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Any old iron?

17 March 2006

How and why did life evolve on planet Earth? This fundamental question has occupied the minds of scientists since before Darwin ...

The most robust evidence for life on the planet is body fossils – the remains of dead organisms preserved in rocks. But the preservation of body fossils is a rather special process and there are strong reasons to believe that the further you go back in time the less likely body fossils are to be preserved.

Unequivocal body fossils of single-celled microbes have been found in rocks that are about 2.5 billion years old. Earlier finds, from rocks as old as 3.5 billion years, are much more controversial and may not be biological in origin. In the absence of definite body fossils, scientists have turned to more subtle evidence for life, principally in the chemical traces the metabolisms of organisms leave behind in rocks.

Isotopes are atoms of the same element with slightly different atomic weights due to differing numbers of neutrons in their nucleii. The chemical process by which many elements are broken down (metabolised) by organisms, preferentially uses the lighter of the isotopes available. For example, the extraction of carbon from the environment to build cellular material by modern photosynthesisers results in carbon in cells that are isotopically enriched in carbon-12, relative to carbon-13.

These results are the clearest evidence yet for microbial metabolism in sediments that are 2.7 billion years old

The presence of carbon enriched in carbon-12 in rocks as old as 3.8 billion years has been used by some scientists to suggest that photosynthesis evolved at this time – but this assertion remains highly controversial. All life is a fight against thermodynamics in that the building of cellular organic matter is something that all life must do, but it requires huge amounts of energy. Photosynthesis uses light as an energy source. Animals (including ourselves) use the energy released when we react oxygen with dead organic matter (food).

There is evidence that other ways of metabolising food are extremely ancient. One of these involves the conversion of iron (very plentiful on planet Earth) from an oxidised form (like rust) to a reduced form that is soluble in water. This process releases energy which the organism can use. Another is the conversion of oxidised sulphur (in the form of sulphate ion) to its reduced form (in the form of hydrogen sulphide).

Until quite recently, for technical reasons, iron isotopes were not available to scientists as a tool for looking for early signs of life. Developments over the past 10 years however, including pioneering work by Bristol scientists, have changed this. What the team has done is to probe very old sedimentary rocks for signs of life in the isotopes of iron. They have measured the isotopic makeup of iron in sediments that are 2.7 billion years old and found that these samples are variably enriched in light isotopes. 

One way to explain this enrichment is that it was caused by microbes ‘eating’ the iron and converting it to a lighter form. Importantly, the enrichment in light iron isotopes is also coupled to similar enrichments in light sulphur isotopes. Since the isotopic make-up of both iron and sulphur can be changed by non-biological processes, the coupling of these enrichments provides more evidence for the involvement of life.

These results are the clearest evidence yet available for the antiquity of iron reduction as a microbial metabolism and suggest that the tool may be applicable further back in time, or on other planets.

Derek Vance/Earth Sciences

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