When nearby food resources became exhausted, the resulting food shortages might have caused selective pressures for evolutionary change. Some organisms evolved the ability to synthesize their food from simple inorganic substances. These became organisms called autotrophs. The autotrophs began to evolve in many different directions, using different substances, such as carbon dioxide, ammonia, and hydrogen sulfide, as food.
Another group evolved into the photoautotrophs, capable of carrying out photosynthesis, which uses solar energy to incorporate carbon dioxide into organic molecules and releases oxygen as a byproduct. Carbon combined with other elements to promote cellular growth, and the oxygen escaped into the atmosphere, which prepared the environment for the next important step in the evolution of primitive organisms.
As the photoautotrophs multiplied, photosynthesis began to gradually change Earth's original oxygen-poor atmosphere to a more oxygen-rich one. A rapid buildup of oxygen in the atmosphere was delayed, as iron in rocks exposed at the earth's surface was oxidized before oxygen could accumulate in the atmosphere. This allowed many microorganisms to evolve oxygen-mediating enzymes that permitted them to cope with the new atmosphere.
Once atmospheric oxygen concentrations reached about 10 percent, solar radiation converted part of the oxygen to ozone, forming a shield against ultraviolet radiation. Life, still primitive and vulnerable, was now protected and could expand into environments that formerly had not been able to harbor life, setting the stage for the appearance of aerobic organisms.
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