Throughout Earth's history the potential has existed for geological and astronomical processes to catastrophically disrupt terrestrial life. Scientists continue to find evidence that many such processes actually occurred—yet life endured and even thrived. A number of times glaciers advanced to cover the entire surface of the planet, resulting in snowball-Earth events.
A growing body of evidence indicates that the advent of a permanent oxygen component in the planet's atmosphere instigated these periodic snowball Earths.
As I describe in a previous TNRTB, the Earth's remarkable design ensures that these normally catastrophic occurrences do not destroy, but enhance our planet's capacity to support life.