SUN-WORSHIP began even earlier than we thought. The world's oldest sedimentary rocks suggest an early form of photosynthesis may have evolved almost 3.8 billion years ago, not long after life appeared on Earth.
A hallmark of photosynthesis in plants is that the process splits water and produces oxygen gas. But some groups of bacteria oxidise substances like iron instead – a form of photosynthesis that doesn't generate oxygen. Evolutionary biologists think these non-oxygen-generating forms of photosynthesis evolved first, giving rise to oxygen-generating photosynthesis sometime before the Earth's atmosphere gained oxygen 2.4 billion years ago (New Scientist, 8 December 2012, p 12).
But when did non-oxygen-generating photosynthesis evolve? Fossilised microbial mats that formed in shallow water 3.4 billion years ago in what is now South Africa show the chemical fingerprints of the process. However, geologists have long wondered whether even earlier evidence exists.
The world's oldest sedimentary rocks – a class of rock that can preserve evidence of life – are a logical place to look, says Andrew Czaja of the University of Cincinnati in Ohio. These rocks, which are found in Greenland and date back almost 3.8 billion years, contain vast deposits of iron oxide that are a puzzle. "What could have formed these giant masses of oxidised iron?" asks Czaja.
To investigate, he analysed the isotopic composition of samples taken from the oxidised iron. He found that some isotopes of iron were more common than they would be if oxygen gas was indiscriminately oxidising the metal. Moreover, the exact isotopic balance varied subtly from point to point in the rock.
Both findings make sense if photosynthetic bacteria were responsible for the iron oxide, says Czaja. That's because these microbes preferentially oxidise only a small fraction of the dissolved iron, and the iron isotopes they prefer vary slightly as environmental conditions change (Earth and Planetary Science Letters, doi.org/kh5). His findings suggest that this form of photosynthesis appeared about 370 million years earlier than we thought.
It is "the best current working hypothesis for the origin of these deposits", says Mike Tice of Texas A&M University in College Station – one of the team who analysed the 3.4-billion-year-old microbial mats from South Africa.
William Martin at the University of Düsseldorf, Germany, agrees. "Anoxygenic photosynthesis is a good candidate for the isotope evidence they see," he says. "Had these fascinating results been collected on Mars, the verdict of the jury would surely remain open," says Martin Brasier at the University of Oxford. "But [on Earth] opinion seems to be swinging in the direction of non-oxygen-generating photosynthesis during the interval from 3.8 to 2.9 billion years ago."
This article appeared in print under the headline "Photosynthesis has truly ancient origins"
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