Scientists overhaul corn domestication story with new analysis

Science Notes: Semi-wild proto-corn spread from Mexico to South America before becoming fully domesticated, offering new clues to its future

Smithsonian scientists and collaborators are revising the history of one of the world’s most important crops.

Drawing on genetic and archaeological evidence, researchers have found that a predecessor of today’s corn plants still bearing many features of its wild ancestor, was likely brought to South America from Mexico more than 6,500 years ago.

Farmers in Mexico and the southwestern Amazon continued to improve the crop over thousands of years until it was fully domesticated in each region.

The findings, reported recently in the journal Science, come from international collaboration between scientists at 14 institutions.

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Their account deepens researchers’ understanding of the long, shared history between humans and corn.

The history of corn begins with its wild ancestor, teosinte. Teosinte bears little resemblance to the corn eaten today: Its cobs are tiny and its few kernels are protected by a nearly impenetrable outer casing.

Over time, however, as early farmers selected for desirable traits, the descendants of the wild plant developed larger cobs and more tender, plentiful kernels, eventually becoming the staple crop that corn is today.

For years, geneticists and archaeologists have deduced that teosinte’s transformation into corn began in the tropical lowlands of what is now southern Mexico about 9,000 years ago. The teosinte that grows wild in the region today is more genetically similar to corn than teosinte elsewhere in Mexico and Central America — though all remain separated from the domesticated crop by hundreds of genes.

In the southwest Amazon and coastal Peru, microscopic pollen and other resilient plant remains found in ancient sediments indicate a history of fully domesticated corn use by around 6,500 years ago, and researchers initially reasoned that the fully domesticated plant must have been carried there from the north as people migrated south and across the Americas.

A few years ago, when geneticists sequenced the DNA of 5,000-year-old corn found in Mexico, the story got more complicated. The genetic results showed that what they had found was a proto-corn — its genes were a mixture of those found in teosinte and those of the domesticated plant.

“But you’ve got continuous cultivation of maize in the southwest Amazon from 6,500 years ago all the way up through European colonization,” said Logan Kistler, curator of archaeogenomics and archaeobotany at the Smithsonian’s National Museum of Natural History and lead author of the study. “How can you have this flourishing, fully domesticated maize complex in the southwest Amazon, and meanwhile, near the domestication centre in Mexico, the domestication process is still ongoing?”

In an effort to solve this mystery, Kistler’s team reconstructed the plant’s evolutionary history by undertaking a genetic comparison of more than 100 varieties of modern corn that grow throughout the Americas. The team mapped out the genetic relationships between the plants and discovered several distinct lineages, each with its own degree of similarity to their shared ancestor, teosinte. In other words, Kistler explained, the final stages of corn’s domestication happened more than once in more than one place.

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