1. Precambrian Era (earliest forms of life, such as bacteria and blue-green algae; later in the period, invertebrates such as jellyfish).
2. Paleozoic Era (Pangaea; invertebrate life, such as trilobites, later in the era, followed by development of vertebrates, including fish; development of vertebrates, amphibians, and the beginnings of reptiles; development of simple plants, such as mosses and ferns)
3. Mesozoic Era (Pangaea separates into continents; "Age of Reptiles"; dinosaurs, flowering plants, small mammals and birds)
4. Cenozoic (Present Era) Ice Age; mammoths; gradual development of mammals, birds and other animals recognizable today; humans; flowering plants, forests, grasslands.)
Edit:
Actually the above answer can be disputed. The Precambrian is not considered an era, it is considered a supereon that is divided into three eons. Two of these eons are subdivided into a total of seven eras. Combine that with the three eras of the Phanerozoic Eon, and you have ten.
So this question doesn't really make sense, considering there are actually ten eras.
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