Until the mid-19th century, your average loaf of bread was grainy, heavy, and dark. With innovations like mechanical slicers and refined flour, however, bread became white, fluffy, and in the perfect portions to make a sandwich.
The "best thing since sliced bread" hasn't always meant much, however, and the role of bread in social and political history is much more complex than a simple bread recipe. The Romans were able to distract the masses from social unrest by making sure there was plenty of "bread and circuses." One of the major differences between the Roman Catholic Church and the Eastern Orthodox Church includes the use of unleavened and leavened bread, respectively. In another social application of bread, in the early 20th century, white bread became the subject of racist conversation and visions of ethnic superiority.