By Paul Rincon
BBC News Online science staff
Scientists claim to have found the oldest evidence of photosynthesis - the most important chemical reaction on Earth - in 3.7-billion-year-old rocks.
The rocks from Isua are amongst the oldest on Earth
Photosynthesis is the process by which plants, algae and certain bacteria convert sunlight to chemical energy.
Danish researchers say rocks from Greenland show life-forms were using the process about one billion years earlier than has previously been shown.
Details of the research are 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, Greenland, where they had previously found the earliest evidence of life on Earth.
"What this demonstrates is that the Earth had a functioning biosphere before 3.7 billion years ago," Professor Rosing told BBC News Online.
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 where organisms are releasing oxygen into the environment.
Rosing and Frei conclude 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 that does not produce oxygen as a by-product, is widely thought to have evolved before the oxygenic form.
Professor Rosing does not dispute this, but, he said: "The problem is that one doesn't know 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".
But Roger Buick, associate professor of astrobiology at the University of Washington in Seattle, US, was cautious about the findings.
"Anything of that sort of age has to be somewhat dubious. Those rocks have been put through the geological mill many times - it would be hard to say that anything you're seeing is primary," he said.
However, Rosing contends that lead isotopes in the rocks preserve an accurate "isotopic memory" of uranium and thorium compositions in the past, suggesting the values are primary, or original.
"Minik Rosing knows [the Isua rocks] better than anyone else. I don't for one minute doubt his data, I just wonder how strong an interpretation you can put on that data," Dr Buick added.
Isua's shales have metamorphosed since they were deposited
Studies conducted by Buick on rocks from Pilbara, western Australia, establish photosynthesis at 2.6-2.7 billion years ago. But the latest findings would appear to push these dates back by about one billion years.
"The biochemistry needed for oxygenic photosynthesis requires lots of bacterial evolution. If their findings are correct, life was very sophisticated, very early on in Earth history," said Buick.
"For three-quarters of a billion years after its formation, the Earth was being pounded by meteorites. That bombardment only ends around 3.8 billion years ago.
"You would think those sorts of conditions would be pretty hostile to oxygenic photosynthesisers. But life may be older and more robust than we thought."