Eoarchean World

Scientists in 2003 claimed to have found the oldest evidence of photosynthesis - the most important chemical reaction on Earth - in 3.7 billion year-old rocks.

Photosynthesis is the process by which plants, algae, and certain bacteria convert sunlight to chemical energy. Danish researchers claimed rocks from Greenland showed that life-forms were using the process about one billion years earlier than had previously been shown. Details of the research were published in Earth and Planetary Science Letters.

Professor Minik Rosing and Professor Robert Frei, both of the University of Copenhagen, Denmark, analysed ancient seafloor sediments in Isua, on the south-western edge of Greenland, where they had previously found the earliest evidence of life on Earth. What this research demonstrated was that Earth had a functioning biosphere prior to 3.7 billion years ago.

Uranium signature

The researchers discovered abundant quantities of the element uranium in the ancient sediments, which had most likely precipitated out of ocean water. In a 'reducing' environment where little or no photosynthesis is taking place, the elements uranium and thorium would move around together in the ocean as mineral particles.

But the high abundance of uranium relative to thorium in Isua rocks suggested that uranium had been chemically separated from thorium. This happens under 'oxidising' conditions in which organisms release oxygen into the environment. Rosing and Frei concluded that microbes, much like present-day cyanobacteria, were converting sunlight to chemical energy through oxygenic, or oxygen-producing, photosynthesis.

Anoxygenic photosynthesis, a form of the reaction which does not produce oxygen as a by-product, is widely thought to have evolved before the oxygenic form. Professor Rosing did not dispute this, but he pointed out the problem of not knowing how long life was evolving on Earth before 3.7 billion years ago. The geological record more or less stops there.

Professor Michael Bickle, a geologist at the University of Cambridge, UK, said the existence of photosynthesis at 3.7 billion years ago was 'indubitable'.

'Geological mill'

But Roger Buick, associate professor of astrobiology at the University of Washington in Seattle, USA, was cautious about the findings. Anything of that sort of age was, to him, somewhat dubious. The rocks would have been 'put through the geological mill' many times - it would be hard to say that anything being seen in them now was primary.

However, Rosing contended that lead isotopes in the rocks would have preserved an accurate 'isotopic memory' of uranium and thorium compositions in the past, suggesting that the values were indeed primary, or original. Dr Buick accepted that Minik Rosing would know the Isua rocks better than anyone else and didn't for one minute doubt his data, but he did wonder just how strong an interpretation could be placed on the data.

Studies conducted by Buick on rocks from Pilbara, Western Australia, established the presence of photosynthesis at 2.6-2.7 billion years ago. But the latest findings appeared to push those dates back by about one billion years. The biochemistry needed for oxygenic photosynthesis requires a good deal of bacterial evolution. If the findings were correct then life was very sophisticated very early on in Earth's history.

For three-quarters of a billion years after its formation, Earth was being pounded by meteorites. That bombardment only ended around 3.8 billion years ago. It would be expected that conditions of that sort would be pretty hostile to oxygenic photosynthesisers. But life may be older and more robust than previously thought.

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