Beavers, those icons of industriousness, have been gnawing down trees, building dams, shaping the land and creating critical habitat in North America for at least a million years. They were once one of the continent's most ubiquitous mammals. Then the European fur traders arrived. In Once They Were Hats, Frances Backhouse examines humanity's 15,000-year relationship with Castor canadensis, and the beaver's even older relationship with North American landscapes and ecosystems. Along the way she discovers how we can learn to live with beavers as they return.