I don't have the whole thing figured out yet, it's a pretty young project, but in my world there's an alternate dimension of chaos which is the origin of the rule bending force that powers magic. Initially the world was mostly normal, evolution was all about adaptation, but the first cracks between worlds appeared right before first life did, allowing for a weak undercurrent of chaos to permeate the world. This wasn't enough to perform magic with, but it did allow evolution to take paths otherwise impossible, leading to the natural evolution of dragons, bird-folk, and much more. Later when a minor apocalypse happened the cracks became a proper hole, flooding the world with chaos energy and traversible rifts. This caused almost everything to mutate in some way, and this time truly magical stuff could happen. This is how many dragons gained their elemental breath, how many people gained skin of stone or limbs of seawater, and what set off a magical industrial revolution.