But the high abundance of uranium relative to thorium in Isua
rocks suggested that uranium had been chemically separated from
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
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, USA, was cautious about the
"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
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