The Precambrian Super-eon
The Precambrian is a collection of eons that extends all the way
back to the formation of the Earth about 4.5bya*. Though the Earth
was formed, it did not cool enough to form lasting, solid rocks until
about 3.8 bya.
The first life we have record of is simple bacteria (prokaryotes)
probably dating back to around 3.8 bya. By 3.2 bya bacteria had
become very successful, growing into tall mats called stromatolites.
These fossilized well and give a good record of their extensive
presence in shallow seas. They produced a lot of oxygen and around
2.5 bya, levels rose ‘suddenly’ (by the standards of geologic time) to
levels similar to today’s atmosphere. This oxygen allowed organisms
to harness energy in new ways, and by about 1.5 bya we see
evidence of the first complex or eukaryotic cells.
The types of living things stayed pretty simple, as far as we can
tell, until about 630 mya** when we see the earliest traces of multicellular life. Some fossils of jellyfish-like animals have been found in
Australia, and Africa, showing that life was diversifying by this time.
With many soft bodied animals in the seas, abundant oxygen, and
increasing geologic stability, the stage was set for a dramatic increase
in the diversity of life.
* bya= billion years ago
** mya = million years ago