What happens after you eat chile peppers reads like a list of drug side effects: burning pain, sweating, teary eyes and panic followed by lingering numbness. The plant makes its mouth-torching ingredients, called capsaicinoids, to stop animals from munching its fruit, biologists say. Perhaps strange, then, how gardeners and cooks have avidly embraced the chile for more than 6,000 years.

University of Washington scientists now propose an explanation: Spiciness evolved as a chemical defense against microbial attacks. And people might have developed a taste for the powerful chile to take advantage of its anti-microbial powers. “That may not have been an accident,” said UW biologist Joshua Tewksbury, lead author of the study. “Eating chiles might have been very beneficial.”

Chile peppers in their native habitat wage an ongoing battle against a highly destructive fungus. Aphid-like insects spread the fungus by boring into fruit, and once inside, even tiny amounts of the fungus can quickly destroy all the seed.

In lab tests, chile capsaicinoids poisoned the fungus, blocking its growth. The UW researchers and a Bolivian colleague also measured the spiciness of a wild chile growing at different locations throughout Bolivia and found that capsaicinoid levels rose with increasing numbers of the fungus-spreading bugs. In areas with few insects – and less danger of fungal attack – most of the plants lacked heat entirely.

…a big part of the chile’s allure is sensory – the mind-blotting instant of pain followed by mild euphoria.

The plants hit an evolutionary home run when they began producing capsaicinoids, which defeat microbes and repel mice and other small animals that might eat seeds. But the compounds don’t faze the birds that chile plants need to spread their seeds. “It shows just how nuanced evolution can be,” Tewksbury said.

Success as a spice has given the South American native a secure niche worldwide. Before Columbus, Asians and Europeans had no experience with chile peppers. After Columbus, the plant swept India and much of Asia in less than 20 years.

Paul Sherman, a Cornell University professor who was not involved with Tewksbury’s study, said it provides more evidence that our taste for pungent plant ingredients arose because of the substances’ ability to kill or retard the growth of microbes in food, an idea he has long championed. Without refrigeration, bacteria and fungi multiply rapidly, releasing toxins that can make people sick or reaching numbers that overwhelm a person’s immune system once ingested.

“Humans have borrowed the plant’s evolutionary recipes for survival and reproduction to use for essentially the same thing: to make us healthier by cleansing what we will eat,” Sherman said.

Paul Bosland, director of the Chile Pepper Institute at New Mexico State University, said the idea seems plausible. Ancient Mayan texts name chiles as ingredients in remedies for a variety of infectious illnesses. “We’ve known for a long time that capsaicins in humans have anti-microbial effects,” Bosland said.

But Linda Perry, a researcher at the Smithsonian National Museum of Natural History, says spicing up bland food was reason enough for people to exploit the chile pepper, an easily grown fruit that is simple to preserve by drying.

Last year, Perry and colleagues reported the earliest evidence to date of chile cultivation and seasoning of food: 6,250 years ago in Ecuador. Since the site was far from the wild plant’s origin in South America, domestication took place even earlier.

“In areas where staple crops are bland, peppers offer an excellent way to spice up cuisine,” says Perry, who has her own criterion for the perfect heat level: “I like where it will make my nose run, but it won’t make me cry.”

Tewksbury doesn’t doubt that a big part of the chile’s allure is sensory – the mind-blotting instant of pain followed by mild euphoria. “It’s fantastic,” he says. “It’s not so different from a runner’s high, except you don’t have to go running.”

Images: Wellcome Collection

I wrote this as a news story for The Oregonian, where it was first published on August 12, 2008.