Earth's atmosphere contained enough oxygen for complex life to develop nearly 1.2 billion years ago – 400 million years earlier than scientists previously believed.

The findings, reported in the Nov. 11 Nature, could lead scientists to reconsider the prerequisites for animal life, on Earth and other planets.

"It means that the conditions were in place for complex life to arise," said geologist John Parnell of the University of Aberdeen in Scotland, lead author of the new study. "There might be animals in that earlier window that we have not yet found."

Geological records show there was one major increase in the amount of oxygen in Earth's atmosphere around 2.3 billion years ago, and another around 800 million years ago.

That second spike in oxygen levels was thought to be connected to the Cambrian explosion, the swift development of most of the major animal groups that came around 550 million years ago.

Parnell's results suggest oxygen can't be the whole story.

"It may have been that something else gave evolution the kick-start which caused animals to evolve," he said. "Oxygen in the atmosphere was already there for quite a long time."

To figure out how much oxygen was in the early atmosphere, Parnell and his colleagues searched 1.2 billion-year-old rocks from what was once a lakebed in Scotland for the chemical signatures of ancient bacteria.

Before there was a useful amount of free oxygen around, these bacteria used to get energy by converting sulfate, a molecule with one sulfur atom and four oxygens, to sulfide, a sulfur atom that is missing two electrons.

Geologists can get a glimpse of how efficient the bacteria were by looking at two different sulfur isotopes, versions of the same element that have different atomic masses. Converting sulfate to sulfide leaves the rock with a lot more of the isotope sulfur-32 than would be there without the bacteria's help.

The geologists extracted pyrite, also known as fool's gold, from the rocks. They then pulled sulfur from the pyrite by chemical processing and by zapping the rocks with a laser. The amount of sulfur-32 was much higher than bacteria could have produced without oxygen.

Parnell suggests the bacteria were able to use oxygen in the atmosphere to convert between the two different forms of sulfur (sulfate and sulfide) many times.

"Their metabolism was becoming more complicated," he said. "The more cycles of that [reaction] that they caused, the more sulfur-32 you ended up with."

The team concluded that the amount of oxygen in the atmosphere 1.2 billion years ago approached the levels at the time of the Cambrian explosion, roughly 10 percent of current oxygen levels. Ten percent may be enough to start complex life, Parnell says.

"It's only when you can start processing oxygen in a complex way that you can then start to produce different cells that do different things," Parnell said. "That's what gives rise to animals."

The evolution of large animals could have been triggered by changing geological conditions, like the end of a dramatic ice age about 600 million years ago, he says.

Parnell also hinted that the results could have implications for sulfur-eating bacteria on other planets like Mars, although because he has another paper in preparation, he didn’t want to go into very much detail.

"If there are microbes on Mars either today or in the past, this kind of metabolism is one which would be readily available to them," he said. "The stage of chemical reduction from sulfate to sulfide is completely feasible on Mars."

"I'm pretty thrilled by the paper," said geochemist Michael Russell of NASA's Jet Propulsion Lab, who was not involved in the new study. "I'd like to see this kind of thing done ever further back in time, so we can get a sense of just how much oxygen there was in the atmosphere."

Image: 1) The cave near Lochinver in the north-west Highlands of Scotland where Parnell and his colleagues collected sulfur-rich rocks. 2) Slivers of fool's gold that hold clues to Earth's early atmosphere. Credit: Stephen Bowden, University of Aberdeen

See Also:

Follow us on Twitter @astrolisa and @wiredscience, and on Facebook.