Dr. Cornelius is a chimpanzee archaeologist and historian, appearing in the original novel of Planet of the Apes (La Planète des singes), and also the first three installments of the classic movie series of the same name, from the 1960s and 1970s. He was portrayed mainly by actor Roddy McDowall, but also by David Watson in the second Apes movie.
In 1968's Planet of the Apes, Cornelius is introduced as the fiancé of Dr. Zira, an animal psychologist and veterinarian (who specializes in working with humans), both of whom are on the scientific staff of Dr. Zaius. While supporting the status quo, Cornelius has begun to question the infallibility of the Sacred Scrolls, which give the traditional history of the ape society, and is considering the validity of evolution (specifically, from human to ape) to explain the scientific gaps in the scrolls.
When Zira brings Taylor, an injured astronaut who is mistaken for a primitive human, home from the zoo for Cornelius to see, Taylor begins to communicate with them, first through gestures and then through writing on paper. While Cornelius dismisses Taylor's story that he flew through the stars to their world (since he has no proof), he agrees that Taylor is worthy of consideration as an intelligent being. When Taylor recovers his voice, he tries to plead his own case before a council session called to arrange his "disposition". When the ape leaders won't allow Taylor to speak, Cornelius defends him.
With Taylor marked for elimination, and at risk of their own careers, Cornelius and Zira escape with Taylor to the Forbidden Zone—the wasteland where humans are said to come from, to a site where Cornelius had been digging the year before. When Dr. Zaius pursues them, Taylor turns the tables, first capturing Dr. Zaius, then forcing him to examine Cornelius's evidence for a human society predating their own. Cornelius marvels at Taylor's explanations of the artifacts found, accepts that apes didn't evolve from humans, and that humans invented the technology that apes were rediscovering. Taylor departs from the apes (after kissing Zira goodbye, to Cornelius's consternation), who return to their city.
Between the first movie and its follow up, Beneath the Planet of the Apes, Cornelius and Zira first undergo a show trial for heresy at Zaius's instigation (who then pleads for clemency on their behalf), then are married and continue their careers. Another astronaut, Brent, appears at their doorstep in the second movie. Zira tends to a gunshot wound he suffered at a gorilla's hands, and Cornelius first shows Brent a map leading to where they left Taylor, then gives him the best possible advice: Never speak if he's captured, or "they will dissect you, and they will kill you—in that order." Zaius also visits to announce that he's accompanying an expedition to the Forbidden Zone, and asks for Cornelius and Zira's promise not to cause trouble in his absence, which they give him. (In the novelisation of the movie, though, they lead a chimpanzee revolt after he departs.)
In the third movie, Escape from the Planet of the Apes, Cornelius and Zira themselves become 'astronauts', escaping their world in Taylor's spacecraft, found and restored by their friend Dr. Milo, and traveling back in time to a few months after Taylor's departure. They discover a world run by humans, and deduce (correctly) that this is not only where Taylor came from, but is their own planet from the apes' prehistory. Cornelius has learned the truth about how the apes rose, from reading secret scrolls Dr. Zaius had kept under lock and key, and is able to answer questions put to him by the human leaders—and offer tantalising clues to both Taylor's fate, and the planet's.
Discovering Zira is pregnant by Cornelius, and fearing a possible ape takeover, scientist Dr. Otto Hasslein takes it upon himself to make sure the baby is never born, and the pair's arrival won't spark a human downfall—inadvertently setting that downfall in motion. Milo, the son of Cornelius and Zira, is born and taken into hiding, while the two try to escape with another ape baby as a decoy. They and the baby are murdered, but their own son (played in the next two movies, again by McDowall) grows up, given the name Caesar by his human foster-father, and leads the apes as the human society destroys itself.
Cornelius and Zira have no roles in Conquest of the Planet of the Apes, except in references to Caesar's "real parents", but appear in video stills examined by Caesar and two companions in the final film, Battle for the Planet of the Apes, as Caesar tries to learn what they knew about Earth's future. Caesar also gives his son (by wife Lisa) the name Cornelius, after his father. This Cornelius was killed by Aldo when he cut the branch Cornelius was on.