By about 3200 B. C., humans were farming along the Indus River. The culture is called Harappan civilization because many discoveries were made near the city of Harappa. The culture of these people covered an area larger than either Mesopotamia or Egypt. By about 2500 B. C., these people began building the city of Mohenjo-Daro. In Mesopotamia, cities were a jumble of winding streets. In the Indus Valley, however, the builders carefully planned their cities. They made a grid of streets. They built an area called a citadel that was easy to defend. All the important buildings were here. They also had systems for carrying water and sewage. Because the houses were mostly alike, scholars think that the Indus culture did not have big differences between social classes. These early people left an important mark on the region. Some religious objects include symbols that became part of later Indian culture. Historians also think that the people of the area had extensive trade with people in the region and with the people of Mesopotamia. What conclusions can be drawn about Harappan civilization?