This is the part I think a lot of people miss when they estimate life in the universe.

Our planetary system has been exceptionally stable thanks to a calm main sequence star and a giant planet as protector. Yet even with the ideal scenario, and single celled "life" starting on Earth almost immediately after the late heavy bombardment allowed, it still took nearly 4 BILLION years for multi-celled life to evolve and flourish in the Cambrian explosion 500 million years ago.