A "see-saw" effect in Earth's atmosphere 2.5 billion years ago may have set the stage for the development of complex life, British and U.S. researchers say.

The Earth's early atmosphere periodically flipped from a hydrocarbon-free state into a hydrocarbon-rich state, and this switch between "organic haze" and a "haze-free" environment was the result of intense microbial activity that would have had a profound effect on the climate, they said.

The findings provide clues to Earth's surface environment prior to oxygenation of the planet, a release from Britain's Newcastle University said Monday.

"Models have previously suggested that the Earth's early atmosphere could have been warmed by a layer of organic haze," Newcastle researcher Aubrey Zerkle said.

"Our geochemical analyses of marine sediments from this time period provide the first evidence for such an atmosphere.

"However, instead of evidence for a continuously 'hazy' period we found the signal flipped on and off, in response to microbial activity."

The conditions that enabled the organic haze to switch on and off ended when the atmosphere became oxygenated about 100 million years ago, researchers said.

"What is most surprising about this study is that our data seems to indicate the atmospheric events were discrete in nature, flip-flopping between one stable state into another," researcher James Farquhar of the University of Maryland said.

"This type of response is not all that different from the way scientists think climate operates today, and reminds us how delicate the balance between states can be."