|MadSci Network: Earth Sciences|
You've only heard part of the story.
Geologists working out the motions of the continents through geologic time have discovered that so-called "supercontinents" have existed at least twice in the past. One time (but not the only time) was during the Mesozoic Era and the supercontinent was called Pangea. This was the time of the dinosaurs. Another time a supercontinent existed was much earlier during the late Proterozoic Era, when the most advanced forms of life were bacteria and algae, and the supercontinent was called Rodinia.
The Earth didn't form with one supercontinent as you may have incorrectly heard or assumed. The Earth formed with a molten crust and the continental crust grew very slowly through a process of solidification of magma (molten rock), collisions of these slabs of crust, partial melting, and then resolidification over a period of many hundreds of millions of years during the Archean.
Visit the Paleomap Project to view maps of the Earth's continents at various times in the geologic past and future.
Try the links in the MadSci Library for more information on Earth Sciences.