Development of life

What has to be said, is that simple models of the evolution of life and its development do not apply. Webs and chains in the past are so intricate and full of random and chaotic events. All scientists have to make assumptions especially on this matter.
As stated before, Earth formed approximately 4.6 billion years ago. It had a to endure heavy bombardments of cosmicdebris. These heavy bombarments ended about 3.8 billion years ago (Kasting, 1993). We know this because the oldest rocks date from that period. The large impacts before this date melted the Earth's surface so no solid rock could exist. Still the oldest rocks to hold cellular fossils date from approximately 3.5 billion years ago(Gould, 1994). Which means that life on Earth evolved quickly and is really old. Those first fossils were of bacteria. Bacteria represent more or less the simplest forms of life, so the only way to expand was in width and height. Or to put it differently, to expand in diversity and in complexity.
After the bacterial cells the cells belonging to the plant- and animal kingdom began to to evolve around 2 billion years ago. Yet, life remained unicellular for the first five sixths of its history. Some of the multicellular algae evolved a bilion yars ago. But no record can be found of multicellular animal organization for the span of 3 bilion years. Even more surprisingly, all major stages organizing animal life's multicellular design occured in a very small timespan. It began less than 600 milion years ago and lasted until 530 million years ago, but the steps are not gradually, they're discontinous. Though it actually took only five million years of intense creativity to develop, called the Cambran explosion,followed by 500 million years of variation.It is not known how nature came up with these anatomical designs so quickly. But this first period of both internal and external flexability gave a range of invertabrate anatomies that may have outnumbered the full range of animal form in all Earth's enviroments today.
The question is why did most of these early experiments die out, while others survived. It's more by luck than by a predictable struggle for existance that those organisms we know survived. For mass extinction mark the boundaries of divisions of geologic-timescale. It is thought that these extinctions were mainly caused by impacts of large extraterrestrial objects which smashed into the Earth (the last of these, about 65 million years ago, is thought to have wiped out the dinosaures). Mass extinctions are not randomly distributed in their inpact on life. Some descendants die and others survive as a practical outcome on presence or absence of evolved characteristics. But if the triggering cause of the extinction is sudden catastrophic, reasons for death or life may lie very close to eachother, they may be random. (Gould, 1994).