Maize
Maize relies on humans to survive. Its kernels stay locked to the cob, unlike its wild ancestor teosinte, whose seeds scatter to the ground on their own. Left alone in a field, a corncob would simply rot where it fell. This is a plant that cannot reproduce without us, and yet it feeds more of the world by weight than any other grain. In 2020, the world grew 1.16 billion tonnes of it, more than wheat and more than rice. The story begins in southern Mexico about 9,000 years ago, where indigenous peoples coaxed a tall stout grass out of a short bushy weed. How did two plants that look nothing alike turn out to be the same lineage? Why does this grain, eaten plain, leave people sick? And how did a crop that needs human hands to plant it spread to nearly every climate on earth?
The Russian botanist Nikolai Ivanovich Vavilov proposed in 1931 that maize descended from teosinte, and the American Nobel Prize-winner George Beadle argued the same case in 1932. The two plants look like strangers. Maize grows a single tall stalk with many leaves, while teosinte is short and bushy. Yet the gap between them comes down largely to two genes, called grassy tillers-1 and teosinte branched-1. In the late 1930s, Paul Mangelsdorf offered a rival idea, that maize came from a cross between an unknown wild maize and a plant in the related genus Tripsacum. Modern genetic testing has refuted his hypothesis. In 2004, John Doebley pinned down the closest living relative of modern maize as Balsas teosinte, native to the Balsas River valley in Mexico's southwestern highlands. Doebley and colleagues had already shown in 2002 that maize was domesticated only once, then spread across the Americas. The evidence sits in the ground itself. Stone milling tools still carrying maize residue were found in an 8,700 year old layer of deposits in a cave near Iguala, Guerrero, waiting to be read like a receipt.
Maize pollen dated to 7,300 years ago turned up at San Andres, Tabasco, on the Caribbean coast. The crop moved fast for a plant that cannot sow itself. It reached highland Ecuador at least 8,000 years ago, lower Central America by 7,600 years ago, and the valleys of the Colombian Andes between 7,000 and 6,000 years ago. Archaeological remains of early maize ears at Guila Naquitz Cave in the Oaxaca Valley are roughly 6,250 years old, and the oldest ears from caves near Tehuacan, Puebla, are 5,450 years old. The earliest plants grew a single small ear apiece. The Olmec and Maya cultivated many varieties throughout Mesoamerica, cooking, grinding, and processing the grain through nixtamalization. By 3,000 years ago, maize sat at the center of Olmec culture, woven into their calendar, language, and myths. Around 4,500 years ago it began moving north, first cultivated at sites in New Mexico and Arizona about 4,100 years ago. The Mapuche people of south-central Chile grew it alongside quinoa and potatoes, and before the Inca Empire expanded, maize was carried as far south as 40 degrees south at Melinquina in Argentina, probably brought across the Andes from Chile.
Maize protein is low in two essential amino acids, tryptophan and lysine, which keeps it from being a complete protein source. There is a deeper trap. The grain contains niacin, a B vitamin, but the body cannot absorb it unless an alkali frees it first. Indigenous Americans had solved this puzzle by at least 1200 to 1500 BC, soaking maize in water made alkaline with ashes and lime. They were trying to loosen the corn hulls. The process, called nixtamalization, happened also to release the niacin, and the lack of that vitamin causes the disease pellagra. When Europeans adopted maize as a staple without the alkali step, malnutrition followed wherever it became the main food. Once people understood alkali processing and added variety to their diets, pellagra vanished from the developed world. High-lysine maize and more balanced eating finished the job. The disease still appears in food-poor areas and refugee camps where people survive on donated maize, the same grain that fed civilizations once they knew the trick.
James L. Reid developed Reid's Yellow Dent in the 1860s, one of the earliest and most successful efforts by farmers who selected their best plants and sold the seed onward. These first methods relied on mass selection, choosing plants after pollination so only the female parents were known. The science sharpened over decades. C. G. Hopkins introduced ear to row selection around 1896, G. H. Shull made hybrids from selected inbred lines in 1909, and D. F. Jones built the highly successful double cross hybrids from four inbred lines around 1918 and 1922. Since the 1940s, the best strains have been first-generation hybrids bred from inbred lines tuned for yield, nutrition, and tolerance of drought, pests, and disease. The work continues at the International Maize and Wheat Improvement Center, known as CIMMYT, whose conventional breeding program began in the 1980s and distributes hybrid seed in Africa through its Drought Tolerant Maize for Africa project. Some discoveries border on the strange. A strain called oloton has formed a partnership with nitrogen-fixing microbes that supply between 29 and 82 percent of the plant's nitrogen, a deal struck long before any breeder thought to look.
Barbara McClintock used maize to prove her theory of jumping genes, the transposons that move within the genome, and the work earned her the 1983 Nobel Prize in Physiology or Medicine. Maize remains a central model organism for genetics and developmental biology. The plant is diploid with 20 chromosomes, and 83 percent of the allelic variation in its genome comes from its teosinte ancestors, a legacy of how freely Zea species outcross. In 2005, the U.S. National Science Foundation, the Department of Agriculture, and the Department of Energy formed a consortium to sequence the maize genome, depositing the data straight into the public GenBank repository. Sequencing finished in 2008, and the consortium published its results in 2009. The genome holds 32,540 genes, and 85 percent of it is made of transposons. Much of it has been duplicated and reshuffled by helitrons, a family of transposable elements inside maize's own DNA. The same restless genetics McClintock first glimpsed turned out to fill most of the genome she was studying.
Two insects each cost the United States a billion dollars a year in maize losses, the European corn borer and the corn rootworms. The fall armyworm is another serious threat, and in Asia the rice ear-cutting caterpillar attacks the crop. Stored grain has its own enemy in the maize weevil. Fungal and bacterial diseases press from every side, from corn smut to ear rots and stalk rots, with northern corn leaf blight damaging maize across its range. In the United States in 2022, the most damaging disease was tar spot, which destroyed 116.8 million bushels. The grain's defense begins early. Immature shoots build up a powerful antibiotic compound called DIMBOA, which shields the plant against a wide range of pests. Yet maize has a fatal weakness in its shallow roots, which leave it prey to drought, poor soils, and strong winds. That same fragility shows up at the table. In many parts of Africa, periodic drought regularly causes the maize crop to fail, and famine follows the loss of a plant that, despite everything, thrives in cold, hot, dry, or wet conditions when the water holds.
In Mesoamerica, maize is seen as a vital force and deified as a maize god, usually female, depicted in sculpture in pre-Columbian times. The reverence never fully faded. In the United States Capitol building, maize ears are carved into the capitals of columns, and the Corn Palace in Mitchell, South Dakota, rebuilds a mural every year from cobs and ears of colored maize. In Dublin, Ohio, a sculpture called the Field of Corn stands hundreds of concrete ears upright in a grassy field. A maize stalk with two ripe ears appears on the reverse of the Croatian 1 lipa coin, minted since 1993. The grain has carried darker meanings too. Because maize is fed to chickens, and chickens symbolize cowards, the kernels have sometimes stood for cowardice. In the months before the 1973 Chilean coup, anti-Allende protesters threw maize at military barracks, a wordless dare to depose the president. A plant that cannot plant itself ended up speaking for the people who grew it.
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Common questions
What is maize and how is it different from corn?
Maize, with the scientific name Zea mays, is a tall stout grass that produces cereal grain. It is called corn in North American English. The name maize is preferred in formal, scientific, and international usage because it refers specifically to this one grain, while corn has varied meanings by region.
Where and when was maize first domesticated?
Maize was domesticated by indigenous peoples in southern Mexico about 9,000 years ago from wild teosinte. The likely location of early domestication is the middle part of the short Balsas River valley in Mexico's southwestern highlands. Genetic work by Doebley and colleagues in 2002 showed maize was domesticated only once.
How much maize does the world produce?
In 2020, total world maize production was 1.16 billion tonnes, more than any other grain and exceeding both wheat and rice. The United States led with 31.0 percent of the total, and China produced 22.4 percent.
Why does eating only maize cause the disease pellagra?
Maize contains the B vitamin niacin, but the body cannot absorb it unless an alkali frees it through a process called nixtamalization. When Europeans adopted maize without this alkali treatment, the lack of available niacin caused pellagra. The disease disappeared in the developed world once alkali processing and dietary variety were applied.
What pests and diseases threaten maize crops?
The European corn borer and corn rootworms each cause about a billion dollars in annual losses in the United States. Other serious pests include the fall armyworm and the maize weevil. In 2022, the most damaging U.S. disease was tar spot, which caused losses of 116.8 million bushels.
How is maize used as food and other products?
Maize is a staple food used in Mexican tortillas and tamales, Italian polenta, and American hominy grits. It is also processed into cornstarch, high fructose corn syrup, corn oil, and bourbon whiskey. Beyond food, it serves as animal feed, a feedstock for ethanol fuel, and a base for plastics, fabrics, and adhesives.