Most major animal groups appear for the first time in the fossil record some 545 million years ago in a relatively short period of time known as the Cambrian explosion. Of great worry to Darwin, the explanation of this sudden, apparent explosion persists today as a shadowy mystery. While some scientists believe there was indeed an explosion of diversity (the so-called punctuated equilibrium theory elaborated by the late Stephen J Gould - Models In Paleobiology, 1972), others believe that such rapid acceleration of evolution is not possible; they posit that there was an extended period of evolutionary progression of all the animal groups, the evidence for which is lost in scanty fossil record.
The theory of the Cambrian Explosion holds that, beginning some 545 million years ago, an explosion of diversity led to the appearance over a relatively short period of 5 million to 10 million years of a huge number of complex, multi-celled organisms. Moreover, this burst of animal forms led