The History of Atmospheric Oxygen

Primitive life began in the absence of free oxygen. However, in the first 400 My of the Earth, bacteria-like organisms developed that could take advantage of the

Figure 3 A simple representation of the key chemical reactions controlling the fluxes of oxygen to and from the atmosphere.

light energy from the Sun to initiate photosynthesis, although early production of oxygen likely oxidized crus-tal materials instead of building up in the atmosphere. Geochemical evidence suggests there was little oxygen in the atmosphere during the Achaean (—2.5-4 billion years ago), but near the beginning of the Proterozoic, about 2.5 billion years ago, cyanobacteria - photosynthetic prokar-yotes also known as blue-green algae - started the process that eventually led to a massive increase in the concentration of atmospheric oxygen. The definitive worldwide change that signaled the appearance of a significant increase in free oxygen was the appearance of red beds, stratified layers of sedimentary rocks, characterized by abundant red oxides of iron (ferric oxides), which occurred about 2.0 billion years ago. When the oxygen began to accumulate, the resulting oxidation of other atmospheric constituents totally changed the nature of the atmosphere and the oxygen 'poisoning' was devastating to many of the existing life forms then found on Earth and its oceans. Some bacteria however were able to endure the oxygen atmosphere. A symbiosis between bacteria and the formerly free-living mitochondria enabled eukaryotes to eventually evolve in response to the crisis. Oxygen-based metabolism came into being. The environment changed and life evolved.

Figure 4 provides an estimate of the growth of free oxygen in the Earth's atmosphere, along with the corresponding evolution in the types of various life forms found on our planet. With the increasing proliferation of life, the amount of photosynthesis increased and atmospheric oxygen also continued to increase. The oxygen buildup eventually resulted in the appearance of the first higher order cells with nuclei, called eukaryotes or eukaryotic cells (some analyses suggest this occurred earlier than shown in the figure). These cells depend on

First land animals

First mammals

First land animals

First mammals

Origin of flowering plants Origin of land plants

First eukaryotic cells

First prokaryotic cells l"M I I I

4 3 2 1 0.5 0.2 0.1 Billions of years before present

Figure 4 Estimated growth of free oxygen in the Earth's atmosphere. Based on graph from http:// www.ldeo.columbia.edu/edu/dees/U4735/projections/ free_o.html.

Origin of flowering plants Origin of land plants

First eukaryotic cells

First prokaryotic cells l"M I I I

4 3 2 1 0.5 0.2 0.1 Billions of years before present

Figure 4 Estimated growth of free oxygen in the Earth's atmosphere. Based on graph from http:// www.ldeo.columbia.edu/edu/dees/U4735/projections/ free_o.html.

atmospheric oxygen for the formation of complex energy-producing compounds. Such cells provide the makeup of all nonbacterial (or nonbacteria-like) forms of like on Earth.

Atmospheric oxygen grew to its present levels of roughly 21 % of the atmospheric content about 400 million years ago (Ma). While it was likely slowed in reaching this state by consumption in terrestrial weathering processes, the amount of atmospheric oxygen, as suggested by the paleontologic record, appears to have varied little since then. The variations that have occurred (while we have little evidence of actual variations, concentrations may have varied from as little as 15% to as much as 30% during the last 500 My) are likely related to periods when there were strong variations in the deposition of organic matter.

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