During the extinction event in which a meteorite crashing into Earth caused the extinction of the dinosaurs and provided the conditions necessary for the evolution of mankind, the impact was so powerful that it created a parallel dimension with its own Earth; here, it conversely caused the extinction of mammals and gave rise to intelligent dinosaurs within that dimension. Dinosaurs living at the meteorite impact site were exposed to its geomagnetic energy, gradually evolving into the Portal Guardian cult. The Guardians were devoted to worshiping the monolith and maintaining the gateway between worlds, secretly ruling through the Monarchy. They harnessed its energy into the grid that powers their great city.
However, their deteriorating dimension forced the planet's population into the capital city, straining resources and fracturing confidence in the Monarchy. The Portal Guardian cult lost influence and faith, further dividing the already desperate people. General Koopa, head of military command, proposed merging the dimensions to claim the long-coveted stability and resources of our side. The Portal Guardians refused, though Koopa was given oversight of the city's power grid for research into conservation efforts.
General Koopa used this position to secretly develop the de-evolution technology for a military coup that saw the slaughter of the entire Portal Guardian cult and the King turned into Fungus. Only Daisy's mother escaped, fleeing into our world with her unborn daughter and the meteorite pendant necessary for Koopa's planned merge. The General pursued her into our dimension, first witnessing New York City.
Koopa soon became obsessed with his vision of recreating the modern metropolis in his own dimension. However, he was unable to accomplish it and instead created a grotesque parody, which deepened his desire to rule Earth instead. While Koopa maintained that his desire to conquer the parallel Earth was meant to return his people to their former dominance, it is implied that he only sought Earth for himself.
The Dinohattanites are a species that evolved from dinosaurs displaced millenia ago from our world. While most of the population is indistinguishable from mammalian-evolved humans, some do possess reptilian traits such as tails, slitted pupils, scales, and oviparity.
Typical dinosaurs also appear in the city, yet are treated as pets or wild animals. Koopa himself calls Yoshi a "throwback", suggesting a sense of racial superiority to their ancestors. Other animals, such as real-world lizards and snakes as well as fictional video game Spinys, are seen or mentioned as food sources to this world's humans.