It’s only a white plastic bucket, but I’m sitting on the first phase of a system that takes the concept of sustainability out of most people’s comfort zone. “Humanure” is a low-tech but effective reuse of what even the most committed composter would consider too hot to handle: bodily waste. Not Herb and Barbara Jones.Homesteaders on Denman Island off the coast of British Columbia, the couple grow most of their own food, collect rainwater, power their home with solar energy, and compost everything. Everything.Proud outliers in today’s consumer-driven, mechanized, fossil-fueled economy, Herb and Barbara’s place is my first stop on a journey to discover what it means to take permaculture to an extreme and find out whether it’s even possible anymore to live in a way that’s completely sustainable.With humanity straining Earth’s natural resources, the question is no longer rhetorical. “Every civilization runs up against limits imposed by nature,” says the ecologist David Holmgren, a co-originator of the permaculture movement. “People must change—or disappear if they don’t change.”Holmgren, along with the nat …