The cultivation of maize, known globally as corn (Zea mays), represents one of the most profound transformations in agricultural history. This grain is now the world’s most produced crop, but its current form is the result of a protracted process of human selection. The physical difference between the modern plant and its ancestor was so vast that for decades, the origin of maize was a scientific puzzle. Modern molecular genetics and archaeological findings have now converged to pinpoint exactly where this evolutionary event occurred and how it was accomplished.
Identifying the Ancestral Plant
The wild ancestor of modern maize is a grass known as teosinte, specifically the subspecies Zea mays subsp. parviglumis, which looks different from a field of modern corn. A teosinte plant is highly branched, resembling a bush with many small stalks, whereas domesticated maize typically grows as a single, large stalk with few branches. The most striking difference lies in the grain itself, which is largely protected in the wild plant.
Teosinte kernels are individually encased in a hard, inedible shell, and the tiny “ear” shatters upon maturity, allowing the seeds to disperse naturally. A teosinte ear is only about the size of a human thumb and contains no more than a dozen hard kernels. This stark contrast made it difficult for early scientists to accept the evolutionary link, but genetic evidence ultimately confirmed that this wild grass is the direct progenitor of the modern crop.
The Central Mexican Origin Point
Scientists have pinpointed the lowlands of southwestern Mexico as the location where this domestication event took place approximately 9,000 years ago. This region is the Central Balsas River Valley, which runs through the modern-day state of Guerrero. Genetic analysis confirmed this area as the indigenous habitat of Zea mays subsp. parviglumis, the teosinte variety most closely related to maize.
Archaeological research provides physical confirmation of the timeline and location. Researchers have uncovered microfossils of early domesticated maize, including starch grains and phytoliths, in occupation layers in the Xihuatoxtla rockshelter within the Balsas Valley. These remains were found on stone milling tools, suggesting that early hunter-gatherers were already processing the crop. Radiocarbon dating indicates that domestication was underway between 8,990 and 8,610 calendar years ago, establishing the earliest known evidence for the presence and use of domesticated maize.
Genetic Changes Driving Domestication
The physical transformation from teosinte to maize occurred through selection acting on a small number of genes. Changes in just a handful of genetic regions account for most of the visible differences between the two plants. One of the most significant changes was driven by the teosinte branched 1 (tb1) gene, which acts as a repressor of branching.
In teosinte, the tb1 gene is expressed at low levels, resulting in a highly branched plant. In the domesticated maize plant, a regulatory change causes the tb1 gene to be expressed much more highly. This increased expression suppresses lateral branches, leading to the single, dominant stalk that concentrates energy into one or two large ears.
Another gene, teosinte glume architecture 1 (tga1), controls the formation of the hard, protective shell around the kernel. A single-nucleotide mutation in tga1 caused the hard casing to become soft and exposed, transforming the kernels from being covered and protected to being easily accessible. Furthermore, the loss of the dispersal mechanism resulted in the non-shattering cob, which made harvesting far more efficient for early farmers.
The Spread of Maize
Once domesticated in the Balsas River Valley, maize began dispersal throughout the Americas along two major paths. Within 1,500 years of its origin, the grain had been carried south, arriving in South America by at least 7,000 years ago. This initial wave of maize was still a partial domesticate, but it served as a foundation for secondary improvement centers in the Andes and Amazon basin.
Migration northward was slower, with maize reaching the southwestern United States approximately 4,000 years ago. Later, a second wave of improved, temperate-adapted varieties emerged, facilitating the crop’s wider adoption across eastern North America around 900 CE. By the time European explorers arrived in the 16th century, Indigenous cultures across the Americas had adapted maize into countless varieties suited to a wide range of environments. Global introduction occurred after the Age of Exploration, turning the plant from a regional staple into the world’s most widely grown grain.