Between roughly 720 and 635 million years ago, Earth experienced the most intense glaciations in its known history. Geologically designated the Cryogenian Period, this interval encompassed two distinct glaciations: the Sturtian (c. 720–660 Ma) and the Marinoan (c. 650–635 Ma). In both events, glaciers advanced to equatorial latitudes and ocean surfaces were sealed beneath thick ice. Joseph Kirschvink's 1992 Snowball Earth hypothesis remains the most coherent framework reconciling carbonate rock chemistry with glacial deposits found at tropical palaeolatitudes.

What triggered the glaciations remains debated; leading candidates include enhanced chemical weathering driven by the tropical breakup of the supercontinent Rodinia (which consumes CO₂) and large shifts in ocean circulation. Once underway, an ice-albedo feedback loop became self-reinforcing: ice reflects incoming sunlight, driving further cooling.

How did Earth escape? Volcanoes kept releasing CO₂ beneath the ice. With chemical weathering suppressed, atmospheric CO₂ accumulated over millions of years until it crossed a threshold that abruptly warmed the planet through greenhouse forcing. The thaw, on geological timescales, was remarkably rapid.

The Snowball intervals created the evolutionary pressure that set the stage for the Ediacaran biota and then the Cambrian explosion. Organisms that survived beneath the ice developed new metabolic strategies; the nutrient-rich oceans unveiled by the melt provided fertile ground for the rapid diversification of multicellular life.