The character of Superman was created by Jerry Siegel and Joe Shuster, and has been continually published in a variety of DC Comics book titles since its premiere in 1938. There have been several versions of Superman over the years, both as the main hero in the stories as well as several alternative versions. In mainstream comic continuity Originally, there was only one Superman. However, beginning in the late 1940s, demand for comics shifted from superheroes as war, horror, science fiction and romance comics became more popular. Most of the DC Comics superhero titles were cancelled or began featuring the more popular genres. Superman, along with Batman and Wonder Woman, continued to be published. To explain how Superman could have been active as a young man in the 1930s when later stories show Superman still youthful in the 1960s, DC Comics developed a multiverse, the existence of several realities. The original Golden Age Superman was retconned to Earth-Two, while the then-currently published hero was assigned to Earth-One. In addition to these main two "official" variations of the standard Superman character, a number of characters have assumed the title of Superman in many variant stories set in both primary and alternative continuity. Following the storyline of The Death of Superman and during the subsequent Reign of the Supermen storyline, a number of characters claimed the mantle. In addition, Bizarro, for instance, is an imperfect duplicate of Superman. Other members of Superman's family of characters have borne the Super- prefix, including Supergirl, Krypto the Superdog and, in some instances, Superwoman. Pre-Crisis 1938-1986 Superman was first published in 1938. In 1986, DC Comics published Crisis on Infinite Earths, a 12-issue series designed to clean up and establish a new continuity for DC, affecting not only Superman, but all of the DC Comics characters. The versions of Superman from this time period are traditionally divided into three main periods. Golden Age 1938-1950s The first version of Superman began being published by DC Comics in 1938 appearing in Action Comics #1. To explain discrepancies in the aging of Superman across several decades, his earliest stories were retroactively portrayed as having taken place on an alternative world called Earth-Two. These stories take place from 1938 until the late 1950s, although the exact dividing line is unclear and some stories happened identically to both the Golden Age and Silver Age Supermen. The Golden Age Superman started his