World's oldest sedimentary rocks reveal photosynthesis' early origins

London: Scientists studying the world's oldest sedimentary rocks have claimed that about 3.8 billion years ago, an early form of photosynthesis may have evolved.

In photosynthesis in plants, the water is split by the process to produce oxygen gas. But some bacteria oxidise substances such as iron - a photosynthesis which doesn't generate oxygen, New Scientist reported.

Evolutionary biologists believe that oxygen-generating photosynthesis were given rise to by non-oxygen-generating forms of photosynthesis, which evolved first.

But the question when did non-oxygen-generating photosynthesis evolve, was answered by the fossilised microbial mats, which formed in shallow water 3.4 billion years ago in what is now South Africa, that show the process' chemical fingerprints.

Andrew Czaja of the University of Cincinnati in Ohio said that the world's oldest sedimentary rocks - a class of rock which is able to preserve evidence of life -is a logical place to look.

The sedimentary rocks - found in Greenland - are almost 3.8 billion years old and have vast deposits of iron oxide which was puzzling to him as he couldn't think as to what could have formed the giant masses of oxidised iron.

To investigate, he analysed the isotopic composition of rock samples taken from and found that some isotopes of iron were more common than they are going to be if oxygen gas was indiscriminately oxidising the metal. Moreover, the isotopic balance varied faintly from point to point in the rock.

Czaja said that both findings made sense if photosynthetic bacteria were responsible for the iron oxide as microbes oxidised only a small fraction of the dissolved iron, and the iron isotopes they prefer vary slightly as environmental conditions change.

Czaja's findings have suggested that this form of photosynthesis appeared about 370 million years earlier than had been thought.