for National Geographic News
Life as we know it may have evolved because Earth's early oceans ran low on nickel, a new study suggests.
The metal is an important nutrient for a class of bacteria known as methanogens, which produce methane, a gas that reacts easily with oxygen.
Methanogens flourished on early Earth, presumably filling the atmosphere with methane gas, said study leader Kurt Konhauser, a geomicrobiologist at the University of Alberta.
"The methanogenic bacteria were very happy until 2.5 billion years ago," he said.
At that point, rocks from the era show that atmospheric oxygen started to rise, which would suggest that methane levels dropped. But the exact reason for the oxygen spike has been a long-standing puzzle.
Konhauser's team looked for an answer in rocks found in Ontario, Canada, that formed on the ancient sea floor.
"Studying them gives a history of seawater," he said.
What they found is that older rocks had metal concentrations high in nickel, while the rocks that formed after them had less of the nutrient.
The scientists suggest that cooling of the Earth's mantle decreased eruptions of nickel-rich volcanic rock, which meant that less nickel was being weathered from the rocks and dissolved in the oceans.
This nickel "famine" starved the methanogens, spurring a gradual decline in atmospheric methane, the researchers think.
Meanwhile, a class of photosynthetic bacteria known as cyanobacteria, which had been on the scene since 2.7 to 2.8 billion years ago, was continuing to produce oxygen.
With less methane around to tie it up, oxygen accumulated in the atmosphere, eventually creating conditions for oxygen-breathing life to dominate.
The finding, Konhauser added, would help explain why there was a 300- to 400-million-year delay between when cyanobacteria appeared and oxygen began building up in the atmosphere.
Findings appear in this week's issue of the journal Nature.
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