system
Given the following post, respond as if you're commenting on Reddit:
human
[Nausicaa of the Valley of the Wind] What exactly polluted the Earth, and where did the massive insects come from?

The insects couldn't have evolved from present day bugs in a mere 1000 years. Even supposing that the human apocalypse happened hundreds of years after modern day, it would take tens of millions of years to see something like the Ohm evolve. Perhaps the technology that created the Giant Warriors was a genetically enhanced spore that precipitated expedited evolution?
gpt
A lot of this is directly answered in the manga, which has a much more expansive story than the film:

- The God-Warriors are nuclear-powered, or at least emit hard radiation around themselves when they are awake and functioning. Said radiation proves fatal to at least one important character and sickens a couple others.

- Additionally, in the film the God-Warrior just kinda roars and crawls around. In the manga, they are self-aware and can function semi-autonomously - not to mention they can fly (and, again, spread radiation when they do).

- Even with the far greater damage they are obviously capable of, there are a couple of other references to the Earth being polluted before the apocalypse - probably talking about industrial waste and pollution generated by humans.

- The Ohmu and the Sea of Corruption are explicitly bioengineered by survivors of the Seven Days of Fire, who then uploaded their minds into a bio-computer to await the time when the Sea would cleanse the pollution and return the Earth to a pristine, livable state. The Ohmu's enraged mass-rampages are actually a sort of deliberate programming - an attempt to extend the sea into areas it previously hadn't reached so the cleansing can be completed worldwide.

- Interestingly, the Doroks first attempt to 'breed' an Ohmu in a tank before they resort to just capturing one and dragging it around.