Caesar was an orphan - his parents were killed almost immediately after his birth by humans who feared that they would spawn the race of intelligent apes that one day would rule the earth. He was raised in a circus, by the kindly Armando, a man who loved animals, and treated him almost as he would a son. When Caesar finally came of age, Armando took him into the city to see the society humans had made - a society in which apes were a new class of slaves. Originally brought into human homes to replace the beloved cats and dogs which died of a mysterious plague, apes quickly learned a new role as servants, and humans became accustomed to having them take care of menial tasks.
Caesar's outrage at this situation resulted in Armando's death under interrogation and his own capture by humans who believed him to be a normal, mute ape. Eventually he led the revolution that brought down human society and made the nightmare of the humans who killed his parents come true.
Alternately, Caesar is the only survivor of a genetic experiment conducted by a San Francisco biotech company. His mother, the most successful test subject for a virus intended to stimulate neuron regrowth in humans, passed the virus on to him while in utero. Because Caesar's mother, "Bright Eyes", went berserk while trying to protect Caesar, she was shot, and her fellow test subjects euthanized. Lab tech Robert Franklin found him and gave him to scientist Will Rodman for "a few days", until a suitable habitat could be found.
Because Caesar showed effects of the virus Rodman wanted to study, the effects of which improved the mental functioning of Rodman's father, Charles, he was raised by the two men and lived with them for eight years. At that point, Charles' Alzheimer's returned, and when the old man got in to a fight with a neighbor, Caesar bit him while defending Charles. This action caused the court to order Will Rodman to place Caesar in a "primate sanctuary" (actually a kennel for unwanted and retired adult apes), where Caesar learned hatred of humans at the hand of his jailer's sadistic son.
While in the sanctuary, Caesar stole a switchblade, broke out of his cell, established dominance over a simian bully by allying himself with the resident gorilla, broke out of the sanctuary, stole a virus with effects similar to those that affected him from Will Rodman's refrigerator, broke back in to the sanctuary, dosed his fellow inmates with the new virus, established himself as a "general", communicated in recognizably human speech, contributed to the death of his human bully, and broke all the apes out of the primate sanctuary.
He was the leader of the movement to break the San Francisco zoo's gorillas out of their enclosure and relocate the apes to the redwood forest north of San Francisco. Because of Caesar's direction, there were only one ape and three human fatalities during the battle of the Golden Gate Bridge. He chose to stay with the apes rather than returning to suburbia with Will Rodman, using human speech to call the forest "home".