It might seem crazy, but at one point Shazam was the most popular comic book superhero in the world. Back when he was known as Captain Marvel, the hero and his title, Marvel Family Comics, were some of the best-selling comics on Earth, outselling even Krypton's most famous son at times. It’s fitting as the two powerful heroes have been inexorably connected starting from the moment Captain Marvel debuted in 1939 as Fawcett Comics' flagship character…and Detective Comics, Inc. found themselves very unhappy about the similarities that they saw between the broad-chested new hero and Superman.

Superman and Captain Marvel both represented a very popular, yet specific, wish fulfilment: the dream of becoming the most powerful person on Earth. Superman’s take on this idea was that of a young man who thinks he's normal but discovers that, in fact, he's completely unique and may just be the key to saving the entire planet. Captain Marvel turned the concept on its head by creating a character who literally represented the reader (who were almost all kids at the time). Billy Batson was an otherwise perfectly normal boy who could transform into an adult hero that just happened to be a lot like Superman.

Detective Comics, Inc. ended up taking Fawcett to court, arguing that Captain Marvel was derivative of Superman. The lawsuit would stretch on for over a decade—during which time DC even changed its name—and eventually ended with the company now known as National Comics Publications coming out on top, which meant that Fawcett could no longer publish comics about their most famous hero. Years later in 1972, DC would license the Marvel Family, including the hero that they believed was inspired by the most iconic member of their trinity.

Once DC took on the Marvel Family, Captain Marvel ended up joining the ranks of the DC Universe and would soon become one of its most powerful and unusual characters. Now under the banner of the same publisher, the two heroes were destined to become even more closely connected than before. The similarities between the heroes made them natural partners or foils, and the comic book canon of Shazam and Superman began.

In December of 1972, DC introduced Captain Marvel in a new title called Shazam! #1, and from his very first appearance—which was rebranded due to copyright issues over the similarly-named character at Marvel—he was aligned with DC's most famous hero, Superman. Though Big Blue only featured on the front cover, it was the beginning of a long relationship between the two heroes which would become vital to the journey of Shazam, especially as the hero wouldn't properly join main DC continuity until a few years later.

The first full crossover of Superman and Captain Marvel also happened to be the first time that the entirety of the Marvel Family was introduced into the DC Universe. Justice League of America #135-137 was a three-issue story arc known as "Crisis on Earth-S," which saw the nefarious King Kull attempt to take over Earth-S, which is, of course, the universe where the man who would be known as Shazam resides. The first issue saw the introduction of Captain Marvel's Squadron of Justice, who would fight alongside the Justice League to save the day.

In the second issue, fans met Billy Batson's friends and fellow heroes, Freddie Freeman and Mary Batson, who'd been struck down by the misfortune of having the power of the gods taken from them, leading to the Justice League, Justice Society of America and Squadron of Justice having to take their places as the heroes of Earth-S. The final issue saw the team's powers restored by Johnny Thunder, and that was lucky because the third part of the story was historic for another reason: it saw Captain Marvel and Superman fight for the first time after the Man of Steel was turned evil by Red Kryptonite. That dichotomy of friend and foe would follow Billy and Clark throughout their comic book careers, all the way through to future animated projects.

Another place that fans saw Superman and Shazam come together was the DC Comics Presents series. A particularly notable issue is #49, which saw Billy Batson have a startling dream about becoming Captain Marvel, including the Seven Deadly Enemies of Men A.K.A. the Seven Deadly Sins, and brought the classic Fawcett / Captain Marvel villain Black Adam to the pages of DC. Though he only appeared in one issue of the original Captain Marvel series—which was reprinted in DC's Shazam! #8—his iconic black and gold costume and evil reflection of Billy's powers made a huge impact on fans of the Golden Age character, and has stayed at the forefront of Shazam lore since he joined the DC Universe.

DC's lauded animated properties kept the pairing of the two heroes alive with series like 2004's Justice League Unlimited, which saw Shazam join the famed team. In Superman/Batman: Public Enemies, Billy Batson got his first larger role in a DC animated film. But Billy's most famous animated team-up with Krypton's last son came in Superman/Shazam!: The Return of Black Adam. The 2010 short coincided with Shazam's newly minted name and refined backstory, which would be cemented in the realm of comics with the line wide New 52 relaunch.

Of course, the reason that we're all here is because of this year's Shazam film. The hit movie has one of the biggest and funniest Easter eggs of all-time with a late stage cameo from the Man of Tomorrow. It's a nice nod to the connection between the two characters and hints at the sprawling history they share while still feeling like a fresh way of connecting the heroes. With over one hundred and sixty years of history between them, Shazam and Superman are nothing less than a powerhouse pairing whose strange past has led to an unusual shared story which has shaped both of their journeys from publisher to page to screen.

Shazam!, starring Zachary Levi, Asher Angel and Mark Strong, is now in theaters. To discover more about DC's latest movie, click here.

Rosie Knight writes about comics, movies and TV for and Be sure to also check out her article on Shazam!'s Seven Deadly Sins and follow her on Twitter at @RosieMarx.