The Great Oxygenation Event, the Cambrian explosion!!
What caused the ‘great oxygenation event, the Cambrian Explosion, believed to be the biggest evolution of new species in Earth’s history?
The theories that go with this question are diverse and continue to grow.
The ‘Age of Chemistry’, the Precambrian, heralded in the new era, a time of hard bodied, aerobic organisms and a plethora of diversity. So, what are the factors that conjured up these new ripples of life?
Well, as with all evolutionary changes, it was a multitude of ecological factors that allowed this big bang of life to occur. From geological movements such as the break up of the super continent Rodinia, to global glaciations, to the development of genetic potential, life was able to take advantage of the abundant oxygen. Yet, it is the change in the chemistry of planet Earth that is behind these opportunities for life. There is believed to be numerous simultaneous reactions that occurred.
One reaction was the result of a rapid decline in atmospheric methane. Why did the methane levels drop? One angle, supported by the journal Nature, reports that the nickel, that existed within the primordial seas, was closely correlated to the emergence of oxygen producing organisms due to its use by methanogen organisms. The dominant organisms before the great oxygenation were the methanogens, single celled organisms that produced methane as a byproduct of metabolism. Large quantities of methane prevented oxygen from building up in the atmosphere, as the gas reacts with oxygen to form carbon dioxide and water. These organisms utilized nickel present in the ocean, gobbling it up in vast quantities. There were 400 x greater amounts of nickel in the oceans 4.6 BYA, than there are presently, and by 2.5 BYA, this quantity had halved. As this nutrient declined, the methanogens also declined, and hence the methane in the atmosphere plummeted. This allowed the photosynthesizing organisms, cyanobacteria, to proliferate allowing the push towards oxygen based life on plant earth. And hence, began the Cambrian explosion.
Another explanation for the great oxygenation of the atmosphere by scientist Gaillard (2011) is major geographic upheaval at the end of the Archaen era (2.5BYA). Falling sea levels, and a profoundly changing landscape saw the geographic repositioning and emergence of continents and an increase in sub aerial volcanism. These volcanoes released vast quantities of magmatic volatiles found in the magma, specifically carbon dioxide, sulphur and chlorine. These volatiles entered the atmosphere rather than the ocean, and were released at a significantly lower pressure. The oxidation of sulphur was affected by this change in pressure and produced a decrease in hydrogen sulphide and an increase in sulphur dioxide. This change of state saw the increase of dissolution sulphate in the oceans, of which fed the sulphate- reducing cyanobacteria and encouraged the proliferation of atmospheric oxygen. The changes in the chemistry of the ocean that enabled complex life forms to gradually develop hard shells and skeletons, created the first ‘animals’, the Edicarans and the Vendians.
These are just some of the potential ecological processes that gave rise to life, and most likely are part of a large story of change and evolution. Paleo- ecology is a fascinating way to look at the world, and help us understand how changes in the composition of the ocean and atmosphere can affect life on the planet, in the past and into the future.
ps. Click on the top artwork for a clear image.