Maize - Origin

Origin

See also: Origin of maize and interaction with teosintes

Maize is the domesticated variant of teosinte. The two plants have dissimilar appearance, maize having a single tall stalk with multiple leaves and teosinte being a short, bushy plant. The difference between the two is largely controlled by differences in just two genes.

Several theories had been proposed about the specific origin of maize in Mesoamerica:

1. It is a direct domestication of a Mexican annual teosinte, Zea mays ssp. parviglumis, native to the Balsas River valley in south-eastern Mexico, with up to 12% of its genetic material obtained from Zea mays ssp. mexicana through introgression.
2. It has been derived from hybridization between a small domesticated maize (a slightly changed form of a wild maize) and a teosinte of section Luxuriantes, either Z. luxurians or Z. diploperennis.
3. It has undergone two or more domestications either of a wild maize or of a teosinte. (The term "teosinte" describes all species and subspecies in the genus Zea, excluding Zea mays ssp. mays.)
4. It has evolved from a hybridization of Z. diploperennis by Tripsacum dactyloides.

In the late 1930s, Paul Mangelsdorf suggested that domesticated maize was the result of a hybridization event between an unknown wild maize and a species of Tripsacum, a related genus. This theory about the origin of maize has been refuted by modern genetic testing, which refutes Mangelsdorf's model and the fourth listed above.

The teosinte origin theory was proposed by the Russian botanist Nikolai Ivanovich Vavilov in 1931 and the later American Nobel Prize-winner George Beadle in 1932. It is supported experimentally and by recent studies of the plants' genomes. Teosinte and maize are able to cross-breed and produce fertile offspring. A number of questions remain concerning the species, among them:

1. how the immense diversity of the species of sect. Zea originated,
2. how the tiny archaeological specimens of 3500–2700 BC could have been selected from a teosinte, and
3. how domestication could have proceeded without leaving remains of teosinte or maize with teosintoid traits earlier than the earliest known until recently, dating from ca. 1100 BC.

The domestication of maize is of particular interest to researchers — archaeologists, geneticists, ethnobotanists, geographers, etc. The process is thought by some to have started 7,500 to 12,000 years ago. Research from the 1950s to 1970s originally focused on the hypothesis that maize domestication occurred in the highlands between Oaxaca and Jalisco, because the oldest archaeological remains of maize known at the time were found there. Genetic studies led by John Doebley identified Zea mays ssp. parviglumis, native to the Balsas River valley and also known as Balsas teosinte, as being the crop wild relative teosinte genetically most similar to modern maize. However, archaeobotanical studies published in 2009 now point to the lowlands of the Balsas River valley, where stone milling tools with maize residue have been found in a 8,700-years old layer of deposits. A primitive corn was being grown in southern Mexico, Central America, and northern South America 7,000 years ago. Archaeological remains of early maize ears, found at Guila Naquitz Cave in the Oaxaca Valley, date back roughly 6,250 years; the oldest ears from caves near Tehuacan, Puebla, date ca. 3,450 BC. Little change occurred in ear form until ca. 1100 BC when great changes appeared in ears from Mexican caves: maize diversity rapidly increased and archaeological teosinte was first deposited.

Perhaps as early as 2500 BC, maize began to spread widely and rapidly. It was first cultivated in what is now the United States, at several sites in New Mexico and Arizona, about 2100 BC. As it was introduced to new cultures, new uses were developed and new varieties selected to better serve in those preparations. Maize was the staple food, or a major staple (along with squash, Andean region potato, quinoa, beans, and amaranth), of most pre-Columbian North American, Mesoamerican, South American, and Caribbean cultures. The Mesoamerican civilization was strengthened upon the field crop of maize, through harvesting it, its religious and spiritual importance and how it impacted their diet. Maize formed the Mesoamerican people's identity. During the first millennium AD, maize cultivation spread from Mexico into the U.S. Southwest and during the following millennium into the U.S. Northeast and southeastern Canada, transforming the landscape as Native Americans cleared large forest and grassland areas for the new crop.

It is unknown what precipitated its domestication, because the edible portion of the wild variety is too small and hard to obtain to be eaten directly, as each kernel is enclosed in a very hard bivalve shell. However, George Beadle demonstrated that the kernels of teosinte are readily "popped" for human consumption, like modern popcorn. Some have argued it would have taken too many generations of selective breeding to produce large, compressed ears for efficient cultivation. However, studies of the hybrids readily made by intercrossing teosinte and modern maize suggest this objection is not well founded.

In 2005, research by the USDA Forest Service suggested that the rise in maize cultivation 500 to 1,000 years ago in what is now the southeastern United States corresponded with a decline of freshwater mussels, which are very sensitive to environmental changes.