without a designer is inconceivable and this objection to evolution has been refined in recent years as the more sophisticated irreducible complexity argument of the intelligent design movement, formulated by Michael Behe. Biochemist Michael Behe has argued that current evolutionary theory cannot account for certain complex structures, particularly in microbiology. On this basis, Behe argues that such structures were "purposely arranged by an intelligent agent". Irreducible complexity is the idea that certain biological systems cannot be broken down into their constituent parts and remain functional, and therefore that they could not have evolved naturally from less complex or complete systems. Whereas past arguments of this nature generally relied on macroscopic organs, Behe's primary examples of irreducible complexity have been cellular and biochemical in nature. He has argued that the components of systems such as the blood clotting cascade, the immune system, and the bacterial flagellum are so complex and interdependent that they could not have evolved from simpler systems. In the years since Behe proposed irreducible complexity, new developments and advances in biology such as an improved understanding of the evolution of flagella, have already undermined these arguments The idea that seemingly irreducibly complex systems cannot evolve has been refuted through evolutionary mechanisms, such as exaptation (the adaptation of organs for entirely new functions) and the use of "scaffolding", which are initially necessary features of a system that later degenerate when they are no longer required. Potential evolutionary pathways have been provided for all of the systems Behe used as examples of irreducible complexity. Cambrian explosion complexity argument The Cambrian explosion was the relatively rapid appearance around of most major animal phyla as demonstrated in the fossil record, and many more phyla now extinct. This was accompanied by major diversification of other organisms. Prior to the Cambrian explosion most organisms were simple, composed of individual cells occasionally organized into colonies. Over the following 70 or 80 million years the rate of diversification accelerated by an order of magnitude and the diversity of life began to resemble that of today, although they did not resemble the species of today. The basic problem with this is that natural selection calls for the slow accumulation of changes, where a new phylum would take longer than a new class which would take longer than a new order, which would take longer than a new family, which would take longer than a new genus would