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Geologic time explains how organisms have changed through time. Most
living organisms are from the Phanerozoic Eon, which is
divided into three eras: the Paleozoic, Mesozoic,
and Cenozoic. Each era is characterized by different types of
fossils. The diagram shows a simplified look at the organisms of each
era.
In the Paleozoic (meaning "old life"), the first bony fish,
amphibians, and reptiles appear. Mammal-like reptiles, are first found
in the fossil record toward the end of this era. Land plants appear in
the early part of the Paleozoic. The first plants were simple stick-like
reeds. By the end of the Paleozoic, ferns and pine-like trees and cycads
were the dominant types of plants.
The Mesozoic refers to "middle" life and records
abundant land organisms. Dinosaurs became abundant during the Mesozoic
Era. True mammals and birds also appeared during this time period, as do
the first flowering plants. In the Cenozoic Era, mammals became
dominant, and grasses evolved. The earliest human ancestors first
occurred about four million years ago.
Organisms through time
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