(CNN) The most catastrophic wipe-out on Earth didn't happen to the dinosaurs.

new study found extreme changes in the atmosphere killed almost 100% of life on Earth about 2 billion years ago.

Researchers sampled barite, a mineral more than 2 billion years old, in subarctic Canada's Belcher Islands. Rocks that old "lock in chemical signatures," helpful clues for researchers to uncover what the atmosphere was like when the rocks first formed, co-lead author and Stanford University Ph.D. candidate Malcolm Hodgskiss told CNN.

There is such thing as too much oxygen

The study focused on a phenomenon called the "Great Oxidation Event." It goes like this: Billions of years ago, only micro-organisms survived on Earth. When they photosynthesized, they altered the chemical composition of the atmosphere, creating a glut of oxygen they ultimately could not sustain.

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