In the beginning (or near enough), there was bread. But in the beginning there was no grocery store, baker, fridge or freezer, so a lot of this bread became stale. Probably at some point, somebody got fed up with stale bread and tossed a piece of it in the fire, or at least meant to. Instead, it landed nearby, browned in the glow of the flames, someone found it, bit into it, and voila: toast.