Ancient oxygen discovery rewrites history of life on earth

ILLUS-AncientAir-LisaTheriaultPhotosynthesis evolved earlier than thought.

A study led by a scientist in British Columbia suggests that we need to reshape the way we think about how life evolved on earth. In a study released last Wednesday by biogeochemist Sean Crowe, it was found that oxygen appeared in the Earth’s atmosphere up to 700 million years earlier than what was once thought. 

Before publication of the study, scientists believed that photosynthesis evolved in single-celled organisms approximately 2.7 billion years ago. Photosynthesis is the process used by plants and similar organisms to convert light energy (typically from the sun), into chemical energy. Oxygen is produced during photosynthesis.

Early photosynthetic organisms were thought to have given rise to a time period known as the Great Oxidation Event, which was believed to be the first time the atmosphere began accumulating a significant amount of oxygen. This event was dated as approximately 2.3 billion years ago. This event led to the evolution of more complex multicellular organisms that can survive in oxygen-rich atmospheres. 

If confirmed, this study will be significant because it suggests that life forms able to produce oxygen may have arisen a lot earlier than previously thought.

Crowe, who is an assistant professor in the department of earth, ocean, and atmospheric sciences and the department of microbiology and immunology at the University of British Columbia, claimed that others have found traces of oxygen in samples that are older than 2.3 billion years (before the Great Oxidation Event). Unfortunately, the chemical signals were never strong enough to analyze. This is due to the fact that the oldest samples that were analyzed were often from the bottom of the ocean, which aren’t in direct contact with the atmosphere.

The soils that were analyzed in the study date back to 2.95 billion years ago. They were found in Kwazulu-Natal Province, South Africa. Crowe collaborated with colleagues at the University of Southern Denmark to test the samples for oxygen.

The process used to test for oxygen is a more sensitive technique than previous tests, and involved looking for chromium atoms. These isotopes are sensitive to reactions using oxygen. The heavier form of the atom, chromium-53, is slightly more soluble when oxidized than the lighter chromium-52. Over time, soils that have been oxidized should show depleted levels of chromium-53 due to rainwater washing it away. Sea sediments, where products of weathering typically end up, are enriched in chromium-53.  The tests showed low concentrations, but enough to analyze.

The oxygen levels detected in the samples were only a 10,000th of present day levels, and a 200th of the levels that followed the Great Oxygenation Event. Through computer modelling, oxygen levels during this time were shown to be five times higher than the amount that can be generated without the help of photosynthesis-capable organisms.

The team wishes to test its findings further on other rocks from different parts of the world. This may provide a challenge, as 3 billion year old rocks are hard to come by (at least ones that have not undergone significant alteration).

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