Geological history of oxygen
Encyclopedia
Before photosynthesis
evolved, Earth's atmosphere
had no free oxygen
(O2). Oxygen was first produced by photosynthetic prokaryotic organisms that emitted O2 as a waste product. These organisms lived long before the first build-up of oxygen in the atmosphere, perhaps as early as . The oxygen they produced would have almost instantly been removed from the atmosphere by weathering of reduced minerals, most notably iron. This 'mass rusting' led to the deposition of banded iron formations. Oxygen only began to persist in the atmosphere in small quantities about 50 million years before the start of the Great Oxygenation Event
. This mass oxygenation of the atmosphere resulted in rapid buildup of free oxygen. At current atmospheric rates, today's concentration of oxygen could be produced by photosynthesisers in 2,000 years. Of course, in the absence of plants
, photosynthesis was slower in the Precambrian
, and the levels of O2 attained were modest (<10% of today's) and probably fluctuated greatly; oxygen may even have disappeared from the atmosphere again around These fluctuations in oxygen had little direct effect on life, with mass extinctions not observed until the appearance of complex life around the start of the Cambrian
period, . The presence of provided life with new opportunities. Aerobic metabolism is more efficient than anaerobic pathways, and the presence of oxygen undoubtedly created new possibilities for life to explore. An atmospheric oxygen level about 2% by volume is necessary to make compounds such as collagen
s, used by all animals; thus high atmospheric oxygen levels are needed for large life-forms.
Since the beginning of the Cambrian
period, levels have fluctuated between 15% and 30% of atmospheric volume. Towards the end of the Carboniferous
period (about 300 million years ago) atmospheric levels reached a maximum of 35% by volume, which may have contributed to the large size of insects and amphibians at this time. Whilst human activities, such as the burning of fossil fuel
s, have a vast impact on carbon dioxide concentrations, their impact on oxygen levels is less significant.
, the Cambrian explosion
, trends in animal body size, and other extinction and diversification events.
The large size of insects and amphibians in the Carboniferous period, where oxygen reached 35% of the atmosphere, has been attributed to the limiting role of diffusion in these organisms' metabolism. However, the biological basis for this correlation is not firm, and many lines of evidence show that oxygen concentration is not size-limiting in modern insects. Interestingly, there is no significant correlation between atmospheric oxygen and maximum body size elsewhere in the geological record. Ecological constraints can better explain the diminutive size of post-Carboniferous dragonflies - for instance, the appearance of flying competitors such as pterosaur
s and birds and bats.
Rising oxygen concentrations have been cited as a driver for evolutionary diversification, although the physiological arguments behind such arguments are questionable, and a consistent pattern between oxygen levels and the rate of evolution is not clearly evident. The most celebrated link between oxygen and evolution occurs at the end of the last of the Snowball
glaciations, where complex multicellular life is first found in the fossil record. Under low oxygen levels, regular 'nitrogen crises' could render the ocean inhospitable to life. More fundamentally, an oxygen concentration of at least 40% of present atmospheric levels is necessary for metazoans to produce biochemicals, such as collagen
, that are essential to their existence. Models based on uniformitarian principles (i.e. extrapolating present-day ocean dynamics into deep time) suggest that such a level was only reached immediately before metazoa first appeared in the fossil record. Further, anoxic or otherwise chemically 'nasty' oceanic conditions that resemble those supposed to inhibit macroscopic life re-occur at intervals through the early Cambrian, and also in the late Cretaceous – with no apparent impact on lifeforms at these times. This might suggest that the geochemical signatures found in ocean sediments reflect the atmosphere in a different way before the Cambrian - perhaps as a result of the fundamentally different mode of nutrient cycling in the absence of planktivory.
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
evolved, Earth's atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
had no free oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
(O2). Oxygen was first produced by photosynthetic prokaryotic organisms that emitted O2 as a waste product. These organisms lived long before the first build-up of oxygen in the atmosphere, perhaps as early as . The oxygen they produced would have almost instantly been removed from the atmosphere by weathering of reduced minerals, most notably iron. This 'mass rusting' led to the deposition of banded iron formations. Oxygen only began to persist in the atmosphere in small quantities about 50 million years before the start of the Great Oxygenation Event
Great Oxygenation Event
The Great Oxygenation Event , also called the Oxygen Catastrophe or Oxygen Crisis or Great Oxidation, was the biologically induced appearance of free oxygen in Earth's atmosphere. This major environmental change happened around 2.4 billion years ago.Photosynthesis was producing oxygen both before...
. This mass oxygenation of the atmosphere resulted in rapid buildup of free oxygen. At current atmospheric rates, today's concentration of oxygen could be produced by photosynthesisers in 2,000 years. Of course, in the absence of plants
Evolutionary history of plants
The evolution of plants has resulted in increasing levels of complexity, from the earliest algal mats, through bryophytes, lycopods, ferns to the complex gymnosperms and angiosperms of today...
, photosynthesis was slower in the Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
, and the levels of O2 attained were modest (<10% of today's) and probably fluctuated greatly; oxygen may even have disappeared from the atmosphere again around These fluctuations in oxygen had little direct effect on life, with mass extinctions not observed until the appearance of complex life around the start of the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
period, . The presence of provided life with new opportunities. Aerobic metabolism is more efficient than anaerobic pathways, and the presence of oxygen undoubtedly created new possibilities for life to explore. An atmospheric oxygen level about 2% by volume is necessary to make compounds such as collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
s, used by all animals; thus high atmospheric oxygen levels are needed for large life-forms.
Since the beginning of the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
period, levels have fluctuated between 15% and 30% of atmospheric volume. Towards the end of the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
period (about 300 million years ago) atmospheric levels reached a maximum of 35% by volume, which may have contributed to the large size of insects and amphibians at this time. Whilst human activities, such as the burning of fossil fuel
Fossil fuel
Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...
s, have a vast impact on carbon dioxide concentrations, their impact on oxygen levels is less significant.
Effects on life
The concentration of atmospheric oxygen is often cited as a possible contributor to large-scale evolutionary phenomena, such as the origin of the multicellular Ediacara biotaEdiacara biota
The Ediacara biota consisted of enigmatic tubular and frond-shaped, mostly sessile organisms which lived during the Ediacaran Period . Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms.Simple multicellular organisms such as...
, the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
, trends in animal body size, and other extinction and diversification events.
The large size of insects and amphibians in the Carboniferous period, where oxygen reached 35% of the atmosphere, has been attributed to the limiting role of diffusion in these organisms' metabolism. However, the biological basis for this correlation is not firm, and many lines of evidence show that oxygen concentration is not size-limiting in modern insects. Interestingly, there is no significant correlation between atmospheric oxygen and maximum body size elsewhere in the geological record. Ecological constraints can better explain the diminutive size of post-Carboniferous dragonflies - for instance, the appearance of flying competitors such as pterosaur
Pterosaur
Pterosaurs were flying reptiles of the clade or order Pterosauria. They existed from the late Triassic to the end of the Cretaceous Period . Pterosaurs are the earliest vertebrates known to have evolved powered flight...
s and birds and bats.
Rising oxygen concentrations have been cited as a driver for evolutionary diversification, although the physiological arguments behind such arguments are questionable, and a consistent pattern between oxygen levels and the rate of evolution is not clearly evident. The most celebrated link between oxygen and evolution occurs at the end of the last of the Snowball
Snowball Earth
The Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once, some time earlier than 650 Ma . Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical...
glaciations, where complex multicellular life is first found in the fossil record. Under low oxygen levels, regular 'nitrogen crises' could render the ocean inhospitable to life. More fundamentally, an oxygen concentration of at least 40% of present atmospheric levels is necessary for metazoans to produce biochemicals, such as collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
, that are essential to their existence. Models based on uniformitarian principles (i.e. extrapolating present-day ocean dynamics into deep time) suggest that such a level was only reached immediately before metazoa first appeared in the fossil record. Further, anoxic or otherwise chemically 'nasty' oceanic conditions that resemble those supposed to inhibit macroscopic life re-occur at intervals through the early Cambrian, and also in the late Cretaceous – with no apparent impact on lifeforms at these times. This might suggest that the geochemical signatures found in ocean sediments reflect the atmosphere in a different way before the Cambrian - perhaps as a result of the fundamentally different mode of nutrient cycling in the absence of planktivory.
External links
- First breath: Earth's billion-year struggle for oxygen New Scientist, #2746, 5 February 2010 by Nick Lane.