The first two-thirds of Earth's history is called the Precambrian
(meaning "before the Cambrian"), 4500 - 550 million
years ago. It is also known as the Archaeozoic Era. The Earth
was formed about four and a half billion years ago. Only after
the Earth's crust had hardened and the atmosphere and hydrosphere
were formed were the conditons created for life to appear about
three billion years ago. The oldest life forms were single celled
cyanobacteria called stromatolites. The next advance came about
2100 million years ago when the first single cells with a nucleus
present (eucaryotes) appeared.
The evolution of multicelled organisms (metazoans) is poorly
documented in the fossil record. Only two sites in the world
have been discovered that reveal the earliest multicelled fauna.
One in the Flinders Ranges of South Australia at a placed called
Ediacara has revealed a variety of jellyfish, worms, sea-pens
and bizarre enigmatic forms collectively known as Ediacarids.
A second location, in Russia, has produced variety of similar
enigmatic forms including a flat worm called Dickensonia which
is also found at the Australian site. These animals are dated
about 600 million years old. Based on the complexity of their
forms, one can assume they actually appeared long before this