There is evidence of life before that, but it was mostly algae and bateria.
The explanation for this is that when the atmosphere was first formed...while all this primordial ooze was going on, it was primarily made up of carbon-dioxide. Therefore the first organisms were those that converted or "breathed" CO2. They were around for tens of millions of years taking in carbon dioxide and converting in into a little by-product called oxygen. After a while, there was enough oxygen in the atmosphere, and enough O3 (ozone) to cool the planet a few degrees and provide for an environment that could support oxygen "breathing" organisms. Oxygen breathing organisms tend to be large, more sophisticated creatures and this is what brought on the Cambrian Explosion (primarly). There are other theories about genetic evolution during this period creating the proper road-map for larger organisms. The explosion actually lasted about 30 million years and it was during this time that the larger animals that began the lineage to todays oxygen breath
ing species developed.
