Some 350 million years ago the Earth's landmasses were all clumped together into a supercontinent that today we call Pangea. The supposed appearance of this huge landmass is shown above, with today's continents pencilled in. In fact, before Pangea, there had been yet another supercontinent called Rondinia, which fragmented around 650 million years ago. Well, all this is almost too much to believe, but it's all been worked out and the proof for it is wonderful to learn about, and the consequences of it are mind-boggling to think about.
You can learn all about continental drift and geological processes associated with it at the Berkeley University Plate Tectonics Site, where you can even view an animation showing how the the continents have drifted during the last 750 million years!