I tend to be a fan of the this is all happening way in the future after the apocolypse of a world that used to be earth or earthlike.
Thundarr?
(I like
Thundarr by the way. I used to think the show silly, but recently I've been rewatching it. That show was well written and way ahead of it's time in relation to plot/storyline. The other morning I saw an episode dealing with modern Chimeras, though they didn't call it that (they just said "an experiment dealing with creating a new form of life." It implied a lot about up-coming and futuristic technology and science which is only now starting to pan out. It had a lot of silly science, like cars or charter planes lasting for 2000 years, but it also was quite inventive and insightful on some other issues.)
We play and I DM/GM six RPG worlds, or settings, or milieus.
D&D/Fantasy - Humans set in Constantinople circa 800 AD. Not humans set in geographically identical world with very different cultures than human cultures.
Pulp - Set in America, circa 1930, but traveling around the world. Often uses real history as basis for scenarios.
Military/Law Enforcement/Espionage/Intel - set in present time during War on Terror (with background on Cold War) and using real, current or projected events as bases for mission/scenarios.
Superhero (more like genetically or otherwise modified individuals) - set in same time frame as game immediately above.
Sci-Fi - two games, both set in future. One in the Federation, one outside the borders of the Federation and only involving intelligent, artificial, and/or alien life. Extrapolates upon current science.
So I guess time wise it's a wash. As for whether these worlds are like ours or not, four are our world (minus the superhero and related stuff) and two are not our world, but maybe something kinda roughly similar will develop. But I doubt it. Then again you never really know.