The prologue states that an extraterrestrial armada (later known as 'Grakks') having destroyed sixty percent of the human population, a society of godlike 'Benefactors' rebuild Earthly civilization, in exchange for military help against the attackers. To replace the missing sixty percent, the Benefactors bestow humanlike intelligence and habits upon Earth's gorillas, and thus produce a combined civilization on Earth.
The prologue details an alien invasion by the Grakks that wiped out 60% of humanity.  Benefactors, powerful beings who rebuilt society, offered military aid to counter the invaders. To compensate for the lost population, they bestowed human-like intelligence and traits upon gorillas, creating a new civilization on Earth.