I will continue to celebrate Stan Lee's legacy in comic books (and more) with this series, The Life and Times of Stan Lee.
Today, the Marvel Cinematic Universe is likely the most successful example of an interconnected universe of stories, with an upcoming Avengers film poised to use the entire 11-year history of the MCU to its advantage next year. However, the very notion of an interconnected film universe wouldn't be possible if it were not for the success of the interconnected Marvel Comics Universe, something that was created in the 1960s through the efforts of Stan Lee, Jack Kirby, Steve Ditko and many other Marvel artists of the era.
In early 1940, MLJ Comics released Pep Comics #4 (by Harry Shorten and Irv Novick), which ended by telling readers to go read Top Notch Comics #5, where the villains that the Shield faced off against in Pep Comics would also take on the Wizard...
And sure enough, in Top Notch Comics #5 (by Will Harr and Edd Ashe), the Wizard and the Shield meet each other...
That, right there, was the first time that superheroes from the same comic book company shared a story together. This was soon followed by friends Carl Burgos and Bill Everett having their Human Torch and Namor characters fight against each other in Marvel Mystery Comics #9 before they ultimately teamed up.
National Comics then upped the ante with All-Star Comics #3 in late 1940 that took a number of characters from National Comics' various titles (mostly the ones put out by Max Gaines' All-American Comics, which was a subsidiary of National Comics that actually briefly split from National for a little bit before Gaines decided to get out of superhero comics and sold his superhero titles to National so that he could pursue what he believed would be his big money-maker, Picture Stores From the Bible) and introduced a framing sequence to the series (that was initially an anthology featuring solo stories of different superheroes) that suggested that the heroes were all part of a big superhero team known as the Justice Society of America.
So the idea of a shared superhero universe was something that existed long before Stan Lee and Jack Kirby created the Fantastic Four in 1961. However, what would soon be known as the Marvel Comics Universe was a much different animal than any of these other comic book universes and it would soon change the way people read comic books forever.