I give you a tale that is part origin story, part conspiracy theory.
“I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones.” - Albert Einstein
Humanity strode its way into the 22nd century with it head held high. Financial order was in place, jobs were prevalent, and many of the diseases that had plagued the planet in the previous centuries had now been eradicated or made so ineffectual that they could be cured with over-the-counter medicines. (“*cough, cough* Hmm. That darn cough is back again... could be lung cancer or maybe even Motaba. I'd better take an aspirin just to be safe. That ought to clear it up.”) It was a time of great expansion. Of great improvement! Mankind wanted for nothing. Well, except oil. Mankind seemed to still really love oil. And war. Oil and war were the last two things it really cared about. This of course led to the classic scenario of “I want that and if you don't give it to me I'll nuke you until you glow like a firebug.” And as one war turned into two, and two into four, and those four, billowing and splintering off into a small series of civil wars, the Earth took a massive beating. Its resources were depleted and each day the clock ticked closer & closer to Armageddon. In response to society's war-lusting kill-party, the world's richest and most elite spent all their funds developing clean energy-efficient fuels. Flying cars and robots sprang up in the houses of the mega rich as they tried their best to ignore the growing turmoil of the outside world.
As the doomsday clock ticked on and on, every hand of power was perched over the shiny BIG RED BUTTON. Sensing the impending doom the wealthy decided it would be best to skip out on the apocalypse, ascending from the Earth, seeking refuge in the safety of the clouds. C. Arthur Speckly (later known as C. Arthur Spacely, after changing his name to fit the astral themed motifs of the sky-ward people) manufactured space sprockets - the little cogs that paved the way to the salvation of humanity. When the first bomb dropped, a large chunk of New York dislodge from the Earth and came crashing down near Anchorage, Alaska. Following this dozens of A-bombs were dropped and vile chemical gases were spewed onto the flora and fauna. The Earth was rocked with explosions as shells were lobed back and forth by each warring country. But the rich rose above the chemical dust and the nuclear fallout. They rose up above it all in their houses of the sky. In their homes of the future.
Up in the heavens society moved on, pretty much as it had on the surface of the planet. Over time the economic structure shifted and a middle class emerged. Things returned to a more simple time, a time of slavish, yet sassy robots, flying cars, and talking dogs (it was a veritable golden age). In school they did not teach the children of the wars that led to their escape to the skies, it was believed that dredging up the past was in bad taste and that the dead world below was best left forgotten. It was too unpleasant to think of all those who had died in the radioactive inferno. It was too burdensome to think about the death of their civilization.
But little did they know...
In the years following the war the world below was met with some interesting changes. For a long time mutants roamed the scorched earth, spending their time doing what mutants often do: eating each other. As decades passed the super mutants ate themselves into extinction, and arising from all the mutant-eating-mutant action came a more peaceful and submissive race. A primitive breed of humans, just barely past the level of Neanderthals in appearance; but with minds hungry for knowledge, and hearts hungrier still for the quiet past that had been blasted away from the Earth. What rose from the radioactive rubble was, like the society above, a simple American middle class. Technology was primitive, constructed crudely from stone, it mimicked the comforts of the past in its own simple way. The dinosaur-like creatures, a heavily mutated byproduct of the chemical wars (some even gifted with torpid forms of speech) were used as farm equipment (and later as household appliances). The good life returned. It was a veritable golden age of foot-powered cars, stone newspapers, and mutated talking dinosaurs. A modern stone age.
Sometime later, the two societies would collide. The Jetsons, a middle class family, whose patriarch George Jetson worked for the descendant of C. Arthur Spacely, would travel to the ruined world below meeting another family known as the Flintstones. The Jetson's son, a scientifically gifted 6-year-old named Elroy, believed he had invented a time machine that had carried his family into the past, but the sad and somewhat Twilight Zone-like irony was that his time machine was nothing more than a teleporter, and that the crude humans they were meeting were not early primitive man, but the broken (yet proud) remains of their own abandoned culture.
The two societies would never meet again.
A sad tale...
But still, they had talking dogs and dinosaurs for lawn mowers, so it wasn't that bad for either side.