Earth went through a heavy asteroid bombardment as changes in the solar system flung asteroids from the belt between Mars and Jupiter on a collision course with Earth.
The heavy bombardment, between 4.5 to 3.2 billion years ago, resulted in craters that fundamentally constituted the geography of the ancient Earth.
Some asteroids were as large as 40km in diameter, much larger than the 12-15km one 65m years ago that resulted in the extinction of dinosaurs.
Read more at Purdue University