A huge proportion of the plants grown for food today are new world in origin; in Africa, over 50% of all food in cultivation is from the New World. Corn and potatoes have changed the world.
As to the concept of the sandwich, most medievals baked ingredients into their bread rather than slicing it and placing ingredients between the slices; medieval bread was often more crumbly and heavier than current bread styles and thus, was not especially conducive to slicing. Instead, if people wanted meat, cheese, nuts or vegetables in bread, they would bake them into the bread at the outset.
To some degree, also, the sandwich is also premised on the concept of leftovers; in earlier times, people ate bread when it was warm and yummy rather than waiting for it to age for more than a day.
Of course, many elements of the sandwich already existed -- people often spread ingredients over bread -- lard, butter, etc. Thus, one could say that the open-faced sandwich has existed since bread came into being 10000 years ago.
As to the concept of the sandwich, most medievals baked ingredients into their bread rather than slicing it and placing ingredients between the slices; medieval bread was often more crumbly and heavier than current bread styles and thus, was not especially conducive to slicing. Instead, if people wanted meat, cheese, nuts or vegetables in bread, they would bake them into the bread at the outset.
To some degree, also, the sandwich is also premised on the concept of leftovers; in earlier times, people ate bread when it was warm and yummy rather than waiting for it to age for more than a day.
Of course, many elements of the sandwich already existed -- people often spread ingredients over bread -- lard, butter, etc. Thus, one could say that the open-faced sandwich has existed since bread came into being 10000 years ago.