T
his chapter examines the more complex life-forms and the develop-
ment of the continents during the Proterozoic eon from 2.5 billion to
545 million years ago. Major differences exist in the character of biol-
ogy and geology of the Proterozoic compared with that of the Archean.The
Proterozoic featured a shift to much calmer conditions as Earth progressed
from a tumultuous adolescence to a stable adulthood. Marine life was distinct
from species of the Archean.It represented a considerable advancement in
ev olution,with the development of complex organisms.
The global climate was cooler. Earth experienced its first major ice age
more than 2 billion years ago along with a major extinction that eliminated
many primitive species attempting to evolve during this time. The Proterozoic
ended following a second period of glaciation and another mass extinction.
Afterward, an explosion of species, representing nearly every major group of
marine organisms, set the stage for the evolution of more modern forms of life.
THE AGE OF WORMS
Life in the Proterozoic was more advanced and complex than in the Archean.
Organisms evolved very little during their first billion years on Earth because
PROTEROZOIC
METAZOANS
THE AGE OF COMPLEX ORGANISMS