<P> The explosion may not have been a significant evolutionary event . It may represent a threshold being crossed: for example a threshold in genetic complexity that allowed a vast range of morphological forms to be employed . This genetic threshold may have a correlation to the amount of oxygen available to organisms . Using oxygen for metabolism produces much more energy than anaerobic processes . Organisms that use more oxygen have the opportunity to produce more complex proteins, providing a template for further evolution . These proteins translate into larger, more complex structures that allow organisms better to adapt to their environments . With the help of oxygen, genes that code for these proteins could contribute to the expression of complex traits more efficiently . Access to a wider range of structures and functions would allow organisms to evolve in different directions, increasing the number of niches that could be inhabited . Furthermore, organisms had the opportunity to become more specialized in their own niches . </P> <P> The "Cambrian explosion" can be viewed as two waves of metazoan expansion into empty niches: first, a coevolutionary rise in diversity as animals explored niches on the Ediacaran sea floor, followed by a second expansion in the early Cambrian as they became established in the water column . The rate of diversification seen in the Cambrian phase of the explosion is unparalleled among marine animals: it affected all metazoan clades of which Cambrian fossils have been found . Later radiations, such as those of fish in the Silurian and Devonian periods, involved fewer taxa, mainly with very similar body plans . Although the recovery from the Permian - Triassic extinction started with about as few animal species as the Cambrian explosion, the recovery produced far fewer significantly new types of animals . </P> <P> Whatever triggered the early Cambrian diversification opened up an exceptionally wide range of previously unavailable ecological niches . When these were all occupied, limited space existed for such wide - ranging diversifications to occur again, because strong competition existed in all niches and incumbents usually had the advantage . If a wide range of empty niches had continued, clades would be able to continue diversifying and become disparate enough for us to recognise them as different phyla; when niches are filled, lineages will continue to resemble one another long after they diverge, as limited opportunity exists for them to change their life - styles and forms . </P> <P> There were two similar explosions in the evolution of land plants: after a cryptic history beginning about 450 million years ago, land plants underwent a uniquely rapid adaptive radiation during the Devonian period, about 400 million years ago . Furthermore, Angiosperms (flowering plants) originated and rapidly diversified during the Cretaceous period . </P>

Which of the following is used to mark the beginning of the cambrian period