The History of Shared Universes in Film

Dating back as far as the 1930s, the concept of two or more distinct movie franchises "sharing" the same continuity has been played around with for nearly a century, but it's only really caught on as a trend among blockbusters in the past decade with the success of the Marvel Cinematic Universe.
Universal Pictures were the first to cross-over the leads from Frankenstein, Dracula, and Wolf Man franchises, while Toho followed this up decades later by pitting Godzilla against (and later teaming up with) Mothra and Rodan.