The Cambrian period represents a boundary line in the emergence of complex life on earth. Creatures that had remarkably good vision and mobility were by then moving throughout the seas of the planet. Predator and prey had clearly emerged with the associated developments of armoured limbs and protective shells.
The numerous and varied creatures that have been unearthed have generated much debate among palaeontologists. How and why the Cambrian biota emerged in such diversity is considered pivotal in understanding how the mechanics of life work. Many of the creatures that arose during this period seem to have no clear ancestry. Some Cambrian lifeforms became extinct whilst others went on to form the basis of all modern life. Notably the first representations of modern animal phyla are to be found in the Cambrian deposits.
Because of the wide variety of creatures that suddenly appeared during this period it is often referred to as the Cambrian Explosion. Other such periods of evolutionary expansion - or radiation - have now been recognised, most notably after the great Permian Extinction (preceding the rise of the dinosaur) and after the K-T impact that ended the Cretaceous period (facilitating the rise of mammals). Whereas evolution was originally thought to be a continuous, gradual process it is now thought to ebb and flow. The fossil record shows that long periods of relative stasis have been punctuated by periods of rapid evolution. There seems to be a clear connection between environmental disruption and surges in biodiversity, especially if large extinctions take place. Vacant habitat draws evolution onwards, prompting a race in adaptation to fill in the environmental gaps.
Two ideas now underpin an explanation for the sudden appearance of multiple complex lifeforms during the Cambrian period. Firstly the apparent lack of fossil evidence for complex life before this period (now resolved somewhat by the Ediacaran discoveries) could be explained by the lack of any hardened shells - so no traces were left to be found. Secondly, just before the Cambrian period, a global catastrophe may have occurred, effectively wiping out most of the life on earth, leaving behind an environment that resembled a blank canvas.
There are a number of Cambrian fossil deposits currently under investigation around the world such as the Chengjiang Biota Shale in China and the Emu Bay Shale in South Australia. Work also continues at the most famous Cambrian fossil site: the Burgess Shale in Canada. New discoveries continue to emerge from each of these sites generating ongoing debate about the Cambrian period and the mechanics of evolution.