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Bacterial traces push back dawn of complex life on Earth

11 November 2010, by Tom Marshall

A key stage of the oxygenation of Earth's atmosphere, which eventually allowed animals to evolve, happened far earlier than previously thought.

Earth's atmosphere

Earth's atmosphere

New research has shown that bacteria were using oxygen to release energy from the chemicals around them 1.2 billion years ago, suggesting a rise in atmospheric oxygen – some 400 million years earlier than was thought possible until now.

This increase in atmospheric oxygen was needed for the evolution of complex multi-celled organisms and - eventually – large animals.

'Our findings will give impetus to further investigations into the timescale of the development of complex life, which followed this event,' says Professor John Parnell, a geologist at the University of Aberdeen and lead author of the paper, which appears in Nature.

When Parnell and colleagues at Aberdeen and the Scottish Universities Environmental Research Centre (SUERC) examined some of Scotland's oldest rocks, they found the unmistakable chemical signature of bacteria whose metabolism involves the reduction of sulphate to hydrogen sulphide - a compound with a familiar rotten egg smell. These bacteria have one of the most ancient evolutionary lineages of any living thing, and are still found today.

But the crucial evidence that the researchers uncovered was that other sections of the microbial community had to be earning their keep from the opposite reaction – turning the sulphides back into sulphates through an oxidation reaction. Essentially, they were getting energy by metabolizing the sulphides that sulphate-reducing bacteria had made.

To do this, they needed a lot more oxygen than most scientists had previously thought was available 1.2 billion years ago, when the rocks were laid down as sediments on a lakebed. 'We're not just talking about oxygen in the atmosphere here; it must have been percolating down into the subsurface environment for these bacteria to be able to use it,' Parnell explains.

'This opens up the way for more complex life to begin burrowing beneath the surface, so we're not just seeing the evolution of life itself but also of behaviour,' he adds. There may not have been much oxygen by modern standards – probably less than a tenth of the amount in the atmosphere today – but at the time this was a revolutionary development.

Secrets in the sulphides

The proof came from looking at the proportions of different kinds of sulphur in the rocks. Many elements naturally occur in several forms, called isotopes. Each isotope has the same number of protons in the nucleus of every atom, but the number of neutrons varies.

The researchers worked by measuring the ratio between two different isotopes of sulphur, known as 34S and 32S. Because 32S is lighter and therefore a little easier to process, sulphate-reducing bacteria use it in preference to 34S. This means the hydrogen sulphides they produce end up containing a disproportionate amount of the lighter isotope. Scientists can measure this difference accurately.

Until this study, it was thought that the difference between the starting sulphate and the product sulphides didn't change much for around 2 billion years, until around 800 million years ago. At that point, the theory went, oxygen arose in the atmosphere, and sulphide-oxidising bacteria evolved to take the opportunity.

'Once these oxidising bacteria evolved, the isotopic difference between the sulphates and sulphides almost doubled,' says Dr Adrian Boyce of SUERC, another of the paper's authors. 'What our research on these ancient Scottish rocks shows is that this must have been happening 400 million years earlier, as we measured the larger values typical of the new type of bacterial network,' he adds.

The team's samples came from near Lochinver in the Highlands. 'In Scotland we're lucky enough to have some of the oldest rocks in Europe,' Parnell explains. 'We knew there was an unusual opportunity here, as these rocks were laid down in a terrestrial environment at the bottom of a lake, whereas most other rocks this old were laid down on the seabed. This means these rocks were formed much closer to the atmosphere.'

The team are now planning to carry out similar research into rocks dating from several crucial points through the last two billion years. They aim to improve our understanding of how the planet's atmosphere progressively became the comparatively oxygen-rich environment we know today.

Early oxygenation of the terrestrial environment during the Mesoproterozoic, Nature 468, 290-293. DOI: 10.1038/nature09538

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