The rapid rise in the atmospheres oxygen roughly 500 million years ago which made advanced animal life possible on Earth is linked to the process that led to the formation of fossil fuels, suggests new research.
The rise in oxygen was associated with a rapid increase in the burial of sediment containing large amounts of carbon-rich organic matter - the raw material of coal, oil and natural gas, the study said.
"Burying the sediments that became fossil fuels was the key to advanced animal life on Earth," said study co-author Shanan Peters, Professor at the University of Wisconsin-Madison in the US.
Multicellular life is largely a creation of the "Cambrian explosion," which coincided with a spike in atmospheric oxygen roughly 500 million years ago, Peters pointed out.
It was during the Cambrian explosion that most of the animals appeared and evolved.
In green plants, photosynthesis separates carbon dioxide into molecular oxygen (which is released to the atmosphere), and carbon (which is stored in carbohydrates).
But photosynthesis had already been around for at least 2.5 billion years. So what accounted for the sudden spike in oxygen during the Cambrian?
The study, published online in the journal Earth and Planetary Science Letters, linked the rise in oxygen to a rapid increase in the burial of sediment containing large amounts of carbon-rich organic matter.
The key is to recognise that sediment storage blocks the oxidation of carbon, Peters said.
Without burial, this oxidation reaction causes dead plant material on Earth's surface to burn.
That causes the carbon it contains, which originated in the atmosphere, to bond with oxygen to form carbon dioxide. And for oxygen to build up in our atmosphere, plant organic matter must be protected from oxidation.
And that is exactly what happens when organic matter -- the raw material of coal, oil and natural gas -- is buried through geologic processes, the researchers explained.
The researchers warned that today, burning billions of tonnes of stored carbon in fossil fuels is removing large amounts of oxygen from the atmosphere, reversing the pattern that drove the rise in oxygen.
And so the oxygen level in the atmosphere falls as the concentration of carbon dioxide rises, the study said.