Whatever the benefits, chilies spread around the world with astonishing speed, thanks in part to Christopher Columbus. In 1492, the explorer encountered some plants cultivated by the Arawak Indians in Hispaniola. Convinced he had landed in India, he referred to them as "pepper," an unrelated spice native to the subcontinent. "The land was found to produce much ají, which is the pepper of the inhabitants, and more valuable than the common sort [black pepper]," he later wrote. "They deem it very wholesome and eat nothing without it." Columbus took chilies back to Spain, but they initially were unappreciated in Europe. The Portuguese got acquainted with chilies at their trading post in Pernambuco, Brazil, and carried them, with tobacco and cotton, to Africa. Within 50 years of Columbus' voyages, Pernambuco chilies were being cultivated in India, Japan and China. Chilies made it to the American Colonies with the English in 1621.
In the United States, where chilies were once an exotic spice, consumption increased by 38 percent between 1995 and 2005. The rise reflects both the influx of immigrants from countries where spicy food is common and more adventurous eating among the general population. According to the U.S. Department of Agriculture, the average American now consumes 5.9 pounds of chilies a year, more than the per capita consumption of asparagus, cauliflower or green peas.
When people call chilies "hot," they're not just speaking metaphorically. Capsaicin stimulates the neural sensors in the tongue and skin that also detect rising temperatures. As far as these neurons and the brain are concerned, your mouth is on fire. (Similarly, mint stimulates a type of neural receptor sensitive to cool temperatures.) With enough heat, adrenaline flows and the heart pumps faster. This reaction, according to some physiologists, is part of what makes peppers so enticing.
The scale that scientists use to describe a chili's heat was developed in 1912 by Wilbur Scoville, a chemist at Parke-Davis pharmaceutical company in Detroit. He would dilute a pepper extract in sugar water until the heat was no longer detectable by a panel of trained tasters; that threshold is its Scoville rating. A bell pepper, for instance, merits a zero, while a typical jalapeño falls between 2,500 and 8,000 Scoville heat units (SHUs). Last year, the naga jolokia, which is cultivated in India, rated a whopping one million SHUs. What's remarkable is that this variation can occur within a single species. The cayenne pepper, C. annuum—50,000 SHUs—is the species from which countless domesticated varieties of bell peppers, jalapeños and poblanos were derived.
Tewksbury first studied chilies near the Tumacácori mission in the mountains of southern Arizona—home to the world's northernmost wild variety, chiltepins. The Rev. Ignaz Pfefferkorn had developed a liking for chiltepins there in the 1750s. Pfefferkorn (whose name means "peppercorn" in German) called them "hell-fire in my mouth." In 1999, Tewksbury and Gary Nabhan, who co-founded Native Seeds/Search, an organization that works to preserve indigenous agricultural plants of the Southwest, established the Wild Chile Botanical Area in Tumacácori. That's when Tewksbury started wondering why chilies were hot.
Chilies, like other fruits, lure birds and other animals to eat them and disperse their seeds. But chilies also attract seed predators, like rodents, that crush seeds and make germination impossible. Many plants produce toxic or foul-tasting chemicals that deter seed predators, but these chemicals are usually found in the plant's leaves and roots as well as its fruit. In chilies, however, capsaicin is found only in the fruit—secreted via a special gland near the stem—and its production increases dramatically as the fruit ripens. Tewksbury and Nabhan suspected that capsaicin protects chilies from rodents.
To test the theory, Tewksbury wanted to compare spicy and mild chilies from the same species, if only he could find some. He contacted Paul Bosland, of the Chile Pepper Institute at New Mexico State University, who maintains a huge collection of chili seeds. Bosland told Tewksbury that he had tasted an unusual chili in his greenhouse one day in 1996. "I took a bite of it just to see what it tasted like and I said ‘hmm' that fruit doesn't seem to have any heat," he recalls. "So, I took another one and I said ‘hmm' that one doesn't either." The chili had come from seeds collected in Bolivia 37 years earlier. Bosland took note of it, wrote it off as a mutant and placed the seeds back in the freezer. But after Tewksbury called, he pulled them out again.
Tewksbury used the seeds to grow chiles for his experiments. When he offered the fruits of those labors to laboratory packrats and cactus mice, the rodents ate the mild chilies but avoided the hot ones. Such studies convinced him "that capsaicin is all about parental care," Tewksbury says. "It's all about plants caring for their offspring," the fruits. He later found that capsaicin also has the strange effect of slowing birds' digestive systems, which helps some seeds germinate, possibly by softening the seed coat. (Birds don't mind eating capsaicin; in fact, some backyard birdwatchers spike their birdseed with chili powder to stop squirrels from raiding feeders.) Even so, Tewksbury didn't believe that deterring rodents and slowing bird digestion were enough to explain why spiciness evolved in the first place. Instead, he has come to think that a chili's heat protects it from much smaller foes.
In Bolivia, fungal rot is a more pervasive threat than rodents. More than 90 percent of ripe wild chili fruits contain signs of fungal infection; it is the primary reason seeds die prior to being dispersed. Back in the lab in Seattle, Machnicki has found that just one fungus—from the Fusarium genus, light pink in color—is the main culprit regardless of the chili species. "It's not a random fungus," says Machnicki. "It's strongly associated with these chili peppers."
In looking at pepper populations that contain both spicy and mild plants, Tewksbury and Machnicki have found that the more capsaicin, the less fungal infection. Furthermore, the fungus thrives in humid environments, and Tewksbury and colleagues have found that chilies in Bolivia seem to adjust accordingly: the moister the climate, the spicier the chilies. In the lab, fungus raised from mild peppers is easily inhibited by a little spiciness, whereas fungus from spicier pepper populations can withstand more heat.