People ask this all the time. Or they just assume that because someone is an Atheist that they have to or automatically accept evolution.
There is no official Atheist position on evolution. Remember, Atheism is only a disbelief in god (s). There is no doctrine, there are no set of rules, nothing. You can believe that space aliens populated the earth, you can believe that life just appeared 2 seconds ago, you can believe all those things and still be an Atheist.
However, it is true that most Atheists accept evolution as the most correct way to describe how we got here. This is because Atheists are not held to strict dogmas that rule out scientific discoveries and theories.