Cyanobacteria that occur symbiotically in other organisms commonly lack a cell wall and appear to function essentially as chloroplasts. When a eukaryotic cell containing chloro-plasts divides, the chloroplasts divide at the same time. The cells of cyanobacteria occurring within the cells of other organisms divide in similar fashion, leading to speculation that chloroplasts originated as cyanobacteria or prochlorobacteria (discussed after the cyanobacteria), living within other cells.
Fossils of photosynthetic organisms believed to be 3.5 billion years old and closely resembling present-day cyanobacteria have been found in Australia. It was not until half a billion years later, about 3 billion years ago, that these organisms began producing oxygen as a by-product of photosynthesis. The oxygen slowly began to accumulate, becoming substantial about 1 billion years ago. At the same time the oxygen was accumulating, other photosynthetic organisms appeared, and forms of aerobic respiration developed.
Within the last half billion years, enough ozone, which is a by-product of oxygen, accumulated to become an effective shield against most of the harmful ultraviolet radiation coming from the sun. Photosynthetic organisms, which had been protected from the radiation by their watery environments, were then able to survive on land.2 Accordingly, cyanobacteria appear to have played a fundamental role in almost the entire history of living organisms.
2. See also the discussion in Chapter 18 of the role of certain green algae in the transition of aquatic organisms to land, and the discussion in Chapter 25 of the effects of ozone depletion on our environment.
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