As of 1492, the Americas and Eurasia-Africa, except for occasional connections via the Bering Strait, had been separated for millions of years. During this time, organisms diverged in their evolution, and human beings on either side of the Atlantic developed their own ways of life, including their own crops, domesticated animals, and diseases. When the voyages of Christopher Columbus and other European sailors established contacts between these great land masses, they triggered an exchange of life-forms with massive consequences for the human populations and for the entire biosphere. Weeds, crops, animals, and germs comprised some of the categories of significant exchange.

The Old World had more species of what we call weeds, that is, plants equipped to spread swiftly on disturbed soils, because it was much larger in area and because it had more species of grazing animals, particularly of domesticated ones, to whose teeth and hooves Eurasian and African grasses and herbs had been obliged to adapt. Thus, many of America's most aggressive weeds, especially in the temperate zones, come from European origins: dandelions, crabgrass, wild oats, sow thistle, kudzu, tumbleweed, plantain, cheat grass, and many others. Only a few weeds—armaranth and Canadian water weed, for example—went the other way and established themselves east of the Atlantic.

In 1492, the native crops of the Old and New Worlds were entirely different, with the exception of cotton, which was cultivated on both sides of the Atlantic. The most important Native American cultivars were maize, white potatoes, sweet potatoes, and manioc or cassava. By the late twentieth century these crops accounted for roughly one-third of the world's food production. Among the more important Eurasian-African crops (a longer list, as would be expected of a much larger area) were wheat, barley, rice, sugarcane, oats, rye, soybeans, bananas, carrots, cabbage, and oranges.

The Eastern and Western Hemispheres also exchanged many kinds of wild animals. For instance, black and brown rats were brought to America, and gray squirrels and muskrats to Eurasia. The exchange of domesticated animals, however, was more important. Amerindians were much less effective as animal domesticators than their European counterparts, possibly because they had fewer domesticable animals to work with. Their livestock—llamas, guinea pigs, turkeys, and dogs—included none that were ridden, pulled heavy loads, provided hides or fertilizer in significant amounts beyond the local level, or supplied large quantities of nourishment (i.e., meat and milk) for human consumption. The domesticated animals of Eurasia-Africa, which included horses, cattle, sheep, goats, pigs, donkeys, and chickens, were a major source of nourishment, leather, fiber, power, and fertilizer. After 1492, these domesticated animals were gradually introduced in the Americas.

Most of the troublesome human diseases originated in the Old World. America had its own, like Chagas's disease and something akin to syphilis, but nothing as appalling and influential as the Old World's smallpox, measles, whooping cough, chicken pox, bubonic plague, malaria, yellow fever, diphtheria, amoebic dysentery, and influenza. With its greater area and bigger animal and human populations, Eurasia-Africa was also home to more kinds of germs. In addition, human beings had lived in dense populations, seedbeds for germs, for much longer in Eurasia-Africa than in the Americas. They had practiced irrigation agriculture for much longer, creating conditions for the propagation of waterborne infections, and had traded longer and more intensively, ensuring the wide diffusion of infections. Above all, they had for thousands of years lived among domesticated animals and vermin-carrying creatures like rats, with which they shared and mutually cultivated a great number of pathogens. The germs of smallpox, measles, influenza, plague, and other of humanity's historically most important maladies are very similar to those carried by Old World animals or even, in the case of plague, exactly the same.

No American disease figured importantly in the Old World, with the possible exception of syphilis, a venereal disease that many claim Europeans acquired from the Amerindians of the West Indies during the first Columbian expeditions. Skeletal evidence indicates that syphilis, or some infection like it, was indeed present in the pre-Columbian Americas. Evidence for it before 1492 in the Old World is not so clear, but the question remains unresolved. Some scholars hold that the appearance of syphilis in war-torn Italy soon after the first Columbian voyages was entirely a coincidence.

The effects of the Columbian Exchange on the size and distribution of human populations is clear and spectacular. Even more than that of direct efforts at extermination or subjugation, the impact of Old World diseases on Amerindians radically reduced their number, conceivably by as much as 90 percent, opening their two continents and nearby islands to massive shifts of Europeans and Africans across the Atlantic. The exchange of cultivated plants and livestock increased food production on both sides of the Atlantic, making possible the enormous world population growth of recent centuries. American white potatoes, for instance, enabled farmers in cool, rainy northern Europe to extract more nourishment from the soil than ever before. Hundred of millions of Chinese are dependent on the New World's maize, sweet potatoes, and even such lesser American crops as the peanut. Multitudes of Africans depend on maize, peanuts, and, especially, manioc. It is difficult to imagine how the Americas could support their hundreds of millions and export foodstuffs as well without wheat, rice, beef, chicken, and other foodstuffs of European origin.

The full significance of the Columbian Exchange cannot even now be fully measured, because it reversed more than 100 million years of divergent evolution. What may be described as approximate balances of nature within the ecosystems of the Eastern and Western Hemispheres were radically upset by the additions from overseas of new organisms such as those mentioned here, as well as numerous others.

Alfred W. Crosby, The Columbian Exchange, 1972.Find this resource:

Redcliffe Salaman, The History and Social Influence of the Potato, 2d ed., 1985.Find this resource:

Alfred W. Crosby, Ecological Imperialism, 1986.Find this resource:

Russell Thornton, American Indian Holocaust and Survival, 1987.Find this resource:

William M. Denevan, ed., The Native Population of the Americas in 1492, 2d ed., 1992.Find this resource:

Jared Diamond, Guns, Germs, and Steel, 1996.Find this resource: