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 between 4000 and 3000 B.C.E. in the Fertile Crescent (see Map 1.3) 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 into Europe. At the height of the Roman empire in the early centuries C.E., dense global trade networks connected the peoples of Europe, Africa, and Asia. With the decline of Roman power in western Europe, those connections broke down, but commercial ties continued to thrive among diverse peoples surrounding the Mediterranean Sea. Because Portugal was denied access to that rich Mediterranean world, explorers there sought trade routes along the coast of Africa. Increased trade with African kingdoms by Portugal and other European societies included the purchase of slaves. This lucrative and devastating form of commerce would expand dramatically as European nations colonized the Americas.