Four and a half billion years ago, when the Earth was very young, it was hardly a place conducive to life. Earth was formed from the collision of planetismals, and continued to be bombarded by the debris in the early solar system for about the first billion years of its history.
Evidence of this period of intense meteorite impacts is not preserved on Earth, whose active internal tectonic engine has recycled the crust formed during this time, but is evident on our nearest celestial neighbor, the moon.
In fact, one of the impacts during this time may have formed the moon. The collision of Earth with a Mars-sized object helps to explain the similarity of moon rocks to some Earth rocks, the slightly younger age of Moon rocks, and a major impact of a large object with the Earth can account for the 23 degree tilt of the Earth on its axis.
For an overview of this and other hypotheses of lunar formation see this site.
The illustration is from here.
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