It was meant to be a world very much like the one we know and live in. Just another very closely aligned paralel universe with minor differences at best from what anyone else would call 'the real world'. It had its Roman Empire, it's Rise of Christianity, it's battle of Hastings, Europe discovers the Americas, the Napoleonic era, the whole smear.
Then Chunks of another reality broadsided it hard utterly transforming it in parts, and having ripple effects elsewhere. You could call it a quantum entanglement, or a fragmened dimensional intrusion, or whatever will help you make sense of it and make you sleep better at night. One reality poured into another, and there were consequences that stuck.
Whatever that old world was like, it was fully of superheroes, supervillains, and strangeness it was all to happy to bequeth on its unsuspecting heirs.
The biggest change occured during the Great War. It seems funny, with so much focus on the United States of America as the home of all things super, that one of the biggest initial incursions was in Anatolia aka Asia Minor. That was the end of the Ottomon Empire. That was the end of what we now call the Armenian Genocide. It was the birth (or Rebirth if you believe old Greek Myths) of the Amazon nation.
Depending on how you define Hero, the first hero was a heroine. We know her better now as Mother Justice. She was the first and greatest of the Amazons, and she had enough.
The world's eyes were on Europe. Perhaps that was a mistake.
But then, supers would make their mark known there too.
The Great War was ending, the War between and Superheroes and Supervillains? That would prove to be eternal