Yet there were still many traps along the way. In what is now Iraq, the Sumerian civilization (one of the world’s first) withered and died as the irrigation systems it invented turned the fields into salty desert. Some two thousand years later, in the Mediterranean basin, chronic soil erosion steadily undermined the Classical World: first the Greeks, then the Romans at the height of their power. And a few centuries after Rome’s fall, the Classic Maya, one of only two high civilizations to thrive in tropical rainforest (the other being the Khmer), eventually wore out nature’s welcome at the heart of Central America.

The changes inflicted on soils by human-induced erosion over many years are sig-nificant and have resulted in valuable land becoming unproductive and often eventuallyabandoned (