Before photosynthesis evolved, Earth's atmosphere had no free oxygen.
Photosynthetic prokaryotic organisms that produced O2 as a waste product
lived long before the first build-up of free oxygen in the atmosphere, perhaps
as early as 3.5 billion years ago. The oxygen they produced would have been
rapidly removed from the atmosphere by weathering of reducing minerals, most
notably iron. This 'mass rusting' led to the deposition of iron oxide on the
ocean floor, forming 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 rates of primary
production, today's concentration of oxygen could be produced by
photosynthetic organisms in 2,000 years. In the absence of plants, the rate of
oxygen production by photosynthesis was slower in the Precambrian, and the
concentrations of O2 attained were less than 10% of today's and probably
fluctuated greatly; oxygen may even have disappeared from the atmosphere again
around 1,900 million years ago These fluctuations in oxygen concentration had
little direct effect on life, with mass extinctions not observed until the
appearance of complex life around the start of the Cambrian period, 541
million years ago. The presence of O 2 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.
Since the start of the Cambrian period, atmospheric oxygen concentrations have
fluctuated between 15% and 35% of atmospheric volume. The maximum of 35%
was reached towards the end of the Carboniferous period, a peak which may
have contributed to the large size of insects and amphibians at that time.
Whilst human activities, such as the burning of fossil fuels, have an impact
on relative carbon dioxide concentrations, their impact on the much
larger concentration of oxygen 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 biota, 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. But Haldane's essay points
out that it would only apply to insects. 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 pterosaurs 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 concentrations 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 concentrations, regular 'nitrogen crises' could render the ocean
inhospitable to life. Significant concentrations of oxygen were just one
of the prerequisites for the evolution of complex life. Models based on
uniformitarian principles suggest that such a concentration 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. References
External links First breath: Earth's billion-year
struggle for oxygen New Scientist, #2746, 5 February 2010 by Nick Lane.
The Mystery of Earth's Oxygen New York Times, 3 October 2013 by Carl Zimmer.