I don't think we know all of the details,
especially about where our atmosphere came from,
but the current view is that Earth formed from the
collisions of a huge number of asteroids that
slowly aggregated into a planet. This process
probably took tens of millions of years.
Most objects in space, including stars and
planets, are formed by a process called
accretion. Accretion occurs when small
particles in space attract each other due to
gravity, and begin colliding and clumping into
larger particles. After a long, long time, enough
particles merge together to become the size of a
planet or star. When the earth formed, it
eventually developed an atmosphere of gas and
water on the surface, conditions needed for the
first life on earth to develop and begin evolving.
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