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INTERPRETING GEOLOGIC thought that these were sudden extinctions. Most took place
HISTORY—A SUMMARY over millions of years. Some of the extinctions aff ected as
much as 90 percent of the things living at the time.
When interpreting the fossil record in any part of the world,
there are several things to keep in mind: 4. New forms of life evolved that replaced those that went
extinct. The earliest organisms we see in the fossil record

1. We are dealing with long periods of time. The history of were marine organisms similar to present-day bacteria. Th e
life on Earth goes back to 3.5 billion years ago, and the oldest fossils of these organisms date to about 3.5 billion
evolution of humans took place over a period of several years ago. Th e first multicellular organisms were present by

million years. This is important because many of the about 1 billion years ago. The development of multicellular

processes of sedimentation, continental drift, and climate organisms ultimately led to the colonization of land by plants

change took place slowly over many millions of years. and animals, with plants colonizing about 500 million years

2. Earth has changed greatly over its history. There have been ago and animals at about 450 million years ago.
repeated periods of warming and cooling, and some 5. Although there were massive extinctions, there are many
cooling periods resulted in the formation of glaciers. Since examples of the descendants of early life-forms present

glaciers tie up water, they had the effect of lowering sea today. Bacteria and many kinds of simple organisms are
level, which in turn exposed more land and changed the extremely common today, as are various kinds of algae
climate of continents. In addition, the continents were not and primitive forms of plants. In the oceans, many kinds
fixed in position. Changes in position affected the climate of marine animals such as starfi sh, jellyfish, and clams are

that the continent experienced. For example, at one time descendants of earlier forms.
what is now North America was attached to Antarctica 6. The kinds of organisms present have changed the nature of

near the South Pole. Earth. The oxygen in the atmosphere is the result of the

3. There have been many periods in the history of Earth when process of photosynthesis. Its presence has altered the

most of the organisms went extinct. Cooling climates, amount of ultraviolet radiation reaching Earth. Plants tend
changes in sea level, and meteorite impacts are all suspected to reduce the erosive effects of running water, and humans

of causing mass extinctions. However, it should not be have significantly changed the surface of Earth.

SUMMARY
A fossil is any evidence of former life. This evidence could be in the form further subdivided into epochs. The fossil record is seen to change during
of actual remains, altered remains, preservation of the shape of an organ- each era, ending with great extinctions of plant and animal life.
ism, or any sign of activity (trace fossils). Actual remains of former organ-
isms are rare, occurring usually from protection of remains by freezing, KEY TERMS
entombing in tree resin, or embalming in tar. Remains of organisms are
sometimes altered by groundwater in the process of mineralization, de- angular unconformity (p. 527)
position of mineral matter in the pore spaces of an object. Replacement of Archean (p. 531)
original materials occurs by dissolution and deposition of mineral matter. Cenozoic (p. 531)
Petrified wood is an example of mineralization and replacement of wood. correlation (p. 528)
Removal of an organism may leave a mold, a void where an organism was eons (p. 531)
buried. A cast is formed if the void becomes filled with mineral matter. epochs (p. 531)
Clues provided by geologic processes are interpreted within a log- eras (p. 531)
ical framework of references to read the story of geologic events from fossil (p. 522)
the rocks. These clues are interpreted within a frame of reference based geologic time scale (p. 530)
on (1) the principle of uniformity, (2) the principle of original horizon- index fossils (p. 528)
tality, (3) the principle of superposition, (4) the principle of crosscutting Mesozoic (p. 531)
relationships, (5) the fact that sites of past erosion and deposition have Paleozoic (p. 531)
shifted over time (shifting sites produce an unconformity, or break in periods (p. 531)
the rock record when erosion removes part of the rocks), and (6) the Phanerozoic (p. 531)
principle of faunal succession. Prearchean (p. 531)
Geologic time is measured through the radioactive decay process, Precambrian (p. 531)
determining the radiometric age of rocks in years. Correlation and the principle of crosscutting relationships (p. 526)
determination of the numerical ages of rocks and events have led to the principle of faunal succession (p. 527)
development of a geologic time scale. The major blocks of time on this principle of original horizontality (p. 526)
calendar are called eras. The eras are the (1) Cenozoic, the time of recent principle of superposition (p. 526)
life, (2) Mesozoic, the time of mid dle life, (3) Paleozoic, the time of ancient principle of uniformity (p. 526)
life, and (4) Precambrian, the time before the time of ancient life. The eras Proterozoic (p. 531)
are divided into smaller blocks of time called periods, and the periods are unconformity (p. 526)

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