River valley civilization

Mesopotamia was one of the earliest river valley civilizations, starting to form around 4000 BCE. The civilization was created after regular trading started relationships between multiple cities and states around the Tigris and Euphrates Rivers. Mesopotamian cities became self-run civil governments. One of the cities within this civilization, Ur, was the first literate society in history. Eventually, they constructed irrigation systems to exploit the two rivers, transforming their dry land into an agriculturally productive area, allowing population growth throughout the cities and states within Mesopotamia.[7]

Ancient Egypt also created irrigation systems from its local river, the Nile River, more complex than previous systems. The Egyptians would rotate legumes with cereal, which would stop salt buildup from the freshwater and enhance the fertility of their fields. The Nile River also allowed easier travel, eventually resulting in the creation of two kingdoms in the north and south areas of the river until both were unified into one society by 3000 BCE.[7]

Much of the history of the Indus valley civilization is unknown. Discovered in the 1920s, Harappan society remains a mystery because the Harappan system of writing has not yet been deciphered. It was larger than either Egypt or Mesopotamia. Historians have found no evidence of violence or a ruling class; there are no distinctive burial sites, and there is not a lot of evidence to suggest a formal military. However, historians believe that the lack of knowledge about the ruling class and the military is mainly due to the inability to read Harappan writing.[8]

The Yellow River was settled in 9500 BCE. Many tribes settled along the river, sixth-longest in the world, which was distinguished by its heavy load of yellow silt and its periodic devastating floods. A major impetus for the tribes to unite into a single kingdom by around 1700 BCE (Erlitou culture, a Yellow River civilization) was the desire to find a solution to the frequent deadly floods. The Yellow River is often called "The Cradle of Chinese Civilization".

Caral-Supe (also known as Caral and Norte Chico) was a complex pre-Columbian-era society that included as many as thirty major population centers in what is now the Caral region of north-central coastal Peru. The civilization flourished between the fourth and second millennia BC, with the formation of the first city generally dated to around 3500 BC, at Huaricanga, in the Fortaleza area.[9] It is from 3100 BC onward that large-scale human settlement and communal construction become clearly apparent,[10] which lasted until a period of decline around 1800 BC.[11] Since the early twenty-first century, it has been established as the oldest-known civilization in the Americas, and as one of the six sites where civilisation separately originated in the ancient world.[12]