F Traces of early life

In 1996, Stephen Mojzsis, then a graduate student at the Scripps Institution of Oceanography at La Jolla, California, made a startling announcement in the journal Nature. He claimed to have identified a clear chemical signature for life in carbon compounds from Isua Group rocks. He had analysed minute grains of graphite, a form of carbon, in the rocks, and found an unusually high proportion of carbon-12. The carbon atom has two stable isotopes, carbon-12 and carbon-13. The ratio of these two forms of carbon can indicate the presence or absence of organic residues of previously living organisms: enrichment in carbon-12 relative to carbon-13 is characteristic of photosynthesizing organisms, and the organisms that eat them. Mojzsis was confident he had identified life: 'Our evidence establishes beyond reasonable doubt that life emerged on Earth at least 3.85 billion years ago, and this is not the end of the story. We may well find that life existed even earlier.'

If the interpretation is correct, then the grains of graphite in the Isua rocks prove that photosynthesis was happening 3.85 billion years ago. Photosynthesis is the process by which green plants convert energy from sunlight into food. Carbon dioxide and water combine, and produce oxygen, usually given off as a gas, and sugars, which form the building blocks of the plant. Now, in the early part of the history of the Earth, these photosynthesizing organisms were not trees or flowers, but presumably simple microbes known as cyanobacteria.

Other researchers have argued strongly against this interpretation. They noted, for example, that the Isua graphite was not in the sedimentary rocks of the area, but in the metamorphic rocks. Indeed, the Isua sedimentary rocks contained relatively low proportions of graphite. The alternative argument was then that the Isua graphites were of secondary, inorganic origin and might have formed by heating of iron carbonate. One of the critics, Roger e Buick of the University of Washington, Seattle, said that 'These J rocks have been buried and cooked at least three times. They've f been severely squashed and strained and tied in knots at least e three times too.'

The Isua graphites are still held as evidence for early life, and the debates continue to rage. But how does this chime with current theoretical views about the origin of life?

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