Earth-One was the designation for an Earth, and the Universe it inhabited, that existed prior to the Crisis on Infinite Earths.
Earth-One itself was populated by various individuals who resembled those of its primary Multiversal counterpart, Earth-Two. Some were direct parallels to their predecessors (ex. Superman (Earth-One) and Superman (Earth-Two)), some less so (ex. Atom (Al Pratt) and Atom (Ray Palmer)). The two Earths were closely linked in terms of parallel development, although Earth-Two individuals usually predated their Earth-One counterparts by a few decades.
The primary superheroes were limited to the western heroes of the eighteen hundreds and few others, until Superman arrived from Krypton and ushered in a new age of heroes. On this Earth, Superman had a career as Superboy and teamed with the Legion of Super-Heroes in the 30th century.
From the perspective of the Earth-One Universe, the first travel to another parallel universe occurred when The Flash (Barry Allen) accidentally changed his vibrational frequency to match that of Earth-Two, where he met Jay Garrick, the Golden Age Flash. That meeting resulted in years of interaction between the original super-hero team, The Justice Society of America, and their younger Earth-One counterparts, The Justice League of America.
As Barry Allen was the first traveler, the honor of naming the two Earths fell to him - his universe's Earth was called Earth-One, Jay Garrick's became Earth-Two. As noted years later by the elder Superman, they (the JSA) were too polite to mention that they were around first .
As Allen explained to Garrick in their first meeting:
- "..I vibrated so fast -- I tore a gap in the vibratory shields separating our worlds! As you know -- two objects can occupy the same space and time -- if they vibrate at different speeds!
My theory is, both Earths were created at the same time in two quite similar universes! They vibrate differently -- which keeps them apart! Life, customs -- even languages -- evolved on your earth almost exactly as they did on my Earth! Destiny must have decreed there'd be a Flash -- on each Earth!" 
During the Crisis on Infinite Earths, Earth-One was destroyed. The New Earth that emerged after the Big Bang possessed elements of Earth-One's history, along with elements similar to that of Earth-Two, Earth-Four, Earth-S, and Earth-X.
- The Earth-One Universe is not known to have existed to the general populace of New Earth. Part of the Pre-Crisis Multiverse.
- Earth-Two superheroes appeared in Earth-One comic books.
- No trivia.
- Characters from Earth-One
- Other things related to Earth-One
- Earth-One's Comic Appearances
- Reality Gallery: Earth-One
Links and References
- Earth-One based on the article "Comments on a Finite Number of Earths" in the OFFICIAL CRISIS ON INFINITE EARTHS CROSSOVER INDEX by Lou Mougin and Mark Waid.
- Earth-1's heroes who were active during the Golden Age by John McDonagh
- Earth-One timeline from the Five Earths Project (Note: contains some data from fan fiction)
- Earth-One Character Indexes from Mike's Amazing World of DC Comics