For most of the course of evolution, life has existed as some form of bacteria and nothing else. The first signs of life were as early as 3.8 billion years ago and little changed until the Cambrian explosion 542 million years ago. That is a span of 3308 million years prior to any signs of complex multicellular life! Somewhere within the Cambrian, life exploded into all the major animal phyla within a geologic span lasting no longer than 54 million years and perhaps even less!
This means the evolutionary rate suddenly increased to a minimum of over 61 times what it had been during the Precambrian. There is no adequate way to explain this in terms of simple, incremental mutations, because such mutations had been going on for over 3308 million years prior to the Cambrian. Something else had to have taken place to generate such a rapid shift in the evolutionary rate.
This blog suggests that what fueled that increased rate was the phenomenon of endosymbiosis, whereby large cells incorporate smaller cells and utilize their DNA to upgrade their own. Evolution is a race for survival, and the cells or animals with the most adaptable genomes will usually come up as the winners (barring unforeseen circumstances such as an asteroid attack).
There are two kinds of endosymbiosis: serial endosymbiosis and hierarchical endosymbiosis. One follows an arithmetic progression and the other is geometric. Cellular evolution that follows a geometric progression will blow the other kind out of the water every time. To better understand this statement, please click on the following thumbnail: