The complex societies that emerged in the Americas were made possible by an agricultural revolution that included the establishment of crop systems, the domestication of animals, and the development of tools. These developments had occurred 4000–3000 B.C.E. in the Fertile Crescent (see Map 1.2) in southwest Asia and in China. The increased productivity in these areas ensured population growth and allowed attention to science, trade, politics, religion, and the arts. Over millennia, knowledge from these civilizations made its way northward and westward into Europe, and at the height of the Roman empire in the early centuries C.E., Europe was part of dense global trade networks that connected the peoples of Europe, Africa, and Asia. With the decline of Roman power in western Europe, however, those connections broke down, and Europeans turned inward. It would take many centuries for European societies to recover. When they did, motivated by a desire to gain access to the riches of the East, they began to search for ways to regain their connections to the larger world.