THE case for the existence of photosynthetic life 3.4 billion years ago has been significantly strengthened. The new evidence, which comes from a layer of carbonaceous rock in South Africa, is certain to fuel the controversy about when exactly such life began on Earth.
In 2002, Martin Brasier from the University of Oxford and his colleagues disputed claims that microscopic patterns found in 3.5-billion-year-old rock in Western Australia were microfossils of various bacteria, including photosynthetic cyanobacteria. Brasier claimed the fossils were really patterns formed during the recrystallisation of volcanic glass from a hydrothermal vent that formed tens of metres under water 鈥� too deep for photosynthetic bacteria to live.
Similar questions were raised about a 3.4-billion-year-old layer of carbonaceous rock found in the Buck Reef Chert in South Africa. The chert was once a volcano in the sea that sank into the Earth鈥檚 crust as it cooled. Billions of years later the structure is on dry land. It was squashed between larger rocks, lifted up and turned on its side, giving modern geologists easy access to its long history.
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Now, Michael Tice and Donald Lowe of Stanford University, California, are saying that though the carbonaceous layer formed under water, it was shallow enough for light to penetrate and give photosynthetic life a start. The layer has both large and small grains of rock near each other, but the smaller grains would have washed away without mats of microbes to trap them, the researchers say.
To determine whether these microbes were photosynthetic, Tice and Lowe calculated the ratio of two isotopes of carbon, which can identify the process that produced the carbon-containing compounds. In this case, the ratio indicates that carbon dioxide was converted to sugar, as happens in photosynthesis. But non-photosynthetic microbes also use a similar process, so the ratio alone is not a clincher.
Tice and Lowe did find something else that they believe settles the case for photosynthesis: iron minerals alongside the carbon. If the microbes were non-photosynthetic, they would have oxidised the iron. But none of the surrounding iron is in an oxidised state, suggesting that it was photosynthetic microbial life that produced the carbon 3.4 billion years ago. 鈥淭his evidence for photosynthesis is new and better,鈥� says Tice.