Now for an idea that's been around for the best part of a century — the idea of a shared comic book universe sure is topical right about now. With Batman v Superman: Dawn of Justice attempting to single-handedly wrangle the DC Extended Universe into being, Captain America: Civil War pitting half of the Marvel Cinematic Universe against one another, and the CW's Flarrowverse crossing over with CBS's Supergirl, shared universes are certainly en vogue at the minute.
For our grandparents, though, the idea of two superheroes cohabiting the same screen or page wasn't as much of a given as it is today — with the very earliest comic book heroes tending to keep themselves to themselves. Which, of course, begs the question:
When Was The First Shared Comic Book Universe Born?
Or, rather, when — in the years following the arrival of the first 'modern' superhero, Superman, back in June 1938 — did we first see the lives of our favorite comic book heroes bleed over into each other's adventures?
For many, the beginning of the true 'crossover' era began back in December 1940's All Star Comics #3, best known for its lead strip, The First Meeting of the Justice Society of America.
In which, as you can probably guess from the title, we saw the first meeting of the (Justice League precursor) Justice Society of America.
The thing is, though, while that issue may well have marked the beginning of modern day super-teams as we know them, it wasn't the moment in which a shared comic book universe came to be. After all, the Marvel Cinematic Universe didn't come into being with the arrival of 2012's The Avengers. Instead, it began back in 2008, when Iron Man ended with the surprise appearance of Nick Fury, and his mentioning of the Avengers initiative. It was in that moment that the MCU, and all of its crossover potential, was born.
Which, in turn, means that...
The DC Comic-Book Connected Universe Arrived Earlier Than The JSA's First Meeting
Specifically, it arrived — at the very latest — in August 1940, in Flash Comics #8. As you can see above, the young Johnny Thunder (a superhero whose power was, gloriously, repeatedly calling down a sentient thunderbolt to help him fight crime without knowing how he was doing it) found himself on the receiving end of a glorious burn from his girlfriend Daisy Darling:
You're just a big bluff that's all. Imagine trying to tell me you've got power as great as Superman.
Now, as later generations of superheroes would discover, that's actually a pretty typical response to being told that your goofy significant other is a superhero, but for readers at the time, that was far from the most intriguing thing about that speech bubble. Y'see, up until that moment, fans of Johnny Thunder, The Flash, Green Lantern, or Superman could have been forgiven for thinking that their heroes lived in entirely different worlds and were united only by their presence in similar comic books. Daisy Darling comparing Johnny Thunder to Superman, however, confirmed — well before the arrival of the JSA — that DC's heroes did in fact share the same universe.
The big twist in that tale, though?
It Turns Out That Marvel Beat DC To The Punch By a Couple of Months
Y'see, two of Marvel's greatest heroes — the original Human Torch and Namor, The Sub-Mariner — had met, and fought one another a few months earlier, back in Marvel Mystery Comics #8. With that issue arriving in June 1940, it seems that Marvel's heroes can lay claim to having the earliest full crossover, and Marvel Comics to having a fully connected comic book universe a few months before DC.
Which perhaps makes up a little for DC beating them to that whole "creating a superhero" thing by almost a full year (Namor didn't arrive until April 1939, ten months after Superman's debut).
The big question now, though?