Oxygen is not the main constituent of Earth's atmosphere, comprising 21% of the total compared with Nitrogen's 78%, but its role in the survival of many forms of life elevates its significance. Most people are well aware of the important role oxygen plays in metabolic processes, but the fact that oxygen in the atmosphere owes its existence as much to life as life owes its existence to it is often overlooked. It was life itself that created an oxygenated atmosphere and continues to maintain it.
Oxygen is what is known as a highly reducing gas: it likes to combine with other molecules like atmospheric gases or surface rocks. It is the second-most electronegative atom in the periodic table after flourine ; this means that oxygen has a strong tendency to rip electrons from other atoms. As a consequence, any given oxygen molecule has a relatively short lifetime in the atmosphere. Before the rise of photosynthesis, a process which produces oxygen and continues to this day to replenish our supply, Earth's atmosphere had no appreciable quantity of oxygen whatsoever.
Prior to 3.45 billion years ago, Earth's atmosphere and oceans were anoxic (i.e. without oxygen). This is supported by the existence of mass-independent fractionalization (MIF) of sulfur isotopes in sediments from this time period, for these can only form in the absence of oxygen . Then, between 2.45 and 1.85 billion years ago, molecular oxygen appeared, albeit at a small fraction of the current atmospheric level, due to the evolution of photosynthetic cyanobacteria . Photosynthesis works by combining CO2 and H2O with energy derived from light to form O2, an additional amount of H2O, and glucose (C6H12O6 ) which the plant may then use for energy. The photosynthetic equation may be written as:
6 CO 2 + 12 H 2O + photons → C 6H 12O 6 + 6 O 2 + 6 H 2O
MIF sulfur isotopes disappeared at about 2.41 billion years ago, signaling the presence of oxygen, and additional evidence for the rise of oxygen comes from banded iron formations (BIFs). There is some controversy over the formation mechanism of BIFs, but a widely accepted idea is that oxygen combined with iron dissolved in sea water, causing it to precipitate out as insoluble iron oxide (i.e. rust) which settled down on the seafloor. The banding patterns we see are assumed to have resulted from cyclical variations of oxygen levels in the oceans. The period between 2.4 and 2.0 billion years ago is sometimes known as the Great Oxidation Event.
Counter-intuitively, the rise of oxygen is often regarded by evolutionary biologists as the worst pollution catastrophe in Earth's history. When it first appeared, oxygen was highly toxic to life because it rapidly destroys organic compounds; it killed off many branches of organisms that could not adapt to its presence. Fortunately, the transition from an anoxic atmosphere to an oxygen-rich one was drawn out over several hundred million years, giving some lifeforms time to evolve oxygen-consuming metabolism that could intake with the molecule safely. Additionally, 2.9 billion years ago marked the earliest known ice age, the Huronian glaciation, which was one of the most severe glaciations in geological history. This glaciation may have been triggered by the rise of oxygen . If methane was an important greenhouse gas for the early Earth, the presence of oxygen, which very quickly destroys methane molecules, would have cooled the planet significantly for a time.
Of course, in the long term, the rise of oxygen has benefited life hugely. Oxygen, being as reactive as it is, can liberate a large amount of energy to an organism that has learned how to deal with it, making possible the evolution of larger and more advanced lifeforms. Just as importantly, a byproduct of the rise of O2 in the atmosphere was the rise of O3, creating the ozone layer and shielding out cell-damaging UV radiation. This allowed life to finally leave the protection of the oceans and colonize the land. The Huronian glaciation may even have benefited life in the long term: by creating extreme environmental stresses, it may have hastened evolutionary development.
Oxygen levels have since continued to more-or-less rise, peaking at 30% of the total atmospheric content during the Carboniferous era some 350 million years ago. During this time, the burial rate of organic matter was rapid, preventing oxygen from combining with carbon in dead organisms and keeping it in the atmosphere. The high availability of oxygen during this period may explain the enormous insects of the Carboniferous era; if more oxygen is available to the absorbed into the blood, the blood may deliver the oxygen further in the body, supporting larger body structures.
The most important thing as far as astrobiologists are concerned is the fact that there are no known abiotic mechanisms of producing an O2-rich atmosphere. If we find a planet with O2 in its atmosphere, would be forced to conclude that life was the cause. A remarkable 99.9999% of the oxygen in Earth's atmosphere was produced by life, the tiny remainder being a result of photodissociation of H2O . There is no guarantee that life, if it exists elsewhere, will produce oxygen, but we do know that oxygen metabolism is very useful to organisms that have learned how to deal with it because O2 allows organisms to make use of the highest energy source per electron transfer that could conceivably be available in a habitable atmosphere. 1 The New Worlds Observer team is very interested in detecting O2 in extrasolar planetary atmospheres because the implications of such a discovery would be enormous.
2. Papineau D, Mojzsis S, Schmitt A. (2007). Multiple sulfur isotopes from Paleoproterozoic Huronian interglacial sediments and the rise of atmospheric oxygen. Earth and Planetary Science Letters 255, 188-212.
3. Holland H. (2006). The oxygenation of the atmosphere and oceans. Philosophical Transactions of the Royal Society B 361, 903-915.
4. Léger A. (2000). Strategies for remote detection of life – DARWIN-IRSI and TPF Missions. Adv. Space Res. 25, 2209-2223.