Well, sliced bread obviously implies a certain degree of sophistication in the world. One would first have to collect in groups before grain could be farmed. Even then, while bread was almost certainly one of the first things people made when civilization became a thing, I could only assume that a careful slice was not the first way devised of partitioning it among a group (probably ripping or tearing). If I assume that the slicing is important because it allowed for greater precsion in the parceling which inevitably lead to the sandwich, I would be willing to wager that the loaf of bread itself was the best thing before slided bread.