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The second of the great eras of life on earth, the Paleozoic witnessed the evolution of the first complex, multicellular life forms, ranging from the bizarre marine arthropods of the Cambrian Explosion to the "mammal-like reptiles" of the Permian period. The Paleozoic Era was succeeded by the Mesozoic Era (250-65 million years ago), also known as the "Age of Dinosaurs," and the Cenozoic Era (65 million years ago to the present), also known as the "Age of Mammals." (The first great era of life on earth was the Proterozoic, stretching from 2.5 billion to 542 million years ago, which might well be described as the "Age of Bacteria.")

Here's an overview of the subdivisions of the Paleozoic Era; just click on the appropriate link to read more about prehistoric life on earth during that particular period.

The Cambrian period: (542-488 million years ago) is most famous for the "Cambrian Explosion," a sudden diversification of undersea life that yielded some truly bizarre arthropods and megafauna denizens of the deep. The end of the Cambrian witnessed the evolution of the very first vertebrate animals.

The Ordovician period: (488-443 million years ago) saw an increasing diversification of undersea life, including tiny bryozoans, brachiopods, bivalves and trilobites. The first jawless fish swam the oceans, looking for all the world like giant, armored tadpoles.

The Silurian period: (443-416 million years ago) was the shortest period of the Paleozoic Era. This is when the very first terrestrial plants and invertebrates (i.e., bugs) appeared, while the most important development in vertebrate life was the evolution of jawed fish.

The Devonian period :(416-360 million years ago) is particularly important from a human standpoint; this is when the first tetrapods, descendants of lobe-finned fish, climbed out of the water and learned to breathe on dry land. Fish were especially diverse at this time, which also saw the evolution of the first sharks.

The Carboniferous period: (360-300 million years ago) is famous for its dense "coal swamps" that eventually turned into modern coal deposits. It was during the Carboniferous that the first true amphibians appeared, evolving by the start of the ensuing Permian period into the first reptiles and lizards.

The Permian period (300-250 million years ago) was most important for its terrestrial life, as reptiles branched off into therapsids ("mammal-like reptiles") and archosaurs ("ruling lizards"), the immediate predecessors of the dinosaurs that appeared in the ensuing Triassic period. The end of the Permian also marked the worst mass extinction in the earth's history.