Adanth was originally conceived to answer the question, “How can all of these strange races and creatures come to exist in the same world?”
I don’t remember exactly when I first conceived of the basic premise of Adanth as a campaign setting, but it must have been between 1989 (when Stephen J Gould’s Wonderful Life was published) and 1995 (which I graduated from university and stopped playing D&D for a while).
I was, at the time, overly concerned with imposing logic and order on my campaign world, and it occurred to me that six-limbed beings like dragons and displacer beasts must have evolved on a different world than did four-limbed beasts like pretty much every vertebrate on earth. The same applied to other creature groupings, and I wondered how all of these creatures could end up on the same world as mundane creatures like horses and oxen, while being absent from our own world.
I had also played a bit of Gamma World, and enjoyed the premise of adventures such as “The City Beyond the Gate”, by Robert M. Schroeck, published in Dragon Magazine No. 100. As a result, I had a lot of worlds in my head that I wanted to be able to play in, and began to think of how I would pull them together.
My answer was Adanth, a world which bounced around the multiverse, and occasionally settled adjacent to another world for a period of time, allowing beings to cross from one world to another. Adanth would collect creatures from other worlds which had inadvertently crossed over, allowing for extreme diversity. At the same time, it facilitated adventures in other worlds or alternate universes.
For a variety of reasons linked to the cosmology, the first intelligent beings to establish cultures on Adanth were elves, dwarves and giants. Humans came later, most of which had come from our own Earth, or one very like it, and had crossed over at various times during our history. And that is where the ball really got rolling.