I know it's not part of the official "canon" but in some licensed works it was said that the Federation used fusion reactors to power their warp engines before the discovery of dilithium. It certainly fits in better with the idea introduced in First Contact that humanity's first warp-capable craft was not an Apollo-level project conduced by a United Earth space agency, but a nuclear missile that a team of scientists converted into a spaceship in the aftermath of WW3. Where the heck is somebody in a rural Montana that's been bombed into the 19th century supposed to find antimatter, much less dilithium?