How the earth got Oxygenated

English: Cyanobacteria

English: Cyanobacteria (Photo credit: Wikipedia)

The fact that we are able to breathe on Oxygen, vital to life, in the modern world, is due to a period, over two billion years ago, when chemical warfare was being waged on Earth on a truly gigantic scale. In the time before plants discovered the power of photosynthesis, single-celled life depended for nutrition not on sunlight but chemicals.

These organisms – anaerobes – consumed  hydrogen, methane and sulphur, among others, existing without oxygen, but when blue-green algae known as cyanobacteria evolved into creatures employing photosynthesis to feed, exhaling oxygen in the process, anaerobes were simply killed off by O2 poisoning. Oxygen combined with metals and proteins in the anaerobic cells, killing them off, while cyanobacteria thrived by turning sunlight into sugar, expelling oxygen as a waste product.

 

In what is today referred to as the Great Oxidation Event of some 2.5 billion years ago, the Oxygen levels began to spike, but a growing body of data that suggests the earliest sun-lovers appeared long before this. A number of researchers now believe that those first photosynthetic organisms appeared 3 billion years ago, giving a whole new dimension to the period.

2.95-billion-year-old rocks from the Pongola Supergroup, South Africa  were studied in detail by Yale University geochemist Noah Planavsky and colleagues, who found them to contain molybdenum and iron, which are markers of photosynthesis. These chemical traces in the rocks indicate that cyanobacteria were producing indeed oxygen in ocean surface water, while another recent study on Pongola rocks, suggest atmospheric O2 was about 100,000 times higher than could be explained by non-biological chemical reactions.

 

Pasadena, California Caltech Geo-biologist,  Woodward Fischer cautioned that the quality of interpretations remains a little bit uncertain, as in all fairness, we still do not, even today, fully understand the molybdenum and the chromium cycle, though as more sensitive techniques emerge for looking back through geological time, the researchers feel inclined to ask whether it was indeed microbes that alone introduced Oxygen or did environmental changes contribute, in the form of more land-based eruptions spewing out gas into the atmosphere, during that 500 million years between when the first cyanobacteria evolved photosynthesis and the Great Oxidation Event? Research, obviously, is ongoing.

 


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