Mass extinctions roll the dice on complexity. They redefine the yardstick against which fitness is measured, at least temporarily (but long enough to have a las

Mass extinctions roll the dice on complexity. They redefine the yardstick against which fitness is measured, at least temporarily (but long enough to have a lasting effect.) If the KT meteorite were somewhat larger, it might have bombed Earth back to the Archean. On the other hand, this is one aspect of the Fermi puzzle where we do have more than one example - and they suggest that mass extinction events alone are not sufficient to answer it.