Superhero movies are consistently the top performers at the box office nowadays. The general public is intimately familiar with characters like Deadpool, Scarecrow, and of course Iron Man. But 16 years ago, the blockbuster landscape was markedly different. The Top 10 domestic grossers of 1999 included The Sixth Sense, Runaway Bride, and Austin Powers: The Spy Who Shagged Me. Audiences were routinely turning out for movie star-fronted comedies or buzzworthy blockbusters like The Matrix, but comic book adaptations were still a low tier genre that was a hodgepodge of the violent (Spawn, Blade), the silly (Batman & Robin, Judge Dredd), and the baffling (Barb Wire).

But in 2000, filmmaker Bryan Singer forever changed the way comic book adaptations were perceived by approaching an adaptation of X-Men as a piece of cinema first and foremost. This was evident from the get-go, as Singer made the bold move to open this movie about superpowered mutants with a sequence set in 1944 German-occupied Poland, as the young Erik Lensherr’s parents are sent off to a concentration camp, thus revealing his powers for the first time.

Think about that for a second. To audiences who perceived comic book movies as colorful and silly, the first footage they saw in X-Men was of a Jewish family being torn apart at a concentration camp. The notion of blending grounded, real-world drama with a property like X-Men was unheard of. Sure Richard Donner’s Superman was a profoundly human picture, and Tim Burton’s Batman was ambitiously unique, but in the year 2000 the comic book genre was essentially dead. The circus show that is Batman & Robin had killed the Batman franchise in 1997, and all that was left were niche properties like the R-rated Blade and the dark, horror-infused Spawn. There was no template for what a superhero movie could be, but it definitely wasn’t considered a “serious” genre. Then along comes X-Men, which isn’t as dark as the niche adaptations that preceded it, but doesn’t parade its characters as silly cartoons either—Singer took a realistic approach to introducing the X-Men, and it worked tremendously.

Singer’s “in” with the film is Hugh Jackman’s Wolverine, who serves as the comics illiterate audience’s surrogate. Wolverine is just as dubious as we are by the notion of a superpowered team of mutants using codenames like “Storm” and “Cyclops”, and as he grows to embrace the more comic book-y aspects of the X-Men, so does the audience. And Singer, with cinematographer Newton Thomas Siegel, eschews theatricality or grandiosity in favor of a grounded visual approach that makes the settings feel familiar and, by extension, more human despite the special effects on display.

Moreover, with X-Men Singer dove deep into the thematics of the story, not content with just making a piece of popcorn entertainment. He recognized and emphasized the parallels between the Professor X/Magneto relationship and the relationship between Martin Luther King Jr. and Malcolm X while also leaning heavily on the social relevance of a group of minorities who are maligned simply because they’re different. This all goes back to the comics source material from which Singer and screenwriter David Hayter drew, but in approaching X-Men the same way Singer would approach a film like The Usual Suspects, it allowed for the drama and weight of the themes to bubble to the surface in an organic way.

The true test of this unique approach to X-Men, however, would be the box office, and indeed the film was a success, grossing nearly $300 million worldwide. It of course spawned many sequels and jump-started a franchise that’s still going strong today, but its impact on the world of comic book adaptations cannot be understated. Singer’s grounded approach to the characters and world of X-Men paved the way for Christopher Nolan to double down on that take with Batman Begins in 2005. It led to accomplished filmmaker Ang Lee tackling The Hulk in 2003’s Hulk. And yes, it opened the door to Marvel Studios venturing out on its own with 2008’s Iron Man.

Together with Sam Raimi’s equally ambitious but a tad more colorful Spider-Man in 2002—which broke out huge at the box office and officially made the superhero genre a “thing”—X-Men prepared audiences for comic book movies that could simultaneously treat the source material with respect but also work as genuinely great pieces of art. No longer was a comic book movie seen as a second-tier genre property—it could be a film, approached with the same sincerity as any other serious drama. For that, we have X-Men to thank.