The transatlantic flow of goods and people, sometimes called the Columbian Exchange, altered millions of years of evolution. Plants, animals, and cultures that had evolved independently on separate continents were now thrown together. Products introduced to Europe from the Americas included corn, tomatoes, potatoes, peanuts, tobacco, and cotton, while people from the Old World brought wheat, rice, sugarcane, horses, cattle, pigs, and sheep to the New. But Europeans also carried germs previously unknown in the Americas.

No one knows exactly how many people lived in the Americas at the time of Columbus’s voyages—current estimates range between 50 and 90 million. The European population in 1492 (including Russia) was around 90 million, the African population was around 40 million, and about 2ro million lived in China and modern-day India. Most inhabitants of the New World lived in Central and South America. In 1492, the Indian population within what are now the borders of the United States was between 2 and 5 million.

Whatever their numbers, the Indian populations suffered a catastrophic decline because of contact with Europeans and their wars, enslavement, and especially diseases like smallpox, influenza, and measles. Never having encountered these diseases, Indians had not developed antibodies to fight them. The result was devastating. Many West Indian islands were all but depopulated. On Hispaniola, the native population, estimated at between 300,000 and 1 million in 1492, had nearly disappeared fifty years later. The population of Mexico would fall by more than 90 percent in the sixteenth century, from perhaps 20 million to under 2 million. As for the area that now forms the United States, its Native American population fell continuously. It reached its lowest point around 1900, at only 250,000.

Overall, the death of perhaps 80 million people—close to one-fifth of humankind—in the first century and a half after contact with Europeans represents the greatest loss of life in human history. It was disease as much as military prowess and more advanced technology that enabled Europeans to conquer the Americas.

Another scene from the Florentine Codex depicts the smallpox epidemic that ravaged the Aztec capital after the arrival of Corte's.

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