By combining information gained from studies of the relative ages of rocks exposed all around the world with absolute ages obtained for a subset of those rocks, geologists have created the geologic time scale. The geologic time scale divides the earth's 4.56 billion years of history into smaller time intervals, each characterized by a unique assortment of life forms. A geologic time scale is shown below.
From about 3.8 billion years ago, when the first life forms appeared, until about 1 billion years ago, only simple organisms, such as bacteria existed. Because such organisms lack shells or skeletons, they do not fossilize very well. As a result, our understanding of early life on earth is limited. However, after about 543 million years ago, organisms with hard parts evolved. Because these life forms are much more likely to leave behind records of their existence in the form of fossils, our understanding of the evolution of life on earth is much more complete from this time forward. This time interval, from about 543 million years ago to the present, is known as the Phanerozoic (which means "visible life") Eon. The Phanerozoic Eon is shown by the vertical blue bar on the leftmost side of the middle column in the graphic above. It is during the Phanerozoic Eon that we observe the progressive evolution of life
It is worthwhile to note that the divisions of the geologic time scale mark extinction events of various magnitudes; the larger divisions mark major extinction events, whereas the smaller ones mark lesser events.