In a cautionary tale about the difficulty of controlling genetically modified plants, researchers in Mexico -- the birthplace of corn -- went climbing ever higher into remote mountain villages looking for completely natural varieties of the 4,000-year-old crop.
Time after time, they couldn't find them.
Sample after sample revealed that just a few years of unlabeled US imports had transferred genetically engineered traits to local corn in the southern state of Oaxaca.
The discovery, confirmed in an article in the science magazine Nature, caused outrage in Mexico, where the vegetable's lofty status is reflected in ancient murals that depict the first human being springing from an ear of corn.
"It's a worse attack on our culture than if they had torn down the cathedral of Oaxaca and built a McDonald's over it," said Greenpeace activist Hector Magallone.
The case has also drawn international attention. In an open letter, 80 scientists from a dozen countries asked the Mexican government to stop the genetic contamination.
There is no evidence that genetically modified grains affect the health of those who eat them. But scientists who oppose them worry more about the possibility that genetically modified strains could displace or contaminate Mexico's genetic warehouse of over 60 corn varieties -- a wealth that enriches staple crops across the world, and includes varieties that have yet to even be catalogued.
US grain growers -- who export about 6.2 million tonnes of corn to Mexico annually, perhaps one-fourth of it genetically modified -- not only aren't worried by the contamination: they want to charge Mexican farmers for it.
"If a locally occurring variety receives some improvement from genetically engineered crops, it's up to the courts to decide whether farmers should be made to pay for that," said Ricardo Celma, head of the US Grain Council's Mexico office. "But we want the patent rights of the owners of that genetic modification to be honored."
Mexican activists see the situation differently. Greenpeace lawyer Maria Colin said her group is considering suing the companies, or the government, for damages.
Greenpeace called for an immediate ban on imports of genetically modified corn, and simultaneous support for natural varieties.
Researchers from Oaxaca's Uzachi agricultural research center weren't looking for genetically modified corn when they went to the Zapotec Indian village of Calpulalpan in late November last year.
They went to the area high in the Sierra Norte mountains because they were sure it was remote enough to offer pure locally occurring hybrids that would serve as a control sample for a project to produce natural, organic corn.
But researcher Francisco Cha-pela recalls that when they analyzed the sample, it contained a genetic "switch" commonly used in genetically engineered plants.
"At first we thought our equipment was malfunctioning, that it wasn't possible," Chapela said. "Then, we thought, `Okay, maybe this field had some problems, we'll go to another one farther back in the mountains.'"
But even in the hamlet of Trinidad, about three hours from the state capital of Oaxaca, they found genetic alterations. After testing six samples in all, they finally found two fields that did not contain traces of modification.
That was no reason to cheer. The mountains of Oaxaca are far removed from the big commercial farms of northern Mexico, where contamination may be much worse, Chapela said.
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