To visit the Shola market, a teeming maze of stalls in Addis Ababa, Ethiopia, you'd never know that the human race's food supply was in jeopardy. Massive sacks stuffed with dried chili peppers overflow onto piles of vegetables-potatoes, beets, carrots, onions, tomatoes. Enticing smells waft from the spice sellers' stalls, crowded with colorful mounds of cumin, turmeric and ginger.

Wandering the market's muddy corridors, Luigi Guarino stoops to smell frankincense and examine fistfuls of legumes. To the casual observer, they're just sacks full of beans. But to the 48-year-old crop scientist, each one is a tiny buffer against worldwide starvation. Guarino fingers the dried seeds with reverence, because he understands how valuable-and fragile-they really are. He has spent much of his career trolling remote and exotic lands, gathering the genetic diversity of the plants that sustain humanity. His work has taken him to roadless villages in Oman's mountains and across the desolate Sahara between Algeria and Niger, all in search of crop varieties with unique traits that could someday save the world's harvest-and its people-from infestation, blight or drought.

Looking up, Guarino explains that in certain countries, industrial progress has left some of the world's most important food crops at risk of obliteration. When it comes to gathering food, "people used to be more mobile," he says, as the beans sift through his fingers. "Now they have nowhere to go. They're more vulnerable to things like drought. As population increases and more farmland is converted to urban areas, you have less land on which to produce more food."

Meanwhile, because the efficiency of modern farming has made crops so genetically uniform, the plants on which humans depend-those we've bent to our will over thousands of years-are at the mercy of chance. They simply aren't diverse enough anymore. To endure, crop plants, like their wild counterparts, need varied gene pools. But the genetic diversity of our food supplies is withering. In the past 200 years in the U.S. alone, 75 percent of the variety within crops has vanished. That's a disaster waiting to happen.