<P> The Snowball Earth hypothesis proposes that Earth's surface became entirely or nearly entirely frozen at least once, sometime earlier than 650 Mya (million years ago). Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical palaeolatitudes and other enigmatic features in the geological record . Opponents of the hypothesis contest the implications of the geological evidence for global glaciation and the geophysical feasibility of an ice - or slush - covered ocean and emphasize the difficulty of escaping an all - frozen condition . A number of unanswered questions remain, including whether the Earth was a full snowball, or a "slushball" with a thin equatorial band of open (or seasonally open) water . </P> <P> The snowball - Earth episodes occurred before the sudden radiation of multicellular bioforms, known as the Cambrian explosion . The most recent snowball episode may have triggered the evolution of multicellularity . Another, much earlier and longer snowball episode, the Huronian glaciation, which occurred 2400 to 2100 Mya, may have been triggered by the first appearance of oxygen in the atmosphere, the "Great Oxygenation Event". </P>

When did a giant global ocean cover the earth
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