Hand-planted maize, beans, and squash sustained the Mayans for millennia, until their culture collapsed about 1,100 years ago. Some researchers have suggested that the Mayans' very success in turning forests into farmland led to soil erosion that made farming increasingly difficult and eventually caused their downfall. But a new study of ancient lake sediments has revealed that most erosion happened well before the culture collapsed and likely played only a small role in disrupting the civilization.
"When you clear a forest, you open up the soil and expose it to rainfall and weathering," says Flavio S. Anselmetti, now of the Swiss Federal Institute of Aquatic Science and Technology in Duebendorf. In the tropical lowlands, he explains, clay soils can drift substantially, but only after forest clearing.