Sequence the events that led to oxygen becoming part of Earth's atmosphere. _Earth's atmosphere contained carbon dioxide, nitrogen, and
water vapor.

_Rains washed carbon dioxide out of the atmosphere and into
the oceans.

_Early life forms in the seas used carbon dioxide during
photosynthesis.

_Oxygen levels in the atmosphere built up.
_Earth cooled and became more solid.

_Early life forms released oxygen and water vapor.