| Nickel crash kick-started evolution|
|Life as we know it may owe its existence to fluctuating levels of a humble metal in the primordial sea, according to an international study.|
The massive influx of oxygen into earth's atmosphere some 2.4 billion years ago, which 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," says Dominic Papineau of the Carnegie Institution's Geophysical Laboratory and a co-author of the study, which appears in the current issue of Nature.
"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 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 by-product 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.
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 nickel crash after its early boom 2.7 billion years ago helped make our planet habitable by complex life," says Professor Mark Barley of the University of Western Australia, one of the researchers in the study.
"When oceanic nickel contents declined after this time this would have caused a famine for methanogens and significantly reduced the methane content of the atmosphere."
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.