How it began
Genetic and archaeological evidence together indicate that maize was domesticated from Balsas teosinte, and that the process began around 9,000 years ago in the Balsas River region of Guerrero and Michoacan, Mexico. In 2009 a team led by Anthony Ranere of Temple University and Dolores Piperno of the Smithsonian reported maize starch grains and phytoliths on grinding stones from the Central Balsas River Valley, showing that maize had been domesticated more than 8,700 years ago.
Before it existed
Teosinte is a branched wild grass with ears on all its branches. Its ears are small and carry only two rows of kernels, each kernel sealed in a hard fruitcase, and the ear shatters when ripe to scatter the seed. Many researchers once believed maize was domesticated in the arid highlands. The Balsas evidence instead pointed to a lowland seasonal forest setting.
How it works
Generations of selection by farmers changed the plant. Maize has a single upright stem with large lateral ears, 8 to 22 rows of kernels, no hard fruitcase, and ears that hold their kernels when mature. Two genes account for much of the change: tb1 controls the switch from a branched plant to a single stalk, and a single mutation in tga1 may have turned hard fruitcases into naked kernels. The maize form of tb1 was established in ancient maize about 5,000 years ago.
How it spread
Maize spread early. The oldest maize microremains outside Mexico, from Cueva de los Ladrones on Panama's Pacific coast, are about 7,800 years old. Cobs from Huaca Prieta and Paredones in coastal Peru date to about 6,800 to 6,500 years ago. Cobs from Guila Naquitz Cave in Oaxaca are about 6,250 years old. The oldest macroremains in the southwestern United States, from New Mexico, are about 4,200 years old.
What lasted
By the time Europeans arrived in the late 1400s, maize was a widely grown domesticated plant with varieties suited to environments from southern Canada far into South America. It has been a staple of Mesoamerican diets from ancient times to the present day.
