Recovered from deep underground, this shale dating from about 2.5 billion years ago contained material untouched by the atmosphere for billions of years. Sedimentary rocks can harbor evidence of oxidation and other chemical reactions that occur as the rocks form.

This oxygen, the scientists believe, was produced via photosynthesis by some of the simple organisms that inhabited the planet at the time, a type of aquatic bacteria sometimes called blue-green algae. Oxygen is vital for most life forms on Earth.

"We couldn't exist in a world that didn't have a lot of oxygen in the atmosphere. So understanding how that came to be is fundamental in understanding how it is the Earth came to be a planet that is hospitable to us," said Arizona State University geochemist Ariel Anbar, one of the researchers.

Some scientists have argued that the sudden evolution of organisms that produce oxygen by photosynthesis caused the Great Oxidation Event. But these new findings suggest that the evolution of oxygen-producing organisms occurred earlier and that oxygen buildup in Earth's atmosphere was more gradual than sudden, the researchers report in the journal Science.

"Our research indicates there were oxygen-producing organisms for many millions of years before the Great Oxidation Event, and then perhaps a 'tipping point' was reached, which launched the Great Oxidation Event," according to a statement from University of Alberta geologist Robert Creaser.

Oxygen today makes up about 21 percent of the Earth's atmosphere. But it has not always been that way.

Earth is roughly 4.5 billion years old. For about the first half of the planet's existence, the environment held almost no oxygen, other than bound to hydrogen in water or to silicon and other elements in rocks.

Then, during this crucial period, oxygen started appearing in the atmosphere and oceans. But it still made up much less than 1 percent of the atmosphere.