Life on EarthAbout 1 to 1.5 billion years after the earth formed, geologists believe, simple living organisms appeared in the oceans. However, more complex forms did not appear in abundance until about 2.5 billion years later—at the beginning of the Cambrian period, only 550 million years ago. Not until the Devonian period, about 350 million years ago, did vegetation become widespread on the land. Land animals were scarce until even later.
Figure 1.9 Two landscape features formed by faults are the graben (A) and the horst (B).
Because life has evolved continuously from Precambrian times, the fossil remains of animals and plants succeed one another in a definite, known order. Geologists have classified rocks in groups based upon this succession, as shown in table 1.1. Geologists have estimated the durations of the eras, periods, and epochs from studies of radioactive minerals. The presence of life may be essential to the petroleum story because, according to the prevailing theory, organic matter is necessary for the formation of oil. Table 1.1
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