The Early Bombardment of Asteroids, Meteorites and So On.

There's evidence to suggest that we had liquid water and land on earth within a few tens of millions of years after the formation of the earth. So as soon as you've got liquid water on earth, then life may have appeared rapidly. We don't have any rocks older than 3.9 billion years because it was destroyed by space. The surface of the earth was made molten and then re-solidified at this point.