The concept was originally formulated by DC editor
Julius Schwartz and writer
Gardner Fox and introduced in
Flash #123, "Flash of Two Worlds", in which the
Golden Age and
Silver Age versions of
the Flash fought side by side. The Golden Age Flash appeared again in issue #129, and the entire
Justice Society of America appeared in issue #137.
The JSA and the Justice League of America then teamed up in JLA #21-22. It was then that the terms Earth-1 (for the Justice League's world) and Earth-2 (for the parallel world the Golden Age heroes inhabited) were introduced.
See also Earth-1, Earth-3, Earth-S, Earth-X, and Earth Prime.