The theory is that long long long ago all of the continents where one big land mass that slowly separated over time. This Super Continent was called Pangaea, or Pangaea Ultima.

So, the world is different now in the sense that land masses are farther apart. This keeps civilizations and cultures from bleeding into one another to a degree because they are all so remote. If makes travel between places more dificult and allows more diverse forms of life to exist around the world.

The world is so different today is because the continents are floating on tectonic plates that has slowly moved (and still is moving) over time.