What event would cause hydrogen to slowly replace oxygen in the athmosphere?
it can be something natural or artifial.
it doesn't have to cause too much mass extinctions. Some sacrifices are ok but not an apocalypse.
So it must be slow enough so living organisms adapt to it, but not slower than 1 billion years, it has to happen way before the sun enters its death phase and eats the solar system.
What would be the most evident and drastic changes in the environment and life on earth?