Domestication of Maize: From Teosinte to Corn in Ancient Mexico

The domestication of maize turned a wild Mexican grass called teosinte into corn, the crop that later fed the civilisations of the Americas. Genetic evidence shows that it happened once, about 9,000 years ago, in the Central Balsas River Valley of south-western Mexico. The transformation was so great that scientists argued for decades about where corn came from at all.

The puzzle of corn’s ancestor

Modern maize cannot survive without people. Its kernels are tightly wrapped in husks on a large cob, so they cannot scatter their own seed. No wild plant looks quite like it, which made its origin a long-standing botanical mystery.

In the early 20th century two main ideas competed. One, championed by the American botanist Paul Mangelsdorf, held that maize descended from a now-extinct wild corn, perhaps crossed with a related grass called Tripsacum. The other held that its ancestor was teosinte, a wild grass of Mexico and Central America.

The geneticist George Beadle, later a Nobel Prize winner for other work, argued from the 1930s onwards that teosinte was the ancestor. He showed that maize and teosinte could be crossed easily and produce fertile offspring, and that their differences were controlled by a fairly small number of genes. Later research proved him largely right.

Genetic evidence: one origin in the Balsas Valley

In 2002 a team including Yoshihiro Matsuoka and John Doebley compared the DNA of hundreds of maize and teosinte plants from across the Americas. They concluded that all maize descends from a single domestication of the lowland teosinte Zea mays subspecies parviglumis about 9,000 years ago.

The teosinte populations most closely related to maize grow today in the Central Balsas River Valley, in the states of Guerrero and Michoacán. This seasonally dry, tropical region was very different from the highland caves where earlier archaeologists had looked for the origins of corn.

Doebley and colleagues also identified key genes behind the change. A gene called teosinte glume architecture1 (tga1) affects the hard case around each kernel. In maize a small change in this gene exposes the kernel, making it easy to eat. Another gene, teosinte branched1 (tb1), helps explain why maize has one strong stalk instead of many branches.

Archaeological evidence

Maize leaves few traces in the tropics, where plant remains rot quickly. Researchers therefore study microscopic remains: starch grains, and phytoliths, tiny silica bodies formed in plant tissues.

At the Xihuatoxtla rock shelter in the Central Balsas Valley, Dolores Piperno, Anthony Ranere and colleagues found maize starch grains and phytoliths on grinding stones in layers dated to about 8,700 years ago. Published in 2009, this is the oldest direct evidence of maize. It matches the genetic estimate and location closely.

The oldest surviving cobs are later. Cobs from Guilá Naquitz cave in Oaxaca, directly dated to about 6,250 years ago, are only a few centimetres long. In the 1960s Richard MacNeish excavated dry caves in the Tehuacán Valley of Puebla, where small cobs roughly 5,000 to 5,500 years old were found. These finds show that maize remained small for thousands of years.

Some earlier finds caused confusion. In the mid-20th century, pollen and cobs from highland Mexico were sometimes taken as evidence for a wild maize. Re-dating with accelerator mass spectrometry, which can date tiny samples directly, showed that many cobs were younger than once believed, and the “wild maize” idea faded.

A slow transformation

Domestication was gradual. Early farmers did not plant fields of large-eared corn. Instead, over many generations, they selected plants with softer kernels, more rows and ears that did not shatter. Teosinte has around 5 to 12 kernels per ear; modern maize can have several hundred.

For a long time maize was probably a minor food among many. Some researchers have suggested that early people valued the sweet stalks, or used the plant for drinks, as much as the grain. Only after about 4,000 to 3,000 years ago did maize-based farming villages become common in much of Mexico, eventually supporting societies such as the Olmec civilization.

Archaeologists and geneticists now agree that people, not nature, drove these changes. Each generation of farmers saved seed from the plants they preferred, slowly reshaping the crop.

The spread across the Americas

Maize moved south and north while still being improved. It reached Central America and northern South America by around 7,000 years ago, and maize remains in Peru date to roughly 6,700 years ago. A 2018 study led by Logan Kistler suggested that maize left Mexico only partly domesticated, and that further improvement took place in South America, especially in south-western Amazonia.

Northwards, maize reached the American Southwest by about 4,000 years ago. In eastern North America it was grown in small amounts from roughly AD 200, but it became a staple only after about AD 900, supporting large Mississippian towns. Along the way it was grown with beans and squash, a combination later called the “three sisters”.

Maize in Mesoamerican life and belief

As maize became the main staple of Mesoamerica, people developed a technique called nixtamalisation. Kernels were soaked and cooked in water mixed with lime or wood ash, which softens the hull, makes the dough easier to grind and releases the vitamin niacin. The resulting dough, masa, is the basis of tortillas and tamales.

The technique mattered for health. When maize spread to Europe without it, populations that relied heavily on maize in parts of Italy, Spain and France, and later the southern United States, suffered from pellagra, a disease caused by a lack of niacin.

Maize also held a central place in religion. Maya art shows a youthful maize god whose death and rebirth mirrored the planting and harvest cycle. In the K’iche’ Maya book known as the Popol Vuh, the gods finally create successful human beings from maize dough. The Aztecs honoured several maize deities, including Centeotl and Chicomecoatl.

Across the Americas, farmers bred hundreds of local varieties, or landraces, adapted to deserts, highlands and tropical lowlands. Many are still grown by Indigenous and rural communities today.

Maize after 1492

Columbus’s expedition encountered maize in the Caribbean and brought seeds to Spain. It spread rapidly through southern Europe, Africa and Asia, where it adapted to many climates. Along with the potato, it became one of the most important crops of the Columbian Exchange, comparable in impact to the domestication of wheat and barley in the Old World.

Today maize is the world’s most-produced cereal by weight, with global harvests exceeding a billion tonnes a year. It is used for food, animal feed, sweeteners and fuel.

Timeline

• c. 7000 BC: Teosinte domesticated in the Central Balsas Valley, according to genetic estimates.
• c. 6700 BC: Maize starch and phytoliths on grinding tools at Xihuatoxtla, Guerrero.
• c. 5000 BC: Maize present in Central America and spreading into South America.
• c. 4250 BC: Small maize cobs left in Guilá Naquitz cave, Oaxaca.
• c. 2000 BC: Maize cultivation reaches the American Southwest.
• c. AD 900: Maize becomes a staple in eastern North America.
• 1493: Maize introduced to Spain after Columbus’s first voyage.
• 2002: DNA study shows a single domestication from Balsas teosinte.

Why it matters

Maize was one of the great independent inventions of agriculture, comparable to the Neolithic Revolution in the Fertile Crescent. From a hard-seeded grass, Indigenous farmers created a crop that sustained Mesoamerican and Andean societies for millennia and now feeds much of the modern world.