Timeline: 4000-2000 BC
September 2016 - Around 4000 BC, there began to be enough people in West Asia, Egypt, China, and Peru for them to live in small cities (about 10,000-50,000 people). These places began to choose kings to govern them, and then to form the first empires. They built the world's first big buildings, the ziggurat temples in West Asia.
New inventions also popped up all over. About 4000 BC, people in Central Asia domesticated horses, and by about 3700 BC they had also invented wooden carts with wheels. In South America, people started farming chili peppers, beans, sweet potatoes, and cotton. In Central America, they started growing avocados. All these people started to use more irrigation to water their fields. While before this people had mainly used tools of stone and wood, about 3000 BC some people began to use metal tools as well. The metal they used was bronze, so we call this the Bronze Age. In West Asia and Egypt, people invented writing to keep records for their new governments, and began to write down stories and poems like the Epic of Gilgamesh.
Around 3000 BC, some of the Indo-Europeans, who had been living in Central Asia, used their new horses and carts to move out of Central Asia, going both east and west. Some settled in Urumchi, in modern China, and others went west into Germany.
By 2500 BC, people were building the Pyramids in Egypt and in Kush, mud-brick temples in India and rammed-earth palaces in China. Along the coast of South America, the Valdivia and Norte Chico people were building stepped platforms too, and people built earthen mounds in North America.
But just before 2000 BC, there seems to have been a terrible drought - a water shortage - all over Asia and northern Africa. Governments collapsed all over the place. In Egypt, the Old Kingdom collapsed into the First Intermediate period. In India, the Harappan civilization came to an end. In West Asia, the Akkadian Empire fell apart. And further north, in Central Asia, the Indo-Europeans began their great migration all over Europe and South Asia.