Earth is the only planet in our Solar System with high concentrations of gaseous diatomic oxygen. Simultaneously, this unique feature of Earth’s atmosphere has allowed the presence of an ozone layer that absorbed UV radiations. But the oxygen content of Earth’s atmosphere has varied greatly through time. For about the first 2 billion years of Earth’s history, the atmospheric oxygen concentration was exceptionally low.
It’s widely assumed that about 2.3 billion years ago, the level of oxygen increased dramatically in a process called the Great Oxidation Event (GOE). This rise in oxygen level occurred during an episode of major glaciation known as the Huronian glaciation. The progressive oxygenation of the atmosphere and oceans was sustained by an event of high organic carbon burial, called the Lomagundi Event, which lasted well over 100 million years, and represents the largest positive carbon-isotope excursion in Earth history (Canfield, 2013). This early oxygen primary production was exclusively conducted by prokaryotes, specifically by cyanobacteria.
However, new geochemical evidence suggested that there were appreciable levels of atmospheric oxygen about 3 billion years ago, more than 600 million years before the Great Oxidation Event, indicating a greater antiquity for oxygen producing photosynthesis and aerobic life.
After the GOE, oxygen levels rose again and then fell in the atmosphere and remained at extremely low levels for more than a billion years. This was probably due to a particular combination of biogeochemical feedbacks that spawned an oxygen-lean deep ocean (Lyons, 2014). The general oxygenation of the oceans began around 750-550 million years ago. This recovery of oxygen levels led to a significant increase in trace metals in the ocean and possibly triggered the ‘Cambrian explosion of life’ (Large, 2014).
But early animals, in general, may have had relatively low oxygen requirements. According to new findings, a sea sponge – the living animal that most resembles the earliest animals on Earth – can live and grow even at atmospheric oxygen levels that are 0.5 percent of today’s levels, which challenges the notion that low oxygen levels were the limiting factor for animal evolution. The study also suggest that the evolution of sophisticated gene regulatory networks, may have controlled the timing of animal origins more so than environmental parameters (Mills, 2014)
Donald E. Canfield, Lauriss Ngombi-Pemba, Emma U. Hammarlund, Stefan Bengtson, Marc Chaussidon, François Gauthier-Lafaye, Alain Meunier, Armelle Riboulleau, Claire Rollion-Bard, Olivier Rouxel, Dan Asael, Anne-Catherine Pierson-Wickmann, and Abderrazak El Albani, Oxygen dynamics in the aftermath of the Great Oxidation of Earth’s atmosphere PNAS 2013 110 (42) 16736-16741; published ahead of print September 30, 2013, doi:10.1073/pnas.1315570110.
Daniel B. Mills, Lewis M. Ward, CarriAyne Jones, Brittany Sweeten, Michael Forth, Alexander H. Treusch, and Donald E. Canfield, Oxygen requirements of the earliest animals, PNAS 2014 ; published ahead of print February 18, 2014, doi:10.1073/pnas.1400547111
Timothy W. Lyons, Christopher T. Reinhard, Noah J. Planavsky. The rise of oxygen in Earth’s early ocean and atmosphere. Nature, 2014; 506 (7488): 307 DOI: 10.1038/nature13068