Asteroids may have stopped pummeling Earth some 600 million years earlier than scientists thought, giving life that much more time to evolve. (Credit: NASA/JPL-Caltech)

The solar system once experienced a meteor shower of epic proportions: Asteroids whizzed around the inner planets, crashing down in a rain of fire that left their surfaces scarred for billions of years. Astronomers typically call this period the Late Heavy Bombardment.

But exactly when that fiery assault happened has been