The Great Oxygenation Event, the beginning
of which is commonly known in scientific media
as the Great Oxidation Event (GOE, also called
the Oxygen Catastrophe, Oxygen Crisis, Oxygen
Holocaust, Oxygen Revolution, or Great Oxidation)
was the biologically induced appearance of
dioxygen (O2) in Earth's atmosphere.
Geological, isotopic, and chemical evidence
suggests that this major environmental change
happened around 2.45 billion years ago (2.45
Ga), during the Siderian period, at the beginning
of the Proterozoic eon.
The causes of the event remain unclear.
As of 2016, the geochemical and biomarker
evidence for the development of oxygenic photosynthesis
before the Great Oxidation Event has been
mostly inconclusive.Oceanic cyanobacteria,
which evolved into coordinated (but not multicellular
or even colonial) macroscopic forms more than
2.3 billion years ago (approximately 200 million
years before the GOE), were the first microbes
to produce oxygen by photosynthesis.
Before the GOE, any free oxygen they produced
was chemically captured by dissolved iron
or by organic matter.
The GOE started when oxygen produced by the
cyanobacteria started escaping into the atmosphere,
after other oxygen reservoirs were filled.
The increased production of oxygen set Earth's
original atmosphere off-balance.
Free oxygen is toxic to obligate anaerobic
organisms, and the rising concentrations may
have destroyed most such organisms at the
time.A spike in chromium contained in ancient
rock-deposits formed underwater shows the
accumulation had been washed off from the
continental shelves.
Chromium is not easily dissolved and its release
from rocks would have required the presence
of a powerful acid.
One such acid, sulfuric acid (H2SO4), might
have formed through bacterial reactions with
pyrite.
Mats of oxygen-producing cyanobacteria can
produce a thin layer, one or two millimeters
thick, of oxygenated water in an otherwise
anoxic environment even under thick ice; before
oxygen started accumulating in the atmosphere,
these organisms would already have adapted
to oxygen.
Additionally, the free oxygen would have reacted
with atmospheric methane, a greenhouse gas,
greatly reducing its concentration and triggering
the Huronian glaciation, possibly the longest
episode of glaciation in Earth's history and
called "snowball Earth".Eventually, the evolution
of aerobic organisms that consumed oxygen
established an equilibrium in its availability.
Free oxygen has been an important constituent
of the atmosphere ever since.
== Timing ==
The most widely accepted chronology of the
Great Oxygenation Event suggests that free
oxygen was first produced by prokaryotic and
then later eukaryotic organisms that carried
out photosynthesis more efficiently, producing
oxygen as a waste product.
The first oxygen-producing organisms arose
long before the GOE, perhaps as early as 3,400
million years ago.Initially, the oxygen they
produced would have quickly been removed from
the atmosphere by the chemical weathering
of reducing (oxidizable) minerals, most notably
iron.
This 'mass rusting' led to the deposition
of iron(III) oxide in the form of banded-iron
formations such as the sediments in Minnesota
and Pilbara, Western Australia.
The saturation of these mineral sinks, and
the resulting persistence of oxygen in the
atmosphere, led within 50 million years to
the start of the GOE.
Oxygen could have accumulated very rapidly:
at today's rates of photosynthesis (much greater
than those in the Precambrian without land
plants), modern atmospheric O2 levels could
be produced in just 2,000 years.Another hypothesis
is that oxygen producers did not evolve until
a few million years before the major rise
in atmospheric oxygen concentration.
This is based on a particular interpretation
of a supposed oxygen indicator used in previous
studies, the mass-independent fractionation
of sulfur isotopes.
This hypothesis would eliminate the need to
explain a lag in time between the evolution
of oxyphotosynthetic microbes and the rise
in free oxygen.
In either case, oxygen did eventually accumulate
in the atmosphere, with two major consequences.
Firstly, it oxidized atmospheric methane (a
strong greenhouse gas) to carbon dioxide (a
weaker one) and water.
This decreased the greenhouse effect of the
Earth's atmosphere, causing planetary cooling,
and triggered the Huronian glaciation.
Starting around 2.4 billion years ago, this
lasted 300-400 million years, and may have
been the longest ever snowball Earth episode.Secondly,
the increased oxygen concentrations provided
a new opportunity for biological diversification,
as well as tremendous changes in the nature
of chemical interactions between rocks, sand,
clay, and other geological substrates and
the Earth's air, oceans, and other surface
waters.
Despite the natural recycling of organic matter,
life had remained energetically limited until
the widespread availability of oxygen.
This breakthrough in metabolic evolution greatly
increased the free energy available to living
organisms, with global environmental impacts.
For example, mitochondria evolved after the
GOE, giving organisms the energy to exploit
new, more complex morphologies interacting
in increasingly complex ecosystems.
== Time lag theory ==
There may have been a gap of up to 900 million
years between the start of photosynthetic
oxygen production and the geologically rapid
increase in atmospheric oxygen about 2.5–2.4
billion years ago.
Several hypotheses propose to explain this
time lag.
=== Tectonic trigger ===
The oxygen increase had to await tectonically
driven changes in the Earth, including the
appearance of shelf seas, where reduced organic
carbon could reach the sediments and be buried.
The newly produced oxygen was first consumed
in various chemical reactions in the oceans,
primarily with iron.
Evidence is found in older rocks that contain
massive banded iron formations apparently
laid down as this iron and oxygen first combined;
most present-day iron ore lies in these deposits.
Evidence suggests oxygen levels spiked each
time smaller land masses collided to form
a super-continent.
Tectonic pressure thrust up mountain chains,
which eroded to release nutrients into the
ocean to feed photosynthetic cyanobacteria.
=== Nickel famine ===
Early chemosynthetic organisms likely produced
methane, an important trap for molecular oxygen,
since methane readily oxidizes to carbon dioxide
(CO2) and water in the presence of UV radiation.
Modern methanogens require nickel as an enzyme
cofactor.
As the Earth's crust cooled and the supply
of volcanic nickel dwindled, oxygen-producing
algae began to out-perform methane producers,
and the oxygen percentage of the atmosphere
steadily increased.
From 2.7 to 2.4 billion years ago, the rate
of deposition of nickel declined steadily
from a level 400 times today's.
=== Bistability ===
Another hypothesis posits a model of the atmosphere
that exhibits bistability: two steady states
of oxygen concentration.
The state of stable low oxygen concentration
(0.02%) experiences a high rate of methane
oxidation.
If some event raises oxygen levels beyond
a moderate threshold, the formation of an
ozone layer shields UV rays and decreases
methane oxidation, raising oxygen further
to a stable state of 21% or more.
The Great Oxygenation Event can then be understood
as a transition from the lower to the upper
steady states.
=== Hydrogen gas ===
Another theory credits the appearance of cyanobacteria
with suppressing hydrogen gas and increasing
oxygen.
Some bacteria in the early oceans could separate
water into hydrogen and oxygen.
Under the Sun's rays, hydrogen molecules were
incorporated into organic compounds, with
oxygen as a by-product.
If the hydrogen-heavy compounds were buried,
it would have allowed oxygen to accumulate
in the atmosphere.
However, in 2001 scientists realized that
the hydrogen would instead escape into space
through a process called methane photolysis,
in which methane releases its hydrogen in
a reaction with oxygen.
This could explain why the early Earth stayed
warm enough to sustain oxygen-producing lifeforms.
== Late evolution of oxy-photosynthesis theory
==
The oxygen indicator might have been misinterpreted.
During the proposed lag era in the previous
theory, there was a change in sediments from
mass-independently fractionated (MIF) sulfur
to mass-dependently fractionated (MDF) sulfur.
This was assumed to show the appearance of
oxygen in the atmosphere, since oxygen would
have prevented the photolysis of sulfur dioxide,
which causes MIF.
However, the change from MIF to MDF of sulfur
isotopes may instead have been caused by an
increase in glacial weathering, or the homogenization
of the marine sulfur pool as a result of an
increased thermal gradient during the Huronian
glaciation period (which in this interpretation
was not caused by oxygenation).
== Role in mineral diversification ==
The Great Oxygenation Event triggered an explosive
growth in the diversity of minerals, with
many elements occurring in one or more oxidized
forms near the Earth's surface.
It is estimated that the GOE was directly
responsible for more than 2,500 of the total
of about 4,500 minerals found on Earth today.
Most of these new minerals were formed as
hydrated and oxidized forms due to dynamic
mantle and crust processes.
== Origin of eukaryotes ==
It has been proposed that a local rise in
oxygen levels due to cyanobacterial photosynthesis
in ancient microenvironments was highly toxic
to the surrounding biota, and that this selective
pressure drove the evolutionary transformation
of an archaeal lineage into the first eukaryotes.
Oxidative stress involving production of reactive
oxygen species (ROS) might have acted in synergy
with other environmental stresses (such as
ultraviolet radiation and/or desiccation)
to drive selection in an early archaeal lineage
towards eukaryosis.
This archaeal ancestor may already have had
DNA repair mechanisms based on DNA pairing
and recombination and possibly some kind of
cell fusion mechanism.
The detrimental effects of internal ROS (produced
by endosymbiont proto-mitochondria) on the
archaeal genome could have promoted the evolution
of meiotic sex from these humble beginnings.
Selective pressure for efficient DNA repair
of oxidative DNA damages may have driven the
evolution of eukaryotic sex involving such
features as cell-cell fusions, cytoskeleton-mediated
chromosome movements and emergence of the
nuclear membrane.
Thus the evolution of eukaryotic sex and eukaryogenesis
were likely inseparable processes that evolved
in large part to facilitate DNA repair.
Constant pressure of endogenous ROS has been
proposed to explain the ubiquitous maintenance
of meiotic sex in eukaryotes.
== See also ==
Geological history of oxygen – Timeline
of the development of free oxygen in the Earth's
seas and atmosphere
Iodide
Medea hypothesis
Pasteur point
Rare Earth hypothesis – Hypothesis that
complex extraterrestrial life is a very improbable
phenomenon and likely to be extremely rare
Stromatolite
