Corn domestication took some unexpected twists and turns

Corn eaten around the world today originated via a surprisingly long and complex process that started in what’s now southern Mexico around 9,000 years ago, a new study finds.

People brought a forerunner of present-day corn plants, also known as maize, to South America from Mexico more than 6,500 years ago. Those plants still contained many genes from maize’s wild ancestor, teosinte, say archaeologist and evolutionary ecologist Logan Kistler of the Smithsonian Institution in Washington, D.C., and his colleagues.  Farmers in Mexico and the southwestern Amazon, in parts of what’s now Bolivia and Brazil, continued to tame the partly domesticated plant over several thousand years, the international team reports in the Dec. 14 Science.

These results, based on a reconstruction of maize’s genetic history, challenge a longstanding idea that farmers in southern Mexico molded teosinte into fully domesticated maize relatively quickly around 9,000 years ago before the crop spread elsewhere.

“We’ve shown that parts of the process were taking place thousands of kilometers [from Mexico] and thousands of years after the whole thing started,” Kistler said in a December 11 news conference.

Kistler’s team analyzed and compared DNA of 108 varieties of modern maize that grow throughout the Americas, 11 DNA samples extracted from ancient maize remains and one DNA sample from ancient teosinte. (Comparisons were also made to previously published DNA evidence for modern teosinte.)

Partly domesticated maize arrived around 6,500 years ago in a southwestern Amazonian region where people were already growing crops such as rice and cassava, Kistler said. Farmers there likely incorporated maize into their crop repertoire and employed local techniques in further domesticating the plant, including growing it in soil enriched with a mixture of charcoal, compost and other ingredients, he suspects.

By around 4,000 years ago, maize had spread more widely in South America’s lowland, farmable areas, the scientists estimate. At that time, early forms of maize also reached what’s now the U.S. Southwest.

Another wave of South American maize cultivation expanded eastward around 1,000 years ago, moving from the foothills of the Andes Mountains to near the Atlantic coast, the new analysis finds. Partly domesticated maize seeds were planted in fields where, in some places, they mixed with wild teosinte. Eventually, fully domesticated forms of maize emerged in different cultivation centers.

The new study highlights a growing realization that pathways toward domestication differed for various plants and animals (SN: 7/8/17, p. 20), says paleoethnobotanist Robert Spengler of the Max Planck Institute for the Science of Human History in Jena, Germany.

“Maize is an amazing example of how plants that evolved to accommodate human seed dispersal and cultivation gained a strong evolutionary advantage,” says Spengler, who did not participate in the new study. Maize is one of the most widespread cultivated plants in the world, dominating crops grown in most of the U.S. Midwest, Central America and parts of South America, he says.