Are the Flinstones us thousands of years in the future?
This theory had its beginnings in the 1987 television crossover special, The Jetsons Meet the Flintstones. In the special, Elroy built a time machine that was supposed to send him and his family into the 25th century. The machine malfunctioned, and they ended up going to Bedrock instead. What this theory suggests, however, is what if the machine actually took them to the 25th century, and what if the Flintstones is actually set in a post-apocalyptic future?
It is believed that a nuclear war occurred hundreds of years prior to the Flintstones, and that the characters from the Flintstones were all that remained of a war-torn world. It is also believed that after this catastrophic war, many people evacuated from the Earth, and built houses in the air. When the radioactive levels went down, several of these sky-ridden individuals returned to the ground to repopulate the Earth. Many animals were also presumably mutated into dinosaurs from the nuclear fallout of the war.
What should also be taken into consideration regarding this theory is the year that the show came out in. The series came out during the height of the Cold War, and because of this fact, it's not too hard to believe that perhaps a bomb from said war was dropped on what would soon become Bedrock, and that the resulting explosion blew the inhabitants back to the Stone Age.
Additionally, the characters are seen celebrating Christmas, even though the stone age happened before the birth of Christ.
Divided Coexistence Theory
What if the time machine malfunctioned so much that it just teleported them to the earth below, and because the dwellers down there are living in such primitive conditions, they assume it's the past? Perhaps after the earth became harsh and hostile post-apocalypse, the rich fled to cities they built in the sky, leaving the less well off to languish in the remnants of a decimated civilization, who make do with the materials that remained and the faint memory of the technology that had existed before and attempt to recreate it in a clumsy and primitive way, a shadow of what once was. The Jetsons would be blissfully ignorant of this, having never been on the ground before, hence why they assume they have traveled far back in time.