The massive influx of oxygen into Earth's atmosphere some 2.4 billion years ago that set evolution on a path to multicellular life was unleashed by a cascade of events in which nickel played a starring role, it argues.

What scientists call the Great Oxygen Event "irreversibly changed surface environments on Earth and ultimately made advanced life possible," said Dominic Papineau of the Carnegie Institution's Geophysical Laboratory and a co-author of the study.

"It was a major turning point in the evolution of our planet."

Scientists generally agree that this oxygen surge was made possible by a rapid decline in atmospheric levels of methane.

But why methane dropped off has remained a mystery.

The researchers, led by Kurt Konhauser of the University of Alberta in Edmonton, Canada, believe the answer lies in rocks that were formed before oxygen was abundant in the air or the sea.

Analysing changes over time in the level of trace elements in these rocks, the scientists noticed that levels of nickel dropped just before oxygen levels soared.

This piece of evidence led them to fit all the pieces of theoretical puzzle together.

The dominant life form before the oxygen big bang was the methanogen -- a single-celled ocean organism that exuded methane as a byproduct of its metabolism.

The enormous quantities of methane these methanogens produced almost certainly prevented the build up of oxygen in the air.

To survive, these creatures gobbled up nickel, which existed 2.7 billion years ago in quantities 400 times greater than today.

By 2.5 billion years ago, nickel levels in the oceans had dropped by more than half.

Starved of this nutrient, the methanogens declined and their output of methane plummeted.

This cleared the way for a class of photosynthesising life forms, called cynobacteria, to gain the upper hand, and pushed the chain of evolution towards complexity.

"The timing fits very well. The drop in nickel could have set the stage for the Great Oxidation Event," said Papineau.

But that still left one key question unanswered: what caused the nickel levels to decline?

The researchers point to geological changes. During earlier phases of Earth's history, when the layer known as the mantle was still hot, lava from eruptions flowing into the oceans were very rich in nickel.

But as the mantle cooled, the lava contained far less of the trace metal.

"The nickel connection was not something anyone had considered before," said Papineau.

"It's just a trace element in sea water, but our study indicates that it may have had a huge impact on the Earth's environment and on the history of life."

The study is published in the British-based journal Nature.

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