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To biologists, the apparent fertility of the upland forest is a sham. This thesis was laid out clearly in Paul Richards’s classic 1952 study, The Tropical Rain Forest. To be sure, Richards said, the Amazon forest is uniquely diverse and beautiful. But its exuberant canopy is a mask covering an impoverished base. The base is the region’s poor soil. No matter what its original condition, the intense rain and heat of the forest have eroded its surface, washed out all its minerals, and decomposed vital organic compounds. As a result, much of the red Amazonian soil is weathered, harshly acid, and almost bereft of essential nutrients—one reason ecologists refer to the tropical forest as a “wet desert.”

Correspondingly, most nutrients in tropical forests are stored not in the soil, as in temperate regions, but in the vegetation that covers it. When leaves or branches fall, the carbon and nitrogen in the debris are rapidly reabsorbed by the hyperefficient root systems of tropical plants. If loggers or farmers clear away the vegetation, they also remove the local supply of nutrients. Normally the forest quickly fills in bare spots, such as those created when big trees fall, and damage is kept to a minimum. But if the opening is too large or the ground is kept clear too long, the sun and rain decompose whatever organic matter remains and bake the surface into something resembling brick in both color and impermeability. In short order, the land becomes almost incapable of sustaining life. Thus the tropical forest, despite its fabulous vitality, exists on a knife edge.

These views were picked up and amplified in Amazonia: Man and Culture in a Counterfeit Paradise, by Betty J. Meggers, the Smithsonian archaeologist. Published in 1971, it may be the most influential book ever written about the Amazon. Agriculture, Meggers pointed out, depends on extracting the wealth of the soil. With little soil wealth to extract, she said, Amazonian farmers face inherent ecological limitations. The only form of agriculture they can practice for a long time is “slash-and-burn,” or “swidden,” as it is sometimes known. Farmers clear small fields with axes and machetes, burn off the chaff and refuse, and plant their seeds. The ash gives the soil a quick shot of nutrients, giving the crop a chance. As the crops grow, the jungle rapidly returns—weeds first, then fast-growing tropical trees. In the few years before forest recovers the plot, farmers can eke something out of the land.

Slash-and-burn, Meggers told me, is “a superb response to ecological limits.” Farmers grab a few harvests, but the soil is not bared to rain and sun long enough to incur permanent damage. Switching from field to field to field, swidden farmers live in the forest without destroying the ecosystems they depend on: a supple, balanced harmony. This ancient lifeway survives today, according to this theory, in the ring-shaped compounds of the Yanomamo. (Most of the Yanomamo actually live around South America’s other huge river system, the Orinoco, but they are seen as emblematic of Amazonia as well.) Gliding nearly nude beneath the trees, cultivating their temporary gardens, the Yanomamo are often said to be windows into the past, living much the same lives as their great-great-great-grandparents. Their long-term existence has not damaged the forest, Meggers told me, a testament to slash-and-burn’s power to keep human groups sustainably within the rigid ecological limits of the tropics.

A second reason that slash-and-burn is required, Meggers told me, is the region’s propensity for suffering “mega-Niño events”—superstrong El Niño climate swings that occur every three hundred to five hundred years. In the archaeological record, she said, “We’re able to pick up severe droughts related to these mega-Niño events. What these did was reduce the food supply. The 1998 El Niño was a small example—they saw droughts, forest fires, and reduction of resources from when trees don’t flower.” Forest fires in the northern state of Roraima