Researchers say exposed rock shows transition from warm, tropical environment to global freeze about 700m years ago
may be a rare record of “snowball Earth” – a crucial moment in planetary history when the globe was covered in ice.
Prof Graham Shields, a senior author on the study, of University College London Earth Sciences, said: “These rocks record a time when Earth was covered in ice. All complex, multicellular life, such as animals, arose out of this deep freeze, with the first evidence in the fossil record appearing shortly after the planet thawed.”
Lasting about 60m years, the Sturtian glaciation was one of two big freezes that occurred during the Cryogenian period, between 635m and 720m years ago. For billions of years before this, life consisted only of single-celled organisms and algae. After this period, complex life quickly emerged, with most animals today similar in fundamental ways to the types of life forms that evolved more than 500m years ago.
Shields said: “The retreat of the ice would have been catastrophic. Life had been used to tens of millions of years of deep freeze. As soon as the world warmed up, all of life would have had to compete in an arms race to adapt. Whatever survived were the ancestors of all animals.”
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