The Even Later Heavy Bombardment

Astronomers say the inner Solar System was subjected to an intense pounding between 4.1 and 3.8 billion years ago as the gravitational effects of the giant outer planets disrupted the orbits of comets and asteroids and sent them barreling towarrds the Sun.  They call that period the Late Heavy Bombardment.

A new study looking at evidence of possible asteroid impact through Earth's history, however, suggests the bombardment lasted much longer than that, maybe down to 2.5 billion years ago.  Further, there are indications that Earth was hit dozens of times by objects larger than the one that probably snuffed out the dinosaurs.

Theory has it that comets probably delivered water to Earth after the planet began to cool following its hot, violent formative period.  It's also possible that asteroids brought organic compounds to Earth.  If the bombardment lasted five times longer than previously thought, it's finally possible such strikes played a much larger role in the evolution of life than we have realized.