The massive transfer of biological species known as the Columbian exchange was one of the most important large-scale processes of the early modern world. In the wake of European maritime explorations, plants, animals, microorganisms, and human populations all migrated to lands distant from their original homes, with profound consequences for natural environments and human societies throughout the world. The question naturally arises: could widespread Eurasian travel during the Mongol era have sparked a similar round of biological exchange? 

Actually, biological exchanges had taken place long before the Mongol era. As early as the third millennium B.C.E., wheat and millet had found their way from Mesopotamia to China. During the early centuries after the emergence of Islam, between about the seventh and eleventh centuries C.E., a large-scale transfer of food and industrial crops took place as Muslim merchants, soldiers, administrators, diplomats, and missionaries traveled throughout much of the eastern hemisphere. Most crops originated in southeast Asia or India and traveled from their homelands to southwest Asia, the Mediterranean basin, sub-Saharan Africa, and western Europe. These crops included citrus fruits, melons, eggplants, artichokes, hard wheat, sugarcane, cotton, and indigo. A few plants traveled from sub-Saharan Africa to Asian lands: sorghum was the most important. By the time Mongols built their empires, this round of biological exchange, sometimes referred to as the "Islamic green revolution," was largely complete. 

Yet the Mongols probably facilitated one particularly important biological transfer-the spread of bubonic plague. The origins and early development of pandemic plague that rocked much of the eastern hemisphere are somewhat obscure. One prominent theory suggests, however, that Mongol armies helped spread plague from southwest China to northern China, then through central Asia to points west. Eventually, plague afflicted populations in India, Persia, Egypt, northern Africa, and the Mediterranean basin, with dramatic demographic consequences in all the lands it visited.