The first living organisms to appear on earth are thought to have been anaerobic unicellular organisms, who used marine organic substances without using oxygen. Later, photosynthetic bacteria and cyanobacteria appeared in the ocean. These bacteria were able to synthesize organic substances using carbon dioxide, which causes oxygen to slowly increase in the atmosphere. Organisms later became multicellular, and eukaryotes emerged. Sponges and green algae later emerged at the end of the Precambrian age.
During the Paleozoic era, amphibians and fishes appeared and flourished in water, and ferns flourished on land. In the Mesozoic era, reptiles flourished, and gymnospermous plants dominated the ecosystem. The Cenozoic era later began when large reptiles gradually became extinct after the earth was struck by a meteorite.
Reference:
http://imnh.isu.edu/exhibits/online/geo_time/geo_time_periods.htm
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