Ember is a city without a past. Or really, without much past to speak of; what are two hundred years in the lives of cities? Ember is an infant city compared to a place like Rome or Cairo. But Ember is all that its citizens have ever known; they don't know the history of the world, or that other cultures exist. So when Lina and Doon begin to learn more about the Builders, it's a total revelation for them. We (the readers of The City of Ember) have been one step ahead this whole time, but it's enough to make us wonder: what might we be missing about the past of our world?
Of all the subjects taught in Ember's school, history is probably the most boring since there's nothing really to teach. Biology clocks in at a close second.
The people of Ember have no real concept of the past in general, because their own past is mysterious to them.