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Around 2.3 billion years ago our planet underwent what was perhaps the most significant and dramatic revolution in its history. However, this was not a revolution that we may be familiar with. There was no political basis to this transition, perhaps involving a suppressed population rising up in unison against a brutal dictator. This was a revolution was on a truly global scale and it was instigated by the chemistry of the planet. In this case, and quite literally, change was in the air.
During much of the Archean eon, between 3.9 and 2.5 billion years ago, the composition of the atmosphere of the Earth was dominated, as it is today, by nitrogen, but also by gases such as methane and hydrogen sulphide. These gases are ‘reducers’ (as opposed to ‘oxidisers’) that ‘donate’ one of their electrons to another substance in reaction. Oxygen, an oxidant, was a minor constituent, representing less than one part per million (1 ppm) of the atmosphere. About 2.3 billion years ago however, this all changed. This momentous and eon-defining event is known as the Great Oxidation to earth scientists, and it represents a truly life-changing transition in the history of the planet. Oxygen concentrations jumped (in relative terms) to between 1 and 10% of the present atmospheric level (PAL for short).
The puzzling thing however, is that oxygenic photosynthesis (OP) had already been active in primitive cyanobacteria (similar to the one above) for at least 300 million years before the Great Oxidation, originating sometime between 3.2 billion and 2.6 billion years ago, with the most convincing evidence placing its genesis at 2.7 billion years before present. Organisms had evolved the extraordinary ability to use photons of light to split water molecules and combine them with carbon dioxide to form complex sugars, in the process releasing oxygen gas as a by product. The evolution of oxygenic photosynthesis was in itself an incredible feat, involving some of the most complex cellular biomachinary ever discovered. Why, then, was the atmosphere of the early Proterozoic eon so oxygen-poor?
The answer, it turns out, is a rather complicated one. Oxygen concentrations during the transitional period before the Great Oxidation, but after the evolution of OP, were spatially variable, concentrated around the organisms responsible for its production as a waste gas. Any oxygen that made it into the air was quickly destroyed by reaction with methane against the backdrop of a highly (ultraviolet – UV) irradiated atmosphere. The reductant-dominated atmosphere was a product of volcanic out-gassing which provided a constant source of reduced gases such as hydrogen, hydrogen sulphide, methane and carbon dioxide. Any oxygen that wasn’t destroyed by reaction with these gases was mopped up by the large swathes of reduced material likely to be present on the surface of the Earth – including iron and iron pyrite (which was transformed from its reduced ferrous state to its oxidised ferric form) and resulting in the iconic banded iron formations (or BIFs) associated with this time. It would appear that oxygen could not maintain a foothold in this reducing world, struggling to maintain a low concentration which would have been spatially patchy and probably variable in time as well. Its sources were balanced by its sinks, as the chemists say.
In order to come to dominate, something would have to break the cycle – an imbalance between the sources of oxygen (i.e. the oxygenic photosynthesisers) and its sinks (methane photolysis and reaction with reduced gases and material from the mantle). Luckily for us as oxygen dependent organisms, something did break the feedback loop and that something was the humble hydrogen molecule.
Hydrogen in its elemental or ‘free’ variety is a reductant, reacting excitedly with oxygen to form water vapour, but in the upper atmosphere it behaves rather differently. At the base of the atmospheric homopause, around an altitude of ~ 100 km, water vapour is absent – frozen out at the stratospheric ‘cold trap’ at 20 km, but hydrogen molecules can be found hitching a lift within the structure of the methane molecule (CH4) which is light enough to rise through the homopause. The methane molecule is attacked by the UV-generated radicals in the upper atmosphere (O, OH) and yields hydrogen atoms that can diffuse right to the very top of the planetary atmosphere, known as the exobase, at around ~ 500 km. Here, some of the light hydrogen atoms – the high energy tail of the distribution – are accelerated to a velocity that allows them to overcome the gravity of the Earth and permanently escape into space. This mechanism, over millions of years, resulted in a net increase in oxygen by the erosion of a finite hydrogen (reductant) reservoir, pushing the balance of sinks and sources of oxygen to the side of the sources. This process is still occurring on the Earth, but at a much lesser rate than in the past due to the fact that methane is now less abundant than it once was.
At a critical point, estimated to be around 1 ppm by volume of oxygen, an ozone layer began to form in the stratosphere. This layer served to shield the Earth below from UV light and thereby prevented the reaction of methane and oxygen in the lower atmosphere, shifting the balance in the favour of oxygen once again and increasing its concentration further still.
The age of oxygen had dawned.
Perhaps I disposed of the political revolution analogy too soon. Let us think of oxygen gas as a suppressed minority but with the great potential to usher in a new age of organism diversity and to completely revolutionise the Earth system. Methane & co. form the oligarchy of the reductants, dominating the atmosphere and violently reacting with any oxidants they encounter (using their oppressive, ruthless army of UV light) to prevent an oxidising majority from forming. They insist upon the use of traditional, inefficient metabolic pathways such as hydrogen sulphide fermentation to power the small, putative organisms over which they rule. But the agents of change, the oxygenic photosynthesisers, are working studiously to infiltrate the atmosphere and gradually, with the protection provided by the revolutionary guard molecule, ozone, they eventually overthrow the reductants, banishing them to the depths of the oceans, lakes or the guts of cows and ushering in a new age of glorious oxygen domination!
Of course, this is just a metaphor and not a particularly good one at that. The real revolution took hundreds of millions of years and firmly without the inferred teleology or determinism that stems from my anthropic bias. Oxygen is a poison, and to the majority of life on the early-Earth, the dawn of the Great Oxidation marked the end of their reign as surface dwelling organisms. However, without it complex organisms would not have evolved. Anaerobic respiration is several orders of magnitude less efficient and therefore unsuitable for powering large, complex bodies that require more energy – energy that can only be provided by an oxygen metabolism.