New anlaysis challenges key evidence about when photosynthesis emerged on Earth

October 23rd, 2008 - 1:05 pm ICT by ANI  

London, Oct 23 (ANI): A fresh analysis of ancient rocks from the Australian outback has challenged key evidence about when photosynthesis emerged on Earth.
Back in 1999, 2.7-billion-year-old shale from the Pilbara Craton in Western Australia was shown to contain hydrocarbon molecules that could have been made only by photosynthesizing organisms.
Geobiologists concluded that photosynthetic organisms must have evolved by the time these rocks formed.
This created a puzzle that if most of the oxygen in Earth’’s ancient atmosphere had come from photosynthetic organisms, why did it take another 300 million years before oxygen levels rose sharply, sometime around 2.4 billion years ago?
According to a report in Nature News, researchers have now reanalysed the shale and claim that there is no puzzle to solve.
The tell-tale hydrocarbons were deposited in the rocks 2.15 billion years ago at most, eliminating the mysterious time-lag.
Birger Rasmussen of Curtin University of Technology in Bentley, Western Australia, led the analysis.
Jochen Brocks of the Australian National University in Canberra, who worked on both studies, said that he was always skeptical of the original analysis.
“The original biomarkers had one big problem,” he said. “The second-oldest biomarkers were only 1.6 billion years old,” he added.
That left an unlikely billion-year gap in the molecular record between those and the oldest markers.
Both experiments relied on the work of a carbon-fixing enzyme called Rubisco, which is crucial for photosynthesis.
It has a strong preference for the lighter isotope of carbon, so the molecules made from the carbon it processes are relatively enriched in carbon-12 compared with carbon-13.
The 1999 paper reported that the hydrocarbons extracted from the Pilbara rocks 700 metres below the surface contained a ratio of carbon isotopes with Rubisco’’s signature.
But the researchers had to rely on the hydrocarbons they could extract from the rock using solvents, and there was a chance that these hydrocarbons could have flowed into the ancient rock from rocks that formed more recently.
“So there was no proof for the age (of these lipids) - only circumstantial evidence and lack of disproof,” said Brocks.
Brocks, Rasmussen, and their colleagues have now repeated the analysis without using solvents. Instead, they used a mass spectrometer to fire a stream of ions at the solid sample to dislodge atoms. They then measured the carbon-isotope ratio in the atoms.
If the original measurement was correct, this technique should have found exactly the same ratio in the new sample. But it didn”t.
Instead, the new hydrocarbon samples had ratios that matched another form of organic matter, called kerogen, which was originally produced by bacteria that used methane instead of carbon dioxide as a source of fuel.
This suggests that the hydrocarbons reported in 1999 had indeed leached into the rock from elsewhere. (ANI)

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