Supporting Online Material

For most of history, few things have mattered more to human communities than their relations with soil, because soil provided most of their food and nutrients. Accordingly, some of the earliest written documents were agricultural manuals intended to organize, preserve, and impart soil knowledge. Indeed, ancient civiliza-tions often worshipped the soil as the foundry of life itself. For the past century or two, nothing has mattered more for soils than their relations with human commu-nities, because human action inadvertently ratcheted up rates of soil erosion and, both intentionally and unintentionally, rerouted nutrient flows. Our distant ancestors found their food by hunting and foraging. They depended indi-rectly on soils to support plant growth, but they did not much alter soils by their ac-tions, except where they routinely burned vegetation. With the transitions to agricul-ture (which probably happened indepen