Some accounts suggest that the city’s inhabitants are actually aliens, inadvertently brought to Earth by the actions of Green Lantern Hal Jordan from a world where apes evolved from man. Either Pierre Boulle’s novel carries some degree of truth, or this is a telepathic fiction created by Grodd or another of the apes for some purpose. Other accounts suggest they are terrestrial hominids, evolved by the actions of an alien visitor. It is possible (and some of the depictions of their physical characteristics support this) that they are not actually gorillas at all, but rather evolved Mangani, their ancestors cousins of the tribe that raised Tarzan.
More intriguingly, there seems to be (or have been) more than one gorilla city. In 1931, Tarzan discovered a replica of London, peopled by gorillas uplifted by a mad geneticist who called himself and was thought of by the apes as “God.” “God's” notes on hybridizing gorilla and human DNA were sought (and possibly found) by both Robert Yerkes and Ilya Ivanovich Ivanov (and possibly Ivanov’s protégé Ivan Kragov, the Red Ghost).
Perhaps twenty years later, Congo Bill discovered a more primitive city of intelligent gorillas who claimed to be from a world with two moons. Bill took this description to mean Mars (an idea perhaps supported by the existence of an alternate earth where Martian apes ruled and were opposed by Jonathan Raven, Ape-Slayer), though perhaps it was Calor. The link between the current Gorilla City and these others is unclear; they may represent periods in the evolution of ape society or purposeful obfuscation of the gorillas’ true nature and history.