Enough Oxygen for Life Found Millions of Years Too Early

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.

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