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1.3Geological Time Periods and Changes in Organisms

The first living organisms to appear on the earth are thought to have been anaerobic unicellular organisms, who used marine organic substances without using oxygen (Fig. 1-2). Let us now look at the changes that occurred in organisms over time according to the divisions of geological time periods.
The period from the formation of the earth until 560 million years ago is called the Precambrian age, and the first life form appeared during this time. Later, photosynthetic bacteria and cyanobacteria appeared in the ocean. These bacteria were able to synthesize organic substances using carbon dioxide, thus causing oxygen to gradually increase in the atmosphere. Organisms became multicellular, and eukaryotes emerged (see Section 4 of Chapter 1). Radiolarians (protozoa), sponges, and green algae emerged at the end of the Precambrian age.

Concurrently, the increased oxygen was changed to ozone by the ultraviolet rays in the stratosphere 10–50 km above the earth. This ozone formed a layer that blocked the harmful ultraviolet rays, preventing them from reaching the earth's surface. This condition enabled living organisms to advance from sea to land. In the Paleozoic era about 400 million years ago, the first organisms to advance to land were bryophytes.
During the Paleozoic era, fishes and amphibians appeared and flourished in water, and ferns flourished on land. In the Mesozoic era, reptiles such as dinosaurs flourished, and gymnospermous plants such as conifers dominated the ecosystem. The Cenozoic era began when large reptiles gradually became extinct after the earth was struck by a meteorite, ushering in the era of angiosperms and mammals, including humans.

Fig. 1-2. Geological Time Scale Divisions and the Emergence of Life

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