As cities grew, they increasingly reached out to each other. Commerce and conflict exposed once-isolated populations to a diversity of culture, religion, philosophy, language, and technology, as well as disease. And major trade routes such as the Silk Road paved the way for massive growth throughout Afro-Eurasia.
Much of the Silk Road trade also took place by sea, between Roman Egypt and the west coast of India. Sailors discovered the "trade winds," which blow reliably from the southwest in the summer, then reverse direction in the winter. That way, the same ships could make the return journey carrying new cargo.
Though probably few, including the great explorer Marco Polo, travelled its entire 16,000 kilometers, the connectivity among cultures along its route held some of the greatest significance to world history. The associated bonds through trade and exchange became particularly important when world zones collided after 1492. Afro-Eurasia societies quickly dominated the rest of the world and led the modern revolution that followed.