Corn is one of the world’s most important crops. We don’t just pop it and munch it on the cob; corn can be turned into flour and syrup, it is fed to livestock, it is transformed into ethanol and it can even be used to make plastic. Between 2016 and 2017, about a billion tons of corn were produced around the globe, and corn yields more than six percent of all food calories for humans.
The story of this humble yet handy starch begins thousands of years ago in Mexico, with the domestication of an ancient grass called teosinte. But according to a new study published in Science, the trajectory of teosinte’s evolution into the golden grain we know today may be more complex than scientists previously thought.
Maize domestication, the commonly accepted theory goes, happened in the Balsas River Valley of south-central Mexico. Around 9,000 years ago, early farmers in this region began selecting for favorable traits of teosinte, which looks very different to modern corn and is not particularly palatable; its cob is small and its few kernels are surrounded by a tough casing. But with human intervention, teosinte evolved into tasty, tender corn, which was subsequently carried to other parts of the Americas. By the time of European colonization in the 15th century, corn was a major food source throughout many parts of the region.
Logan Kistler, the new study’s lead author and curator of archaeobotany and archaeogenomics at the Smithsonian National Museum of Natural History, says that according to this theory, gene flow from wild teosinte was still happening in some domesticated corn, but “in a major, evolutionarily important way, gene flow more or less stopped in the common ancestor of all maize.”
Recent revelations, however, prompted Kistler and his colleagues to rethink this idea. In 2016, two independent research groups analyzed the DNA of 5,000-year-old maize cobs from a cave in Mexico, and found that the ancient corn was still in the midst of the domestication process. The cobs had some genes associated with teosinte, dictating things like seed dispersal and starch production, and other genes characteristic of domesticated corn, like variants responsible for eliminating teosinte’s hard outer casing.
These findings, according to Kistler, were surprising. By the time the cobs ended up on the floor of the ancient cave, maize had already travelled far beyond Mexico, and had been cultivated in the southwest Amazon for around 1,500 years. The grain’s evolutionary story, in other words, appeared to have forked into two different paths.
“You have this paradox, this mismatch, where you already have maize being continuously cultivated in parts of the Amazon for thousands of years, and then it’s still not even finished being domesticated in the center of origin,” Kistler explains. “In order to reconcile the archaeology and the genetics … we had to think about a new domestication model.”
So, Kistler and his fellow researchers decided to take a closer look at corn DNA—and what they found suggests that while the domestication of teosinte did indeed begin in Mexico, we shouldn’t think of maize domestication as a discrete event. Instead, the grain’s evolution was a long and convoluted process, with the final stages of its domestication occurring more than once, in more than one place.