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Life evolved when the ocean nickel levels dropped
evolution of life was preceded by a great rise in the atmospheric oxygen levels. There are a couple of theories as to what triggered that rise. While both talk about a decrease in the number of bacteria that produce methane (methanogens) and a subsequent increase in the number of bacteria that produce oxygen during photosynthesis (cyanobacteria), neither is clear on the why and when.
A new theory attempts to give a clearer picture: A drop in the level of nickel in oceans, 2.7 million years ago, started it all. Methanogens need nickel for metabolism. When the number of these bacteria declined due to the decrease in nickel levels, population of cyanobacteria increased which, in turn, led to the Great Oxidation Event.
Kurt Konhauser, of the department of earth and atmospheric sciences, University of Alberta, Canada, along with researchers from the US, UK, Australia and France, took samples of ancient rock formations from Australia, South Africa and Canada and measured the nickel-to-iron ratio in them.
The researchers estimated a huge decline in oceanic nickel levels about 2.7 billion years ago because of cooling of the earth's mantle. When the mantle was hot, volcanic eruptions spewed out nickel-rich rocks. On weathering, these rocks released the nickel into oceans.
As the amount of nickel-rich rocks declined, the oceanic nickel levels could not be replenished. Methanogens further used up the remaining nickel, reduing its concentration 50 per cent. So the methanogens, unable to survive in the oceans, were relegated to freshwater environments, where the nickel levels were higher, and where they exist to this date. The study was published in the April 9 issue of Nature.
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