T
his chapter examines the earliest history of Earth, its evolving life-
forms, and the formation of continents. The first 4 billion years, or
about nine-tenths of geologic time (Table 2), are referred to as the
Precambrian era, the longest and least understood period of Earth history.The
Precambrian began with only simple creatures living in the sea. It ended with
an explosion of new, highly specialized species, which set the stage for more
modern life-forms to follow (Fig. 15)
The Precambrian is divided nearly equally into the Archean and Pro-
terozoic eons. The boundary is somewhat arbitrary and reflects major differ-
ences between the types of rocks formed during the two periods. Archean
rocks are products of rapid crustal formation. Proterozoic rocks are more rep-
resentative of relatively stable modern geology.
The Archean, from 4.6 to 2.5 billion years ago, covers a time when Earth
was in a great turmoil and subjected to extensive volcanism and intense mete-
orite bombardment.The high internal heat of the newborn planet kept the
surface well agitated, destroying any semblance of a crust.This is why the first
several hundred million years are absent from the geologic record. During this
ARCHEAN ALGAE
THE AGE OF EARLY LIFE