The oxygen that humans and other aerobic organisms breathe comes from different natural processes with living organisms at their center. About 70 percent of the free oxygen in the atmosphere is produced by the photosynthetic activities of green algae and cyanobacteria in marine environments. The remaining 30 percent is produced by terrestrial. While the availability of oxygen is continuously replenished through these natural processes, the exact origin of oxygen on Earth is still a matter of debate.
An Enumeration and Discussion of the Main Theories Explaining the Origin of Oxygen on Earth: The Geological History of Oxygen
Oxygen is one of the most abundant elements in the universe alongside hydrogen and helium. However, on Earth, there was no free oxygen in the atmosphere around 3.5 billion years ago, particularly before the emergence of photosynthetic organisms. Small amounts of oxygen existed through geological and biological processes, but it was not enough to build up because of its reactivity with reducing elements and minerals. Oxygen also existed in compounds like dihydrogen oxide or water, sulfur dioxide, and iron oxide.
1. Early Photosynthesis Theory: Photosynthetic Prokaryotes
It is important to underscore the fact that some types of prokaryotic photosynthetic organisms emerged as early as 3.5 billion years ago. This was well before the first buildup of oxygen in the atmosphere. These organisms are called anoxygenic photosynthetic bacteria. They used photosynthesis but did not produce oxygen was a waste product. However, over time, they evolved to become oxygenic photosynthetic prokaryotes.
Ancient oxygenic photosynthetic organisms produced oxygen as a waste product. However, it was not enough to produce a buildup. Furthermore, because the element is reactive, it formed banded iron formation on the ocean floor. The oceans rusted and turned red due to the deposition and accumulation of iron oxide. This theory challenges the conventional view that cyanobacteria were the first oxygen-producing organisms on Earth.
The aforementioned comes from more recent studies suggesting the presence of minimal amounts of oxygen in shallow oceans before a monumental event that resulted in its massive atmospheric build-up. These same studies also suggest that photosynthesis may have originated a billion years earlier than previously thought. Nevertheless, based on this, the origin of oxygen on Earth can be traced back to the emergence of ancient photosynthetic organisms.
2. Cyanobacteria Theory: The Great Oxidation Event
The evolution and emergence of cyanobacteria about 3.4 to 2.9 billion years ago from anoxygenic photosynthetic bacteria remains uncertain. Some hypotheses include gene acquisition or transfer from oxygenic photosynthetic bacteria, duplication and modification of genes for anoxygenic photosynthesis, or environmental pressure due to increasing levels of carbon dioxide, decreasing levels of hydrogen sulfide, or increasing solar radiation.
Note that cyanobacteria produce oxygen as a byproduct of photosynthesis. The scientific consensus is that the population of this specific bacteria grew about 2.5 billion years ago to a level that their production of oxygen accumulated in shallow oceans and then escaped into the air. The released free oxygen changed the composition and chemistry of the atmosphere and affected life on Earth. This was called the Great Oxidation Event.
The Great Oxidation Event is considered one of the most important events in the history of the Earth. It is also the most dominant theory that explains the origin of abundant free oxygen and explores its specific impacts and outcomes. The phenomenon cooled the atmosphere due to the reaction of oxygen with methane. It also paved the way for the evolution and emergence of diverse multicellular life forms that depend on oxygen.
3. Geological Theory: Notable Geological Processes
The geological theory includes several hypotheses tracing the origin of oxygen on Earth to several geological processes. Some of them also explain the possible factors that caused the Great Oxidation Event. Researchers D. C. Catling, K. J. Zahnle, and C.P. McKay suggested that metamorphic processes resulted in the release of hydrogen and methane gasses that oxidized the crust. This is called the decreasing sink hypothesis.
Changes due to tectonic events were also hypothesized as a contributor to oxygen accumulation. Evidence suggests that oxygen levels spiked whenever smaller landmasses collided to form larger landmasses. Some of these events can break down water molecules into hydrogen and oxygen. This process is called the stoichiometric decomposition of water. Tectonic events also release nutrients to the ocean that feed oxygen-producing cyanobacteria.
Nevertheless, considering the aforementioned, the geological theory suggests that the origin of oxygen partly comes from several geological processes such as volcanic eruptions, hydrothermal vents, or weathering of rocks. These could have produced small amounts of oxygen before the evolution of cyanobacteria. The problem with this theory is that it does not directly explain how oxygen levels became suitable enough to support complex life.
4. Cometary Impact Theory: Oxygen From Cometary Impacts
Some of the oxygen on Earth may have come from comets according to several researchers. These include earlier scientists such as American astronomer Fred Whipple and American planetary scientist John S. Lewis who separately argued and proposed in the mid-1900s that comets could have delivered water, other oxygen-rich compounds like carbon dioxide and carbon monoxide, and other volatiles on Earth upon impact.
Astrobiologists Kevin J. Zahnle David C. Catling and Mark W. Claire published several papers that explored and suggested how cometary oxygen could have played a role in the rise of oxygen in the atmosphere of the Earth around 2.4 billion years ago. Their specific calculations revealed that cometary impacts increased the level of oxygen in the atmosphere by up to 0.1 percent during periods of intense bombardments.
It is worth mentioning that comets release gas and dust whenever they approach a star. Some of these gases include molecular or free oxygen. The exact origin of oxygen in comets is still up for debate. Other studies have also noted that the free form of the element is produced by chemical reactions such as the Eley-Rideal mechanism on the surface of the comet or in its coma which splits water molecules into oxygen and hydrogen.
FURTHER READINGS AND REFERENCES
- Catling, D. C., Zahnle, K. J., and McKay, C. P. 2001. “Biogenic Methane, Hydrogen Escape, and the Irreversible Oxidation of Early Earth.” Science. 292(5531): 839-843. DOI: 1126/science.1061976
- Claire, M. W., Catling, D. C., and Zahnle, K. J. 2008. “Biochemical Modeling of the Rise in Atmospheric Oxygen.” Geobiology. 4(1): 239-269. DOI: 1111/j.1472-4669.2006.00084.x
- Holland, H. D. 2006. “The Oxygenation of the Atmosphere and Oceans. Philosophical Transactions of the Royal Society B: Biological Sciences. 361(1470): 903-915. DOI: 1098/rstb.2006.1838
- Tanai, C. 2008. “Early Archean Origin of Heterodimeric Photosystem I.” Heliyon., 4(3): e00548 DOI: 1016/j.heliyon.2018.e00548
- Zahnle, K. J., Catling, D. C., and Claire, M. W. 2013. “The Rise of Oxygen and the Hydrogen Hourglass.” Chemical Geology. 362: 26-34. DOI: 1016/j.chemgeo.2013.08.004