Despite oxygen gas’ abundance and necessity for complex life today, ancient cyanobacteria almost ended life on earth when they first began producing oxygen. The time it takes for a planet to become oxygenated by photosynthetic life is a critical factor in determining whether complex life will successfully evolve. Should the oxygenation time be too long, a planet’s host star will live and die before complex animal-like life could evolve. Alternatively, if this oxygenation time is too short, any emerging simple life would be wiped out by the toxic oxygen. A sudden increase in atmospheric oxygen can also cause runaway global cooling, leading to an ice-covered world that is not hospitable to life.
There is then a goldilocks range of oxygenation times in which it is probable that aerobic photosynthetic life could oxidize a planet without bringing about planetary extinction. Defining the variables that effect this range would inform the search for life on exoplanets throughout the galaxy.
Background
The Initial Conditions of Archean Earth
De Pastino et al. (2017) describes the earth’s climate and geochemistry during the Archean eon (4-2.5 Ga), in a way that would have been completely hostile to the complex life that exists on Earth today. Most notably, little oxygen gas existed, and any that did would quickly react with the ubiquitous reductants present. Reductants included large concentrations of unoxidized iron in the planet’s oceans and an atmosphere of greenhouse gases such as carbon dioxide and methane. Life did exist during the Archean eon, but it was mostly anaerobic and limited in complexity.
The Evolution of Photosynthetic Cyanobacteria
Moore (2017) says that Cyanobacteria first evolved about 2.8 Ga; about 300 million years before the end of the Archean. It was similar to much of the other life of the Archean in that it was simple and prokaryotic. However, as Green (2020) points out, Cyanobacteria had a special ability that no organism had ever evolved before or would ever evolve again: Oxygenic Photosynthesis. These small microbes began producing large amounts of oxygen gas that would eventually alter the earth on a geologic scale and pave the way for complex animal life.
The Great Oxygenation Event
The Archean Eon ended with perhaps the most pivotal event in the history of life on earth, the Great Oxygenation Event (GOE). Cyanobacteria had continued to divide and photosynthesize over the last 300 million years and a major change was about to occur to Earth’s atmospheres and oceans. The new oxygen source suddenly overtook the previously dominating oxygen sinks as atmospheric levels of oxygen reached 3%. Green (2020) speculates that the GOE might have been the deadliest extinction event of earths history, and so it is sometimes dubbed the oxygen catastrophe.
The Cambrian Explosion
The Cambrian period is well known for the boom in complex animal life at its beginning. Catling et al. (2005) concludes that this new complex animal life was only possible because oxygen gas had finally reached a high enough concentration (about 20%) to accommodate their greater size and activity. The Cambrian Explosion is a good indicator as the end point of oxygenation time on Earth. It took about 3.9 billion years from the formation of the Earth, to the point where atmospheric oxygen was great enough to sustain complex life.
Limiting Factors of a Long Oxygenation Time
The Critical Build up of Oxygen
Based on what is known about aerobic respiration’s importance for complex animal life on Earth, Catling et al. (2005) hypothesized that any complex animal-like life that has evolved on exoplanets would also be highly dependent on free oxygen gas. If an exoplanet is determined to have 3% atmospheric oxygen concentration or more, then it is possible it has undergone a GOE or comparable event. If an exoplanet has 20% atmospheric oxygen concentration or more, then it is possible it has had an explosion in diversity in animal-like life similar to the Cambrian explosion on Earth.
Photosynthesis and Volcanism
According to Catling et al. (2005), the major source of oxygen on Earth is biological photosynthesis, in which oxygen gas is split from liquid water. Therefore, oxygenic photosynthesizers are likely a prerequisite to complex animal-like life on other exoplanets. Despite the correlation between these two types of life, it was not until 2.3 billion years after cyanobacteria arose on Earth that complex animal life did too. Not only was Archean life a long way off from evolving into anything of Cambrian complexity, but also the free oxygen was not even available to support such a jump in complexity. The cause of this delay in oxygen build up was due to the vast oxygen sinks present on Earth. Although cyanobacteria were pumping out oxygen, the new gas almost immediately reacted with unoxidized iron, CO2 and methane. These reductants were also continually being produced by volcanic activity on earth. It is this back and forth of photosynthesis and volcanism that likely delayed the critical build up of oxygen on earth. When looking for complex animal-like life on exoplanets, astrobiologists should keep oxygen sources and sinks in mind. A small planet with abundant liquid water and low amounts of green house gases would have a much shorter oxygenation time than a larger dry planet with high levels of volcanic activity. Hypothetically, an exoplanet could produce such high amounts of reductants that the planet is never oxygenated.
Lifetime of Main Sequence Stars
The reason oxygenation time is crucial to the possibility of complex animal-like life on other planets, is that this life does not have infinite time to evolve. Earth had a 3.9-billion-year oxygenation time, which is nearly within a factor of 2 of the sun’s lifetime (“Main sequence,” 2021). If oxygen had been built up half as quickly, complex animal life would have been unlikely to evolve before Earth was sterilized by the death of its host star. Exoplanets could have wildly variable oxygenation times based on the sources and sinks mentioned above, but they may have variable windows of opportunity as well. Some exoplanets orbit stars with lifespans many times that of the sun (“Red dwarf”, 2021), giving these planets a much longer oxygenation window.
Limiting Factors of a Short Oxygenation Time
The Great Oxygen Catastrophe
The previous section on long oxygenation times might give the impression that the shorter the oxygenation time, the more likely complex animal-like life is to arise and thrive. However, the major extinction event that coincided with the GOE should not be forgotten. If the oxygenation of earth had occurred more suddenly, life may have failed to adapt to the new oxygenated world. Moore (2017) says that Cyanobacteria had already existed 2.8 Ga, but reductants present in the Archean earth’s oceans and atmosphere delayed the GOE until 300 million years later. Without this buffer period, an Oxygen Catastrophe could end life on exoplanets before complex life could evolve.
Toxic Oxygen
Anaerobic life living on other planets would also likely be adapted to anoxic conditions and would be unprepared for a dramatic increase of oxygen gas in their surroundings. It is impossible to say with certainty what the minimum oxygenation time that simple anaerobic life could successfully adapt to, but looking at earth’s history, it can be said that any oxygenation time less than 300 million years would be very dangerous. It is then necessary that an exoplanet harbouring early anaerobic life have sufficient oxygen sinks to delay a GOE as long as the earth’s sinks did.
Snowball Earth
Immediately following the GOE, earth’s global temperature plummeted due to the removal of greenhouse gases by oxygen. This same effect would pose a threat to the GOE-surviving photosynthetic life of another planet. If the entire planet were to be so cold that the poles froze to the equator, as potentially happened on earth during the Huronian Glaciation, there would be no place for photosynthetic life to be sustained. Without these oxygenic photosynthesizers, complex animal-like life could not arise. Catling (2013) suggests one solution to this problem, that the planet remains warm enough that a belt of liquid water be preserved around the equator.
Conclusion
Oxygenation time of an exoplanet must be of moderate length such that complex animal-like life may arise. It is bound on the upper end by the death of the host star if oxygen sinks dominate oxygen sources. Earth’s oxygenation time of 3.9 billion years is likely lower than other potentially inhabited exoplanets as other planets host stars have lifetimes much longer than that of the sun. Oxygenation time is bound on the lower end by the deadliness of a quick GOE. Due to the mass extinction that followed earth’s GOE, it is likely that earth’s oxygenation time approaches that of the minimum oxygenation time exoplanets could have where simple life might survive an oxygen catastrophe.
Evan Nelles Henderson, Revised from undergraduate paper, 2021.
References
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