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The Columbian Exchange was the widespread exchange of animals, plants, culture, human populations, communicable diseases, technology and ideas between the American and Afro-Eurasian hemispheres following the voyage to the Americas by Christopher Columbus in 1492, colonization and trade by Europeans in the Americas, and institution of the slave trade in Africa and the Americas.:163
The term was coined in 1972 by Alfred W. Crosby, a historian at the University of Texas at Austin, in his eponymous work of environmental history.:27 The contact between the two areas circulated a wide variety of new crops and livestock, which supported increases in population in both hemispheres.
Explorers returned to Europe with maize, potatoes, and tomatoes, which became very important crops in Europe by the 18th century. Similarly, Europeans introduced manioc and peanut to tropical Asia and West Africa, where they flourished and supported growth in populations on soils that otherwise would not produce large yields.
Before AD 1500, potatoes were not grown outside of South America. By the 1840s, Ireland was so dependent on the potato that the proximate cause of the Great Famine was a potato disease. Since being introduced by 16th-century Portuguese traders, who brought them from the Americas, maize and manioc replaced traditional African crops as the continent's most important staple food crops. 16th-century Spanish colonizers introduced new staple crops to Asia from the Americas, including maize and sweet potatoes, and thereby contributed to population growth in Asia.
Tomatoes, which came to Europe from the New World via Spain, were initially prized in Italy mainly for their ornamental value (see below), but from the 19th century tomato sauces became typical of Neapolitan cooking and, ultimately, Italian food in general. Coffee from South America and sugar cane from The Indies became the main export commodity crops of extensive Latin American plantations. Introduced to India by the Portuguese, chili and potatoes from South America are today an integral part of Indian cuisine.
Before the Columbian Exchange, there were no oranges in Florida, no bananas in Ecuador, no paprika in Hungary, no potatoes in Ireland, no coffee in Colombia, no pineapples in Hawaii, no rubber trees in Africa, no chili peppers in Thailand, and no chocolate in Switzerland.
Initially, at least, the Columbian exchange of animals largely went in one direction, from Europe to the New World. Horses, donkeys, mules, pigs, cattle, sheep, goats, chickens, large dogs, cats and bees were rapidly adopted by native peoples for transport, food, and other uses. One of the first European exports to the Americas, the horse, changed the lives of many Native American tribes in the mountains. They shifted to a nomadic lifestyle based on hunting bison on horseback and moved down to the Great Plains. The existing Plains tribes were able to extend their territories with horses, and herds became desirable to accumulate.
European exploration of tropical areas was aided by the New World discovery of quinine, the first effective treatment for malaria. Europeans suffered from this disease, new to them, but some indigenous populations had developed at least partial resistance to it. In Africa, resistance to malaria has been associated with other genetic changes among sub-Saharan Africans and their descendants that can cause sickle cell anemia.:164
Before regular communication had been established between the two hemispheres, the varieties of domesticated animals and infectious diseases that jumped to humans, such as smallpox, were strikingly more numerous in the Old World than in the New. Many had migrated west across Eurasia with animals or people, or were brought by traders from Asia, so diseases of two continents were suffered by all. While Europeans and Asians were affected by the Eurasian diseases, their endemic status in those continents over centuries caused many people to gain acquired immunity.
By contrast, "Old World" diseases had a devastating effect on Native American populations, who had no natural immunity to the new diseases. Measles caused many deaths. The smallpox epidemics are believed to have caused the largest death tolls among Native Americans, surpassing any wars and far exceeding the comparative loss of life in Europe due to the Black Death.:164 It is estimated that upwards of 80–95 percent of the Native American population died in these epidemics within the first 100–150 years following 1492; the most affected regions in the Americas lost 100% of their indigenous populations.:165
Similarly, yellow fever is thought to have been brought to the Americas from Africa via the Atlantic slave trade. Because it was endemic in Africa, many people there had acquired immunity. Europeans suffered higher rates of death than did African-descended persons when exposed to yellow fever in Africa and the Americas, where numerous epidemics swept the colonies beginning in the 17th century and continuing well into the 19th century.
Debate on the origins of syphilis has been raging for centuries. New genetic evidence supports the theory that Christopher Columbus brought syphilis to Europe from the New World According to the study, genetic analysis of the syphilis family tree reveals that its closest relative was a South American disease that causes yaws, an infection caused by a sub-species of the same bacteria. 
|Type of organism||Old World to New World||New World to Old World|
It took three centuries after their introduction in Europe for tomatoes to become widely accepted. Of all the New World plants introduced to Italy, only the potato took as long as the tomato to gain acceptance. In large part this was due to 16th-century physicians believing that this native Mexican fruit was poisonous and the generator of "melancholic humours." In 1544, Pietro Andrea Mattioli, a Tuscan physician and botanist, suggested that tomatoes might be edible, but no record exists of anyone consuming them at this time. On October 31, 1548 the tomato was given its first name anywhere in Europe when a house steward of Cosimo de' Medici, the grand duke of Tuscany, wrote to the Medici private secretary that the basket of pomi d'oro "had arrived safely." At this time the label pomi d'oro was also used to refer to figs, melons, and citrus fruits in treatises by scientists.
In the early years, tomatoes were mainly grown as ornamentals in Italy. For example, the Florentine aristocrat Giovanvettorio Soderini wrote how they "were to be sought only for their beauty" and were grown only in gardens or flower beds. Tomatoes were grown in elite town and country gardens in the fifty years or so following their arrival in Europe and were only occasionally depicted in works of art. However, in 1592 the head gardener at the botanical garden of Aranjuez near Madrid, under the patronage of Philip II of Spain wrote, "it is said [tomatoes] are good for sauces." Besides this account, tomatoes remained exotic plants grown for ornamental purposes, but rarely for culinary use. The combination of pasta with tomato sauce was developed only in the late nineteenth century. Today around 32,000 acres (12,950 ha) of tomatoes are cultivated in Italy, although there are still areas where relatively few tomatoes are grown and consumed.
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Plants that arrived by land, sea, or air in "ancient" times (or before 1492 in the UK) are called archaeophytes, and plants introduced to Europe after those times are called neophytes. In addition to the diseases mentioned above, many species of organisms were introduced to new habitats on the other side of the world accidentally or incidentally. These include such animals as brown rats, earthworms (apparently absent from parts of the pre-Columbian New World), and zebra mussels, which arrived on ships.
Invasive species of plants and pathogens also were introduced by chance, including such weeds as tumbleweeds (Salsola spp.) and wild oats (Avena fatua). Some plants introduced intentionally, such as the kudzu vine introduced in 1894 from Japan to the United States to help control soil erosion, have since been found to be invasive pests in the new environment. Fungi have been transported, such as the one responsible for Dutch elm disease, killing American elms in North American forests and cities, where many had been planted as street trees. Some of the invasive species have become serious ecosystem and economic problems after establishing in the New World environments.
Escaped and feral populations of non-indigenous animals have thrived in both the Old and New Worlds, often displacing native species.
Gray squirrels have been particularly successful in colonising Great Britain and populations of raccoons can now be found in some regions of Germany, the Caucasus and Japan. Fur farm escapees such as coypu and American mink have extensive populations in the Old World.
In the New World, populations of feral European cats, pigs, horses and cattle are common.