For millions of years, over 635 million years ago, the Earth was shrouded in ice. At that time, most land was clustered around the equator, but the glaciers left their mark on the rocks even there. Nobody knows what caused the Earth to freeze, but once it did, the shiny white surface of the ice reflected heat and maintained the 'snowball' conditions, until CO2 released by volcanoes created a greenhouse effect strong enough to melt the ice. If the ice had covered the entire planet, life may have died out. But there's evidence slushy pools survived during this period, and primitive but hardy life forms survived in the pools. When the Earth thawed, life exploded out across the planet in a burst of evolution. We may owe our existence to this Snowball Earth event.
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