Life appeared on earth somewhere between three and four billion years ago. While the origin of life is generally recognized as an event of the first order, there is another event in the history of life that is less well known but of comparable significance. The origin of biological diversity and at the same time of complex macroscopic multi-cellular life, occurred abruptly in the Cambrian explosion 600 million years ago. This event involved a riotous diversification of life forms. Dozens of phyla appeared suddenly, many existing only fleetingly, as diverse and sometimes bizarre ways of life were explored in a relative ecological void [28,65].
The Cambrian explosion was a time of phenomenal and spontaneous increase in the complexity of living systems. It was the process initiated at this time that led to the evolution of immune systems, nervous systems, physiological systems, developmental systems, complex morphology, and complex ecosystems. To understand the Cambrian explosion is to understand the evolution of complexity. If the history of organic life can be used as a guide, the transition from single celled to multi-celled organisms should be critical in achieving a rich diversity and complexity of synthetic life forms.