|
|
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 time. |
 |
|
