THE explosion of complex animal life half a billion years ago may all have been down to clay.
Earth鈥檚 atmosphere has contained free oxygen since photosynthesis evolved about 2.8 billion years ago. But until sometime between a billion years ago and the start of the Cambrian period about 550 million years ago, the concentration did not rise above 0.2 per cent of modern levels, and this was nothing like enough to allow multicellular animals to evolve.
鈥淔or billions of years there was oxygen in the air, but nothing like enough to allow multicellular life to evolve鈥�
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The dramatic rise in oxygen levels at the start of the Cambrian is usually attributed to the burial of most of the Earth鈥檚 organic carbon in shallow waters. This would have kept it from reacting with oxygen in the air and allowed levels of the gas to rise, although the mechanism that led to the carbon burial was not clear.
Now Martin Kennedy of the University of California, Riverside, and his team say that it happened because the organic carbon became trapped in clay. He arrived at this hypothesis when he noticed that Precambrian shales differ in texture from shales deposited after the Cambrian. When he analysed the clay content of shales deposited in Australia from 850 to 530 million years ago, he found that the fraction of clay in them increased dramatically during the period to near modern levels. Clay minerals work like kitty litter, trapping and forming bonds with organic carbon debris from dead organisms. The clay eventually ended up in shallow marine environments, permanently removing the carbon (Science, DOI: 10.1126/science.1118929).
鈥淐lay minerals work like kitty litter, trapping and forming bonds with carbon debris from dead organisms鈥�
What started producing the clay? Before life spread on land, physical weathering broke down rocks, but the particles were not fine enough to form clay. Today microbes and fungi produce clay particles by chemically weathering rocks. There is evidence that microbes and fungi began spreading on land about a billion years ago, and Kennedy thinks that clay production increased over a period of tens of millions of years, beginning about 620 million years ago when primitive life spread across the continents. 鈥淔ungi probably tipped the balance, as they have root-like structures,鈥� he says.
Harvard University palaeontologist Andrew Knoll, a specialist on Precambrian life, is not entirely convinced. 鈥淚t is an interesting idea that needs testing with larger data sets.鈥�
