Listed in chronological order.
The first definitive appearance of the universe that would later be known as Earth-2 was in New Fun Comics #6, with the first appearance of Doctor Occult. He was quickly joined by the Crimson Avenger, Superman I, Batman I, the Justice Society of America, and a score of other heroes. All DCU stories from 1935 to 1945 are Earth-2 stories. In 1945 some stories began being set on Earth-1, though this distinction wasn't clear until the 1970s. Into the mid-1950s, more and more comics were set on Earth-1. The first appearance of the Martian Manhunter, in Detective Comics #225, is considered the break point. After 1955, no comic should be considered set on Earth-2 unless clearly marked.
The history of Earth-2 was substantially similar to the real world. Known variations begin circa 1976, when it was revealed that apartheid had been abolished in South Africa much earlier than in our world. (All-Star Comics #58-66, 1976-1977).
In Crisis #10, Earth-2 was combined with four other Earths to become Earth-Sigma (see below). In Planet Krypton several Earth-2 Hypertime ghosts were seen (most notably, Batwoman). Also, in The Kingdom #2, a universe resembling Earth-2 was seen in Hypertime.
The first appearance of Earth-1 was in More Fun Comics #101, which contained the first appearance of Superboy. In several stories written in the 1970s it was made clear that the Superman of Earth-2 had never been Superboy. In the 1940s, of course, this wasn't true - the Superboy shown in More Fun was the same person as the Superman appearing in Action Comics. Nevertheless, "Superman I never was Superboy" is what was considered canon at the time of the Crisis, and presumably is still true today. During the 1950s, more and more comics were set on Earth-1, including the adventures of Captain Comet (Strange Adventures #9, 1951), Martian Manhunter (Detective Comics #225, 1955), Flash II (Showcase #4, 1956), Green Lantern II (Showcase #22, 1959), and the Justice League of America (The Brave and the Bold #28, 1960). After the Manhunter's first appearance, all DCU comics can be assumed to be on Earth-1 unless marked otherwise. In Crisis #10, Earth-1 was combined with four other Earths to become Earth-Sigma (see below). In Planet Krypton several Earth-1 Hypertime ghosts were seen. Also, in The Kingdom #2, a universe resembling Earth-1 was seen in Hypertime.
Earth-Sigma (name proposed by me; "sigma" can mean "sum" in mathematics) was a merging of Earths 1, 2, 4, S, and X. It was also a constant work-in-progress. When initially created, all heroes who had fought the Anti-Monitor at the Dawn of Time remembered their original universes. Shortly they all forgot, leaving the Psycho-Pirate as the only one who remembered. Over the next few years, many major characters had their origins revamped and updated, including most of the most important heroes. Unfortunately, since this wasn't done all at once, massive contradictions arose. Eventually DC decided to clean house again, and Earth-Sigma was destroyed in Zero Hour. In Planet Krypton and The Kingdom several characters are seen who appeared during the Earth-Sigma period.
"Earth-0" is the commonly-accepted name for the Post-Zero Hour universe. It was created at the Big Bang through the efforts of several heroes in Zero Hour #0, the last issue of that miniseries. (It was numbered backwards, from 4 to 0.) It is largely identical to Earth-Sigma, except that the Legion of Super-Heroes was completely "rebooted". Theoretically, all the contradictions of Earth-Sigma have been ironed out. Unfortunately, DC history defies ironing at this point, and new contradictions have appeared. Those characters who were present at the Big Bang continue to remember Earth-Sigma. This quirk has only come up once, when Superman met the new Legion and confused them greatly. There is no dividing line between Earth-0 and the Central Timeline; it is possible they are one and the same.
"How does 'Hypertime' work? Off the Central Timeline we just left. Events of importance often cause divergent 'tributaries' to branch off the main timestream. ...On occasion, those tributaries return -- sometimes feeding back into the Central Timeline, other times overlapping it briefly before charting an entirely new course."
-Hunter, The Kingdom #2
The Central Timeline is the "main" and current DC Universe. The other worlds of Hypertime are not its equals; they're divergent and more ephemeral. CT may be the same as Earth-0. It may also theoretically be the thread that binds the whole Main Sequence together; the Central Timeline might originally have been Earth-2, and changed to each of the other Main Sequence worlds as time passed.
|Jonathan Woodward, firstname.lastname@example.org|
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