The Precambrian period
covers the geological time before the
beginning of the Cambrian period
some 570 to 600 million years BP. Most
major animal phyla appear in the fossil
record within a few million years at the
beginning of the Cambrian period.
This appearance has been called the
“Cambrian explosion” because before
this time, fossil deposits are mostly
devoid of any organisms more complex
than single-celled bacteria. Comparative
now suggest that the rarity of Precambrian
fossils may represent poor fossilization
rather than absence of animal
diversity from the Precambrian period.
Nonetheless, animals make a relatively
late appearance in the history of life on
earth. What were the early forms of life
that generated both the oxidizing
atmosphere critical for animal evolution
and the evolutionary lineage from
which animals would arise?