The Paleozoic era (the era of "ancient life") is the first era of the Phanerozoic Eon, or period of visible multicellular life, which followed the Precambrian time in Earth history. The Paleozoic era began with a massive diversification of species and ended with the Earth's greatest extinction event. The Paleozoic era consists of six geological periods (Cambrian period; Ordovician period; Silurian period; Devonian period; Carboniferous period; Permian period).

During the middle of the Paleozoic, a great deal of continental landmass was centered in the Southern Hemisphere. By the end of the Paleozoic, all the continents coalesced into a single continent, Pangaea. Climatic conditions were cold and dry in the middle and along the southern edge of Pangaea during the Permian period.

During the first two periods, almost all life-forms were aquatic. Probably every animal phylum (including vertebrates in the Chordata) existed by the end of the Cambrian period, as well as seaweeds. Diversity of marine species was even greater during the Ordovician. A global mass extinction occurred at the end of the Ordovician. Evolutionary diversification continued in the oceans during the Silurian period. The first small land plants proliferated in wetlands during the Silurian. By the Devonian period, forests that consisted mostly of relatives of modern club mosses and horsetails covered the extensive wetlands, and fishes diversified in the fresh waters and oceans. Amphibians evolved and terrestrial arthropods such as insects diversified during the Devonian. During the Carboniferous and Permian periods, forests dominated by the first seed plants spread beyond the wetlands, and reptiles diversified. There were no birds or mammals.

The Paleozoic era came to an end when massive volcanic eruptions were followed by an asteroid that collided with the Earth, creating temperature extremes and acid rain that led to worldwide extinctions.

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