Re: Time Frame for Appearance of Superhumans
Before the 20th Century, there were a few super-normals like Zorro or the Lone Ranger floating around, but very few and far between. Masked heroes became more common during Prohibition when the Mob started gunning down lawmen/prosecutors/judges who were too effective at busting rum-runners. If some of these were stronger than normal or faster, it wasn't obvious enough to be noticed. There were stories of superhumans, but these mages and psionic marvels kept to the shadows if they really existed.
Powered people didn't show up until the late 1930s, and they were relatively weak powers at that. By the end of the war, some men could lift tanks or fly with the fastest pursuit fighters, but they were rare.
The power level of Superman or Green Lantern didn't show up until the 1960s, and didn't become common until the 1980s. Well, common as in "more than one per continent".
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For the longest time, I never was really concerned for explaining this slow progression. I like the Galactic Champions theory of returning magic, and have retroactively applied that to my world.
And yes, the power increase is generational; the 1800s allowed exceptional humans to push their limits, but they were limited to human normal. At the turn of the century, an invisible corner was turned and certain humans suddenly had no real upper limits. The fact that most of these people lived in the electrified cities probably has nothing to do with it (electricity is the early "nuclear accident").
The critical mass was reached with the generation born in the early 1920s; some of these were blatantly superhuman. Their children were slightly more powerful; their children even more powerful, etc.