, myths, proverbs, animal fables,  magic spells, and the first epics on clay tablets. Some of those tablets told  the story of Gilgamesh, a king of the city of Uruk who was also the subject of mythical tales. But by the third millennium BCE, Sumer was no longer the only empire around, or even in Mesopotamia. Waves of nomadic tribes poured into the region from the north and east. Some newcomers looked up to the Sumerians, adopting their way of life and using the cuneiform script  to express their own languages. In 2300 BCE, the Akkadian king Sargon conquered the Sumerian city-states. But Sargon respected Sumerian culture, and Akkadians and Sumerians existed side-by-side for centuries. Other invading groups focused  only on looting and destruction. Even as Sumerian culture spread, a steady onslaught of invasions killed off the Sumerian people by 1750 BCE. Afterward, Sumer disappeared  back into the desert dirt, not to be rediscovered until the 19th century. But Sumerian culture lived  on for thousands of years— first through the Akkadians, then the Assyrians, then the Babylonians. The Babylonians passed Sumerian  inventions and traditions through along Hebrew, Greek, and Roman cultures. Some persist today.