Agriculture is rightly recognized as one of the turning points in human history. The practice of tending to specific animals and plants to maintain and even increase their utility helped drive humans into city-building and, from there, into the large, complex, settled societies we know today. Humans, however, are not the only animals that have discovered agriculture. Everything from snails to elephants has some ability to foster and guide the evolution of another creature for its own use. Agriculture, it turns out, is a subset of ecosystem engineering, and a lot of creatures are engineers.