Tectonic forces broke up the ancient supercontinent of Pangaea about 180 million years ago, sending the Americas slowly drifting away from the continents of the Old World (Eurasia and Africa). From that time until 1492, the plants and animals living in the two hemispheres experienced continuous, divergent evolution. Periodic openings of the Bering Strait land bridge during Ice Ages did allow animals (humans prominent among them) to migrate from Asia to America, but despite these occasional contacts the human-shaped ecosystems of the two hemispheres developed very different characteristics.
The people of the New World cultivated a number of highly nutritious plant species, South American potatoes and Central American maize (a.k.a. corn) foremost among them. Thousands of years ago, the people of southern Mexico successfully bioengineered maize from strains of inedible teosinte grass. The Indian creation of corn, one of the world's most calorie-efficient grains, was one of the most significant technological accomplishments of ancient man; widespread cultivation of the highly nutritious corn eventually allowed the agriculturalists of the central Mexican plateau to support the huge, stationary populations needed to sustain a great civilization.
The New World's wealth in produce was offset by a poverty in domesticable animals. Indians throughout the Americas kept dogs, and the natives of the Andes tamed the llama, but no other animal species of the Americas proved amenable to domestication. Indian hunger for meat had to be met by hunting wild game.
In the Old World, the situation was quite different. Plant sources of carbohydrates were less robust, but Europeans, Africans, and Asians learned to domesticate a multitude of useful animals—horses, pigs, cows, oxen, chickens, sheep, goats, and camels, among others—which provided them with meat, milk, clothing and transportation.
The drawback of Old World civilizations' reliance upon domesticated animals came in increased incidence of disease. Many of the world's nastiest illnesses derive from bugs that have leapt back and forth between people and their animals. Humans caught smallpox from their cows, influenza from their fowl, bubonic plague from the rats who lived in their houses. By the time of Columbus, the Old World was wracked by endemic contagions of dozens of deadly diseases, which kept life expectancies low and infant mortality rates high. Largely due to the ravages of disease (especially bubonic plague), the population of Europe in 1492 was lower than it had been 200 years earlier.
Columbus's ships, ferrying people, plants, animals, and diseases between the Old World and the New, instantly reconnected ecosystems that had developed in complete isolation from one another for millennia. According to historian Alfred Crosby, who developed the concept of the Columbian Exchange, the voyages of Columbus and his successors re-knitted the torn "seams of Pangaea," which had ripped apart by continental drift millions of years before. Suddenly, the separate ecologies of the Eastern and Western Hemispheres merged into a single, worldwide, man-made ecosystem. The ecological effects were dramatic, and—in ways entirely unforeseen and misunderstood by both Europeans and Indians at the time—they systematically favored the peoples of the Old World in their encounters with Native Americans.
The profoundly uneven nature of the Columbian Exchange colored all subsequent American history.