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As the continents have moved over Earth’s surface, the world has experienced other physical changes, including large increases and decreases in atmospheric oxygen concentrations. The atmosphere of early Earth probably contained little or no free oxygen gas (O₂). The increase in atmospheric O₂ came in two big steps more than a billion years apart.

The first step occurred at least 2.4 billion years ago (bya), when certain bacteria evolved the ability to use water as the source of hydrogen ions for photosynthesis. By chemically splitting H₂O, these bacteria generated O₂ as a waste product. They also made electrons available for reducing CO₂ to form the carbohydrate end-products of photosynthesis (see Key Concept 10.3). The O₂ they produced dissolved in water and reacted with dissolved iron. The reaction product then precipitated as iron oxide, which accumulated in alternating layers of red and dark rock known as banded iron formations (Figure 24.7). These formations provide evidence for the earliest photosynthetic organisms. As photosynthetic organisms continued to release O₂, oxygen gas began to accumulate in the atmosphere.

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The second step occurred about a billion years later, when some of these photosynthetic bacteria became symbiotic within eukaryote cells, leading to the evolution of chloroplasts in photosynthetic plants and other eukaryotes. This change resulted in continued accumulation of O₂ in Earth’s atmosphere (Figure 24.8).

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Ash and gases produced by massive volcanic eruptions near the end of the Permian blocked sunlight and resulted in rapid cooling of Earth. There was massive die-off of the lush Permian forests, and the decay of these forests used up much of the atmospheric oxygen. In addition, the loss of these photosynthetic organisms meant that less oxygen was being produced to enter the atmosphere.

One group of O₂-generating bacteria, the cyanobacteria, were a major component of complex communities that formed rocklike structures called stromatolites, which are abundantly preserved in the fossil record. To this day, stromatolites are still formed in a few very salty places (Figure 24.9). Cyanobacteria liberated enough O₂ to open the way for the evolution of oxidation reactions as the energy source for the synthesis of ATP.

Thus the evolution of life irrevocably changed the physical nature of Earth. Those physical changes, in turn, influenced the evolution of life. When it first appeared in the atmosphere, O₂ was toxic to most of the anaerobic prokaryotes that inhabited Earth at the time. Over millennia, however, prokaryotes that evolved the ability to tolerate and use O₂ not only survived but gained the advantage. Aerobic metabolism proceeds more rapidly, and harvests energy more efficiently, than anaerobic metabolism. Organisms with aerobic metabolism replaced anaerobes in most highly oxygenated environments.

An atmosphere rich in O₂ also made possible larger and more complex organisms. Small single-celled aquatic organisms can obtain enough oxygen by simple diffusion even when dissolved oxygen concentrations in the water are very low. Larger single-celled organisms, however, have lower surface area-to-volume ratios. To obtain enough oxygen by simple diffusion, larger organisms must live in an environment with a relatively high oxygen concentration. Bacteria can thrive at 1 percent of the current oxygen concentration, but eukaryotic cells require levels that are at least 2–3 percent of the current concentration. For concentrations of dissolved oxygen in the oceans to have reached these levels, much higher atmospheric concentrations were needed.

Probably because it took many millions of years for Earth to develop an oxygenated atmosphere, only single-celled prokaryotes lived on Earth for more than 2 billion years. About 1.5 bya, atmospheric O₂ concentrations became high enough for larger eukaryotic cells to flourish. Further increases in atmospheric O₂ concentrations in the late Precambrian enabled several groups of multicellular organisms to evolve (see Figure 24.8).

Oxygen concentrations increased again during the Carboniferous and Permian periods because of the evolution of large vascular plants. These plants lived in the expansive lowland swamps that existed at the time (see Table 24.1). Massive amounts of organic material were buried in these swamps as the plants died, leading to the formation of Earth’s vast coal deposits. Because the buried organic material was not subject to oxidation as it decomposed, and because the living plants were producing large quantities of O₂, atmospheric O₂ increased to concentrations that have not been reached again in Earth’s history (see Figure 24.8). As mentioned at the opening of this chapter, these high concentrations of atmospheric O₂ allowed the evolution of giant flying insects and large amphibians that could not survive in today’s atmosphere.

The drying of the lowland swamps at the end of the Permian reduced burial of organic matter as well as the production of O₂, so atmospheric O₂ concentrations dropped rapidly. Over the past 200 million years, with the diversification of flowering plants, O₂ concentrations have again increased, but not to the levels that characterized the Carboniferous and Permian periods.

Biologists have conducted experiments revealing the changing selection pressures that can accompany changes in atmospheric O₂ concentrations. When fruit flies (Drosophila) are raised in hyperoxic conditions (i.e., with artificially increased atmospheric concentrations of O₂), they evolve larger body sizes in just a few generations (Investigating Life: The Relationship between Atmospheric Oxygen Concentration and Body Size in Insects). The present atmospheric O₂ concentrations appear to constrain body size in these flying insects, whereas increases in O₂ appear to relax those constraints. This experiment demonstrates that the *stabilizing selection on body size at current O₂ concentrations can quickly switch to directional selection for a change in body size in response to a change in O₂ concentrations.

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