The Tale of the Mystery Corn in Mexico's Hills.

Olga Toro Maldonado was short on corn seed and slightly curious. In the spring of 1998, alongside the corn she had always raised on her hillside plot, she planted 60 kernels purchased from the government store. "The corn looked good," she recalls, so the next year she planted a cross between the two species. The harvest was smaller than the year before--one ear per stalk rather than the usual two--but the corn was tasty enough. She ground it into flour for tortillas and fed the kernels to her chickens.

A few scientists stopped by in fall 2000 and took away samples from her most recent harvest. They returned a week later with some disturbing news. Toro's corn contained transgenes--genes from bacteria and other organisms artificially introduced into the corn to make it resistant to herbicides or insects. Toro, 40, heard the word "contamination" and began worrying about her six children, her chickens and whether the pollen from her corn had spread. "I feel guilty," she says. "But another woman told me she planted it, too. I'm not the only ignorant one. We don't know the damage we can do."

The head scientist was Ignacio Chapela, a 42-year-old Mexican and a microbial ecologist at the University of California, Berkeley. His team collected corn from the mountains of Oaxaca, in southern Mexico, and found that several samples contained transgenes. The finding was startling because the Mexican government bans the planting of genetically modified (GM) corn. And the agriculture industry has long contended that contamination from GM crops was extremely unlikely. "I was dumbfounded," Chapela says. "I knew it was a difficult political fray we were getting ourselves into."

There is no evidence that GM corn is dangerous for human consumption. Chapela and his allies are concerned instead that GM corn might pose a threat to corn's biodiversity. Mexico, where corn was first domesticated 10,000 years ago, is what scientists call the crop's "center of genetic diversity"--a kind of repository of traditional varieties. GM corn, with its engineered advantages, could theoretically overwhelm these indigenous types. That would leave breeders without a source of pristine seed if a plague struck corn crops elsewhere. "World food security depends on the availability of this diversity. Having it contaminated is something humanity should worry about," says Chapela.

Mexico, the corn-consuming capital of the world, has been cautious about corn. Congress banned GM corn crops in 1998 even while allowing GM cotton and tomatoes. The current administration has been considering loosening the ban in an effort to improve agriculture and attract investment. A combination of decades of bad agricultural policy and falling trade barriers with the United States has turned Mexico into an importer of its staple food: 6 million tons of corn a year come from the United States. A panel of scientific advisers recently recommended opening northwest Mexico, which has none of the traditional strains of corn, to transgenic corn crops. "Mexico as a country cannot exclude itself from biotechnology," says Victor Manuel Villalobos, the under secretary of Agriculture. "It is not an intelligent position to say that because there are risks we won't touch it."

Chapela's revelation that GM corn is already growing in the hills of Oaxaca is an embarrassment to the Mexican government, to say the least. After Chapela's paper appeared in the scientific journal Nature in November, a Greenpeace activist declared the contamination "a worse attack on our culture than if they had torn down the cathedral of Oaxaca and built a McDonald's over it." The group began urging indigenous groups in Oaxaca to sue the federal government. Eighty ...