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 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. 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.


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