T
he most fascinating field of geology is the study of our planet’s past.
The history of Earth is written in its rocks, and the history of life is
told by its fossils.Earth history is divided into units of geologic time
according to the type and abundance of fossils in the strata. The fossil record
provides valuable insights into the evolution of Earth. Knowledge of the orig-
ination and extinction of species throughout geologic time is also necessary
for understanding the evolution of life.
When fossils are arranged chronologically, they vary in a systematic way
according to their positions in the geologic column. This observation led to
one of the most important and basic principles of historical geology, whereby
periods of geologic time could be identified by their distinctive fossil content.
Geologists were thus able to recognize geologic time periods based on groups
of organisms that were especially abundant and characteristic during a partic-
ular interval. The occurrence of certain organisms defined each period. The
succession of species was the same on every continent and was never out of
order. These principles became the basis for establishing the geologic time
scale and ushered in the birth of moderngeology.
The text chronicles the formation of Earth and its evolving life-forms,
beginning with the earliest history of the planet during the Archean eon. It
follows the evolution of the more complex life-forms of the Proterozoic eon.
It then examines the invertebrate life-forms of the lower Paleozoic era, the
early vertebrate life of the Ordovician period, and the plant life of the Silurian
period. Next, it focuses on life in the sea and the first vertebrates to come onto
XIINTRODUCTION