The heavily gene altered human cloned stock made by the Birthless in the image of man of old.
this stock is the base from which every humanoid has since developed.
At the time of the cataclysm those firstborn in the more toxic zones and those with limited long-term shelter suffered the alterations that have since become the myriad of races present in the modern world.
While those that had proper cover and cleaner lands became the human looking extremely resistant direct decedents in image of the firstborn of predawn.
The world settled and ever indefatigable Firstborn crawled out of their cave dwellings. as they progressed as they ever will it came to fact that there were now 8 tribes of the first born, and numerous off shoots where their genetic structure did not hold.