The Proterozoic eon is a time in the Earth's history that
occurred from 2.5 billion to 543 million years ago. Many
important events in the geologic history of the Earth occurred
during this eon. For example, stable continental masses first
appear and began to significantly grow in size through the
accretion of sedimentary deposits. Also occurring during this
time is the fossilization of our planet's earliest organisms,
mainly bacteria and archaeans. By about 1.8 billion years ago,
eukaryotic organisms begin to appear in the fossil record as
well. Finally, multicellular eukaryotic life-forms become common
near the end of the Proterozoic. During the middle of
Proterozoic oxygen concentration in the atmosphere increases
significantly. This global change reduced the abundance of many
anerobic bacterial groups, but made possible for the domination
of aerobic eukaryotic life-forms on our planet. These include
multicellular algae, and toward the end of the Proterozoic, the
first animals. Geology of the Proterozoic T