It's a bird, it's a plane, it's a Platonic concept...

Henry Cavill as Superman in <i>Man of Steel</i>. Photo / Warner Bros

Of all the superheroes, it seems most fitting that Superman should have revealed himself in a dream, zooming through his creator's head in a single night of frantic inspiration.

"I am lying in bed when suddenly it hits me," wrote his creator Jerry Siegel years later. "I conceive a character like Samson, Hercules and all the strong men I ever heard tell of rolled into one. Only more so."

The result, first seen 75 years ago this month brandishing a car above his head on the cover of Action Comics #1, was different only in degree from the body-stockinged blue streak who shortly returns to our screens in Zack Snyder's Man of Steel. Siegel and Joe Shuster's Superman was a comparatively earthbound creature whose powers extended to leaping 600ft, running "faster than a streamline train" and deflecting damage from anything smaller than a "bursting shell" - all attributes that speak volumes about the time and the conditions in which he was born.

Borrowing liberally from pulp heroes Doc Savage, John Carter and the genetically engineered strongman Hugo Danner, the first Man of Steel was a wish-fulfillment dream of human power in an age of overwhelming machinery and armament. Time would transform him into the beneficent alien demigod familiar to modern audiences, but by then the concept of Superman was already hovering, impregnable, in the Platonic realm of ideas.

Siegel and Shuster's early comics cast Superman as a defender of the little guy, pitting him against hoodlums, crooked lobbyists, profiteering industrialists and other enemies of the American working class, but the approach of war offered wider horizons. By D-Day, his name adorned vehicles across the Allied war effort; actors playing the character could be heard on radio soliciting for blood drives and war bonds. Soldiers read Superman comics at the Normandy landings, as their hero gamely battled "Japanazis" and "Japoteurs" in four-colour adventures of his own. Militant America took him to its heart as the square-jawed symbol of its growing role as world policeman, but the character soon soared above questions of national identity. In vain did the psychiatrist Fredric Wertham protest, in the mid-Fifties, that Superman comics gave children "fantasies of sadistic joy in seeing other people punished over and over again while you yourself remain immune". That familiar blue streak - Bird? Plane? - had split the skies of popular culture, and across the world a gap had been filled that no one knew existed.

In 75 years, Superman's appeal has remained broadly constant, despite the changing fortunes of the media that carry him. Comics fall from favour, television series and embarrassing films come and go, but Superman T-shirts still sell. Unlike the generations of superheroes spawned by his example, the Man of Steel never loses what the comics writer Grant Morrison has called "that essential, unshakeable quality of Superman-ness the character possesses in every incarnation, which is divinity by any other name". Batman, a hero whose psychic roots lie deep in bereavement and psychosis, can be turned at a writer's whim from camp self-parody to delusional obsessive or hard-boiled vigilante, but Superman is always Superman, wise, benevolent and unswervingly assured of the moral high ground. Some of this has to do with careful image management by DC Comics, but not all. Even elaborate reinventions such as Mark Millar's Red Son, which imagines the Kryptonian landing in the USSR and growing up a Soviet hero, bump up against the gleaming decency of the character. If he were other than he is, he would not be Superman.

This combination of goodness and power can leave writers struggling to create dramatic tension. Siegel and Shuster laid the groundwork by devising Clark Kent, who put a human face on the invulnerable alien. Subsequent caretakers invented Kryptonite to bring their hero to his knees. By the Eighties, writers had evolved Superman's powers to the extent that there was literally nothing he couldn't do - Planet-juggling? Time-travel? No problem - and the character's whole history had to be rewritten so that his adventures could continue.

Superman's myth has also proved open to interpretation and appropriation, so over the years the character has been claimed as a symbol of Jewish identity, Christian solidarity, Buddhist incarnation and (at the David Icke message boards, with reference to the symbolic "serpent-like" S) the all-conquering reptile conspiracy to rule the world. One semi-serious theory claims Superman is Jewish, not just because of his Jewish creators and Kryptonian name (Kal-El, reminiscent of the Hebrew for voice and God) but because his origin myth echoes that of Moses; the baby found in a crashed rocket rather than a reed basket. Larry Tye, author of a diverting Superman biography, observes drily that having "-man" at the end of your name usually makes you "Jewish, or a superhero, or both".

Some Christians think they have an equally strong claim. Richard Donner's 1978 film made high-camp drama of the parallels with the Christian Incarnation, as Marlon Brando minced about in white as Superman-pere, promising to send Earth his "only son". Writers such as Stephen Skelton went further, unearthing the words Lucifer in Lex Luthor and cleric in Clark. And Alvin Schwartz, a former writer for the Superman comics, published a book detailing his own encounter with the character in the form of a tulpa, or Buddhist spirit avatar. The idea of Superman, Schwartz contended, had become strong enough to give the character concrete form. He had bumped into it in the back of a taxi.

But one of the most attractive theories about Superman's lasting appeal comes from the Scottish comics creator Morrison. In his book Supergods, Morrison proposes the recent mass-media frenzy for superheroes addresses a deep, specific cultural need. As technological progress and medical science rushes forward, he suggests, we look to superhero myths for guidance in approaching our own enhanced scope and reach. The heroes offer, he concludes, "a bright flickering sign of our need to move on, to imagine the better, more just and more proactive people we can be". And why not? If Superman first took form as a comforting fable of human physicality in a machine age, could his latest incarnation guide us as we ascend to the next plateau of our own evolution? It is, at least, a heartening frame of mind in which to approach the blockbuster season..

• Man of Steel opens in New Zealand on June 27