DC Comics History
Long-running series all have in common the need to occassionally adjust or revamp their continuities. In 30-odd years, the James Bond movies have sort of ignored any real attempt at continuity, leaving us to wonder if the 007 of 'Goldeneye' has any recollection of the events of 'Dr. No'. In1984, the folks at Toho marked Godzilla's 30th anniversary by simply announcing that the continuity of the first 14 sequels would be ignored. The American "Godzilla" film simply ignores all 22 previous Godzilla films.
But the longest continuous fictional reality belongs to DC Comics, who have an interesting tradition... When their complex continuity conflicts with itself or just needs updating, they explain the update in the context of a story.
DC Comics (known as National Periodical and by other corporate names before adopting the name of its oldest continuous publication, Detective Comics) started in 1935. It wasn't the first company to produce comic books, but it may have been the first to be successful featuring original material. Before DC, comic books were basically collections of newspaper comic strips.
Early DC comics featured characters who resembled popular newspaper comic strip regulars like Dick Tracy and Mandrake the Magician. But in 1938 they decided to start an aditional publication and try something a little different. In Action #1 DC featured a somewhat outlandish character created by a couple of kids (Jerry Siegel and Joe Shuster, who had been unable to get the strip into the newspapers)... Sort of a contemporary Hercules in a Flash Gordon-like costume. The suits at DC weren't really sure about this character, and avoided using him on the next couple of Action covers. But word got back to them that buyers weren't asking for "Action Comics". They were asking for "the one with Superman".
The next year DC struck oil again with another, rather different, costumed vigilante character called Batman. The kids behind this creation were named Kane and Finger.
The success of Superman and Batman would spawn a legion of imitators. The most successful of which was Fawcett Comic's Captain Marvel (SHAZAM!).
Through the early 1940s, DC would introduce more superheroes. The Flash, Hawkman, the Green Lantern, and many others. During World War II, these characters' originally idependant realities, along with Superman and Batman's, were merged as they banded together to form the Justice Society of America and the All-Star Squadron. It was the Golden Age of DC Comics.
After the War, paranoia about Communism and All Things UnAmerican (or anything fun!) brought comic books under scrutiny. In time censorship, or the threat of it, made it nearly impossible to create really interesting stories for superheroes, and by the early 1950s all but the most popular superheroes were being discontinued.
When the climate started getting a little freindlier a few years later, DC began to reintroduce some of the classic superheroes in new versions, with updated costumes, different secret identities and origins. This was the start of the Silver Age of DC Comics. It was also a continuity-buster.
In the early 1960s DC established an explanation of the relationship between the Golden Age and Silver Age superheroes. Barry Allen, the Silver Age Flash used his super-speed powers to travel to a parallel universe, where he met the Golden Age Flash Jay Garrick. Since Jay had interacted with the Golden Age Green Lantern, Hawkman, and others, it only stood to reason that all the Golden Age characters lived in this alternate world, which was designated 'Earth-2'. Barry Allen's own world, with it's Silver Age characters, was designated 'Earth-1'.
But what about characters like Superman and Batman, who had existed continuously through the Golden and Silver Ages with the same origins and secret identities? Jay Garrick and Barry Allen had both interacted with them. The solution was that each Earth had it's own version of Superman and Batman. This worked out particularly well for Superman, as the introduction of Superboy into his history conflicted with his established past. Now the Earth-1 Superman was the grown Superboy, while the Earth-2 Superman had started his career as an adult.
Earth-1 was the setting for DC's primary stories, and there the characters were always kept at their traditional ages with their traditional situations more or less static.
The Earth-2 characters were identified with World War II, and so were allowed to age and change. Because Earth-2 didn't directly affect the primary continuity of the major comics, the writers had the chance to explore how the characters' lives had actually played-out, without having to "reset" them to a traditional status at the end of each story. They could get married, have kids, grow old, change careers, retire, even die.
The concept of alternate Earths was attractive to writers. An Earth-3 was created, where all the Earth-1 heroes existed as villians. Other alternate Earths were created to explain lapses in continuity.
When DC aquired characters from other publishers, they placed them on their own Earths... For instance, Captain Marvel and his associated Fawcett Comics characters were placed on Earth-S, where they could occasionally be used (even crossing dimensional barriers for team-ups with Superman), but would usually be out of the picture.
After a couple of decades, the "Multiverse" concept was getting pretty muddled. More Earths had been created than most folks could hope to keep track of, and characters migrated from Earth to Earth. "That happened on another Earth" became a too-easy solution to any bit of best-forgotten writing. To celebrate their 50th anniversary, DC Comics decided to streamline and unify their continuity.
Naturally, they did it with a storyline.
The Crisis on Infinite Earths was a year-long series in 1985 with countless cross-overs. It detailed how a mad experiment billions of years ago had retroactively shattered the Universe into the miriade universes of the Multiverse. This made reality too unstable to withstand the assault of a cosmic villian called the Anti-Monitor.
The climax of the Crisis had heroes from various Earths going back to the dawn of time to battle the Anti-Monitor and prevent the shattering of the Universe into the Multiverse.
The result of this battle was that, after a flux period whilst history realigned itself, the multiple Earths never existed. There was only one Earth. It resembled Earth-1 more than any of the others, but incorporated aspects of Earth-2 and theoretically all the other alternates. For instance, Jay Garrick, along with his Golden Age buddies, had been the costumed "mystery men" of the 1940s, then retired. Decades later, they saw a new generation adopt the names they had used as the new era of superheroes arose on the same Earth. The 1940s era of the mystery men on the Post-Crises Earth had no Superman or Batman, as they wouldn't arise until decades later as the first of the new superheroes. Along with all the characters of the Post-Crisis Earth (including characters who had once been asigned to various Earths), Superman and Batman were essentially "new", with their histories open to revision. The Post-Crisis continuity stories commenced several years into the careers of Superman and Batman, as this was necessary to allow for the back-stories of associated characters like Dick Grayson (the first Robin). Mini-series and flashbacks were used to fill in the events of those early years in their careers.
So DC's flagship characters each have three significant comic book incarnations. The Golden Age (Earth-2), Silver Age (Earth-1), and Post-Crisis versions. The Earth-2 and Earth-1 versions knew one another. As far as the Post-Crisis versions know, the Earth-2 and Earth-1 versions never existed.
The Post-Crisis verions were later retrofitted by the Zero Hour storyline, which served primarily to condense the events of the characters' histories into a shorter span of time, thus keeping them from being aged as much.
Clark Kent of Earth-2
The original, Golden-Age Superman.
Clark Kent of Earth-1
The Silver-Age Superman.
Post-Crisis Clark Kent
The current Superman.
Bruce Wayne of Earth-2
The original, Golden-Age Batman.
Bruce Wayne of Earth-1
The Silver-Age Batman.
The current Batman.
Dick Grayson of Earth-2
The original, Golden-Age Robin.
Dick Grayson of Earth-1
The Silver-Age Robin.
Jason Todd of Earth-1
The Earth-1 Batman's second Robin.
The Robin of the Earth-1 Batman's hypothetical future.
Post-Crisis Dick Grayson
The current Batman's original Robin.
Post-Crisis Jason Todd
The current Batman's second Robin.
The Post-Crisis Batman's third and current Robin.
The murderer of Bruce Wayne's parents.