Earth wasn't always as we know it today. As a matter of fact, if maps from the Earth existed millions of years ago, you wouldn't be able to recognize it at all! That is because the 7 continents that we know today move in relation to each other, as they "float" with the tectonic plates. Scientists have measured this movement and they managed to play the film backwards.
So, here is how the Earth's crust evolved throughout millions of years of evolution, and how it will probably be in some 100 million years in the future.