Parallel EarthIn the continuity of Sonic X, Sonic and his friends come from an unnamed planet that is in a parallel dimension to Earth (the main setting of the first two seasons of the series). A long time ago, Sonic's world and Earth were once one, but a cataclysmic event split the planet into two and sent them to different dimensions. The flow of time in Sonic's world is considerably slower than on Earth, with one month in Sonic's world equaling an entire year on Earth.
When Sonic came to Earth, the two worlds began to merge into one once again, which would ultimately stop the flow of time completely, thus forcing Sonic and co. to return to their world.
In this series, Earth is based on the setting established in the video game series, with a majority human population and political bodies based on those in the games. However, in Sonic X, anthropomorphic animals are completely unknown on Earth until the arrival of Sonic and friends. On the other hand, Sonic's world is completely populated by anthropomorphic animals, and the only native human is Doctor Eggman (who was himself originally from Earth but crossed over many years prior the series).