* Nuclear War: The film opens with a scene of a burning spaceship, suggesting a catastrophic event, possibly nuclear war. This is further implied by the presence of radioactive dust in the atmosphere and the fact that the apes are wearing radiation-proof suits.
* Environmental Destruction: The film's setting is a post-apocalyptic Earth, suggesting that humans may have contributed to their own downfall through environmental destruction. This is supported by the presence of ruined cities and the scarcity of resources.
* Overpopulation: The film hints at overpopulation as a contributing factor, with the protagonist, Taylor, finding evidence of a massive and overcrowded human society before the apocalypse.
* Disease: It's possible that a deadly disease could have ravaged the human population, weakening them and making them susceptible to the apes' uprising.
The film leaves it open to interpretation as to the exact cause of human decline, suggesting that a combination of factors, including environmental degradation, nuclear war, and overpopulation, led to their downfall.
The sequels and remakes explore different possible causes, including:
* A deadly virus: In *Escape from the Planet of the Apes* (1971), a deadly virus is responsible for the decline of humanity.
* A deliberate plan by the apes: In *Rise of the Planet of the Apes* (2011), the apes are deliberately engineered by humans to be intelligent, ultimately leading to their rebellion and the downfall of humanity.
Ultimately, the exact cause of humanity's downfall is a matter of speculation, but the film's depiction of a world ruled by intelligent apes serves as a powerful allegory for the dangers of environmental destruction, technological advancement, and social inequality.