The Eastern Continent is the original homeland of the dwarven race.
The designation is not racial-centric. Originally, the two land masses were joined in the north. During the war that raged between the Dwarves and humans about 2000 BH, there was cataclysmic damage to the magical energy that covers the planet and the sides were wrenched apart.
The area where they were joined still exhibits magical anomalies and produces rift spice.
It is agreed that the dwarves are the ones that did this, but there is nothing to indicate how it was accomplished. At any rate, the war ended when this happened.