Population Migration / Travel
The first groups of Humans are transported from another Material Plane. Each group is dedicated to a different Pantheon of deities endemic to their plane of origin. The first groups had mastered the use of copper and many had begun to work bronze. They would start to build small settlements, creating the first fledgling city-states within a few generations to emulate those they left behind.
It happened quite quickly, that certain geographic features appeared upon the landscape of each continent. A mountain grew in the center of a vast high plain. A tree of impossible proportions grew from a cold northern forest. A great mud-brick ziggurat appeared as if a mirage from the desert heat. From these, and several others, came groups of Humans. Some confused, others gird for war, they moved across the land they were given by their divine patrons.