Published online 22 October 2008 | Nature | doi:10.1038/news.2008.1184

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Row brews over when photosynthesis emerged

Research contradicts key evidence that Sun-fuelled life arose 2.7 billion years ago.

The Hamersley Ranges Karijini National Park in Australia could be the final resting place of the earliest photosynthetic organisms.Punchstock

A fresh analysis of ancient rocks from the Australian outback has challenged conventional wisdom 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 organisms1. Geobiologists concluded that photosynthetic organisms must have evolved by the time these rocks formed.

This created a puzzle: 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? (see 'Oxygen's place in fossil history')

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. The analysis, led by Birger Rasmussen of Curtin University of Technology in Bentley, Western Australia, is published in this week's Nature2.

Jochen Brocks of the Australian National University in Canberra, who worked on both studies, says that he was always sceptical of the original analysis. "The original biomarkers had one big problem," he says. "The second-oldest biomarkers were only 1.6 billion years old." That left an unlikely billion-year gap in the molecular record between those and the oldest markers.

Isotope mismatch

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 paper2 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," says 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. The instrument could also focus on an area of the sample just 50 nanometres wide, ensuring that they sampled precisely the right part of the rock.

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.

It ain't over ...

But the conclusion has been questioned by Brocks's co-authors on the 1999 study. Roger Summons, of the Massachusetts Institute of Technology in Cambridge, says that the authors "have not disproved, or even acknowledged subsequent carbon isotopic and biomarker work on the [same source] from Australia". The later work, he says, suggests that photosynthesis evolved earlier than the proposed 2.4 million years ago.

Independent experts also have concerns about the Nature paper. David Catling of the University of Bristol, UK, says that strands of evidence that photosynthesis evolved at least 2.7 billion years ago have basically been "swept under the carpet without proper discussion".

He points out, for instance, that the Nature paper did not reference research done in 1992 by Roger Buick into 2.7-billion year old stromatolites — structures formed in shallow water by photosynthetic bacteria3. "I am surprised that the paper got published in its current form," adds Jim Kasting of Pennsylvania State University in University Park.

Given so much contradictory evidence, the field is likely to remain divided for some time. "This debate is far from over," says Summons. 

  • References

    1. Brocks, J. J. et al. Science 285, 1033-1036 (1999). | Article | PubMed | ISI | ChemPort |
    2. Rasmussen, B., Fletcher, I. R., Brocks, J. J. & Kilburn, M. R. Nature 455, 1101–1104 (2008).
    3. Buick, R. et al. Science 255, 74-77 (1992). | PubMed | ISI | ChemPort |
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