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This photo shows rocks from the Belcher Islands in Hudson Bay, Canada, from which Malcolm Hodgskiss collected barite samples dating 2.02 to 1.87 billion years old. Malcolm Hodgskiss

Some two billion years ago, a significant decline of once-abundant oxygen killed as much as 99 percent of all life on Earth in a mass extinction event larger than the one responsible for the dinosaur die-off.

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