(CNN) The most catastrophic wipe-out on Earth didn't happen to the dinosaurs.
A 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.
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.
Micro-organisms exhausted the nutrients they needed to create oxygen, which knocked the Earth's atmosphere off-kilter. This led to an "enormous drop" in the biosphere -- the amount of life on Earth. Scientists weren't sure just how drastic the drop was until now.
The team's calculations showed that anywhere from 80 to 99.5% of organisms were wiped out at the end of the Great Oxidation Event, Hodgskiss said. There were simply too many of them, and they produced too much oxygen.
"Even our most conservative estimates would exceed estimates for the amount of life that died off during the extinction of the dinosaurs approximately 65 million years ago," he said.
How did researchers study what the world was like before humans ever lived in it? They combined a model of how much carbon dioxide and oxygen there might've been in the atmosphere then based on past research with their chemical measurements from the barite to calculate how much life there was then.
The findings are older than most life on Earth, but they're relevant to the planet today because Earth is still vulnerable to atmospheric changes, Hodgskiss said. Oceans are heating up, which affects how numerous some nutrients are. Runoff in oceans disrupts underwater ecosystems, too, threatening photosynthesizing organisms that contribute more than half of the oxygen in the Earth's atmosphere.