In the movie, After Earth:
Humans have been away from Earth for over a thousand years living on other planets outside of our solar system. It is mentioned Humanity was under attack by an alien race, but it did not mention terraforming or any particular change of the Earth's ecosystem.
Why did Humans who were not native to Earth appear to be unable to breathe on Earth without technology?
I don't remember anyone explaining why humans couldn't breathe there. Was there something I missed or is this a plot hole we were expected to hand-wave away?