Several independent lines of evidence support the conclusion that Zea mays originated only in Mesoamerica. Evolutionary studies indicate that wild species of the genus Zea, which are found only in Mesoamerica, are the closest relatives of maize. Botanical studies document that maize is very widespread and extremely diverse in the Americas. Fossil records support an ancient date for domestication of maize in Mesoamerica and nowhere else. Archaeological records chronicle a long history of use of maize among Native Americans. In the historical documents of Europe, Africa, and Asia, there are no definitive records of maize prior to 1492, but there are numerous records of maize dating from the late 15^th century. Although some critics have argued for an independent evolution of maize in Asia, or for pre-Columbian diffusion of maize to Asia, the evidence consists largely of folklore and interpretation of ambiguous Indian texts and sculptures. Some critics have even argued that the supposed primitive agriculturalists of Asia could not have adopted and diversified maize in only five centuries, which most experts agree gives far too little credit to the ingenuity of Asian farmers ⁽¹⁷⁹⁾.

Maize is an American Plant

Because maize originated in one geographical area and spread to Europe, Africa, and Asia during the past five centuries, the historical record is an important source of evidence, even though that evidence is often incomplete. Due to the authors' limited linguistic abilities, our access to the historical record has been restricted to European accounts and to English and French translations of Middle Eastern, South Asian, and Chinese documents. For a complete survey of historical records of maize, we will need to develop collaborations with linguists or native speakers who are familiar with Middle Eastern and Asian languages. Due perhaps in part to language limitations, we have been able to identify few 16^th, 17^th, and 18^th century records of agriculture or of American crop plants in the Middle East and in far-western China. In 16^th and 17^th century India, in contrast, many European travellers report the American crop plants tobacco and pineapple, but few travellers mention maize, even though Indian records document the presence of maize in western India in the 17^th century. Perhaps because these early travellers were traders, merchants, and missionaries, they lacked the interest or the botanical expertise to distinguish maize from the diversity of sorghums, millets, and other Indian cereal crops that were unfamiliar to them.

Maize Crossed Asia Within 100 Years

The historical record supports the conclusion that maize spread from west to east across Eurasia and reached China within 60 years of its introduction to Spain soon after 1492. Historical documents and linguistic evidence support the hypothesis that the earliest introductions of maize to eastern Asia were along overland routes of trade between Asian populations and not the result of direct introduction by Europeans. Once maize was introduced from the Americas to the Mediterranean region, it could move rapidly across the west to east axis because it was already adapted to the latitudes and climates of the regions in which it was spreading. For example, the Bahama Islands where Columbus first landed are at the same general latitudes as Egypt, Saudi Arabia, Pakistan, Nepal, Bangladesh, and Yunnan in southwestern China. The spread of maize across Eurasia in the 16^th century echoes the well-documented spread of wheat and barley several thousand years ago from the Middle East to Ireland in the west and Japan in the east ⁽¹⁸⁰⁾. Accounts of 16^th century Muslim and Christian travellers document maize in Ethiopia by the year 1526, and on major trade routes of the Middle East in the years 1507 and 1574. By the early 17^th century, maize was being recorded in western India as makka or grain of Mecca, the Muslim holy city in Saudi Arabia. Even three centuries later, variations of grain of Mecca remained as vernacular names for maize throughout the Middle East and South Asia. This linguistic evidence supports a role for Muslim traders in disseminating maize from the Mediterranean region to India and further east. Records of the Ming Dynasty document extensive trade missions in the 16^th century from the Middle East and South Asia along the northwestern trade routes of the Silk Road through far-western China. However, we have not found specific references to maize among the trade goods listed in translations of the Ming documents. We have not found any eyewitness reports of maize cultivation along the Silk Road until the early 19^th century when the first British agents were permitted to enter far-western China. Although Ming Dynasty agricultural histories support the overland introduction of maize from western regions of China by 1550-1570, evidence is not sufficient to determine the relative importance of the northwestern trade route from the Middle East and the southwestern trade route from northeastern India.

Maize Travelled by Sea and Overland

During the 16^th century the Portuguese built a network of trade settlements in Brazil, Africa, and Asia. They discovered Brazil in 1500 and introduced maize, from Brazil or Europe, to the western coast of Africa by 1520-1550 and to the eastern coast by 1620. Variations of the Portuguese name milho for maize survive in several African languages. They established the capital city of their Estado da India in Goa on the western coast of India in 1510 and small trade settlements at Bengal on the northeastern coast in 1536. Although the Portuguese coastal settlements are a likely point of introduction of maize, we have found no mention of maize in descriptions of these settlements by 16^th and 17^th century European travellers. Furthermore, we have found no evidence that the Portuguese name milho for maize has survived in any South Asian language. If maize were introduced from Portuguese settlements into the Mughal Empire of northern India, then it could have spread northward to the major trade routes into China. In their eastward search for spices, the Portuguese also established trade settlements on Sumatra, Timor and other islands of the southeast Asian archipelago, where they introduced maize and other American crop plants. Among the natives of these islands, the Portuguese name milho for maize has survived. As they continued east, Portuguese traders reached the southern coast of China in 1517, but they did not build a permanent trade settlement until 1557, which postdates the first record of maize in inland China. Fifty years ago, Charles Boxer determined that supposed records of maize in coastal provinces of China in the 16^th century are mistranslations of the original Portuguese and Chinese documents. Thus, we have found no reliable historical record of a maritime introduction of maize to China by the Portuguese during the 16^th century, whereas the 16^th century overland introduction of maize into China is well-documented.

Adaptability of Maize in Asia

The rapid spread of maize in 16^th and 17^th century Asia was facilitated by its prior adaptation to the latitudes and climates of the region and by the presence of long-established networks of overland trade. The historical record indicates that maize cultivation initially did not displace the long-established and productive irrigated rice systems of coastal India and eastern China, but instead utilized marginal or new agricultural lands. Maize was adopted first by aboriginal tribes and other land-poor farmers in mountainous regions of South Asia and western China. As Buchanan noted in his surveys of South Asia around the year 1800, while the natives of the southern city of Bangalore thought it absurd to consider maize as a cereal grain, the poor farmers of the Himalayan mountains in Kangra and Nepal were already living "much on maize". The extraordinary diversity and versatility of maize contributed much to its success as a new crop in complex Asian agricultural systems with extreme variability in altitude, slope, rainfall, soil, and agronomic practices. As a result, in Nepal today, maize is grown on more than 800,000 hectares comprising 30% of the total cultivated land and remains the staple food of populations in the hill regions ⁽¹⁸¹⁾. Yields of maize in Nepal, however, remain very low, averaging less than two metric tons per hectare, as compared to more than eight metric tons per hectare in the United States. The main challenge to maize research in Asia today is to increase the productivity and nutritional quality of an American crop plant that feeds many of the poorest human populations in Asia and throughout the world.