Uniqueness and Timing The "Cambrian Explosion" refers to the sudden appearance in the fossil record of complex animals with mineralized skeletal remains. It may represent the most important evolutionary event in the history of life on Earth. The beginning of the explosion is generally placed about 542 million years ago, during the Cambrian Period at the start of the Palaeozoic Era (the same time the Ediacarans disappear from the fossil record). While the explosion was rapid in geological terms, it took place over millions of years - the Burgess Shale, at 505 million years old, records the tail end of the event. The explosion is particularly remarkable because all major animal body plans (each more or less corresponding to a distinctive Phylum - Mollusca and Chordata, for example) appeared during this time, changing the biosphere forever. The origin and diversification of animals during the Cambrian Explosion. Dotted lines represent the probable range of particular groups of animals. Solid lines represent fossil evidence. Extinct groups are represented with a circled-cross. Cones represent the approximate origin and diversification of the modern phyla (the crown groups). The basic body plan of major groups of animals (today's phyla) had already evolved by the time of the Burgess Shale. (modified after Xiao and Laflamme, Peterson et al and Dunn et al.).

The Cambrian Explosion and the Origin of Modern Marine Ecosystems The rapid appearance of a wide variety of animals - particularly bilaterians - led to the development of radical new ecological interactions such as predation. Consequently, ecosystems became much more complex than those of the Ediacaran. As the number and variety of organisms increased, they occupied a variety of new marine environments and habitats. Cambrian seas teemed with animals of various sizes, shapes, and ecologies; some lived on or in the sea floor (a benthic lifestyle), while others actively swam in the water column (nektonic). The fundamental ecological structure of modern marine communities was firmly established during the Cambrian. By the end of the Period, some animals had also made the first temporary forays onto land, soon to be followed by plants. Back to top

Explaining the Explosion Why did the Cambrian explosion happen when it did, and why was it such a unique event? While there is no current consensus among scientists, most researchers agree the explosion cannot be ascribed to a single, simple causal mechanism. The potential triggers can be classified in three main categories: environmental, genetic, and ecological. Deciphering the impact of each of these factors remains one of the most important challenges faced by palaeontologists today. Back to top

Was there an Explosion at all? Very few organisms ever enter the fossil record; after death, their remains are usually completely destroyed and recycled. Animals with hard body coverings, such as trilobites, are much more likely to be preserved than those with only soft body parts. So the evolutionary development of mineralized shelly parts by different groups would be marked in the rock record by a sudden jump in fossil numbers. Thus, preservation bias alone could create the appearance of an "explosion" of new life forms at the beginning of the Cambrian. When he published On the Origin of Species in 1859, Charles Darwin puzzled over the apparently sudden appearance of complicated organisms in the fossil record at the beginning of the Cambrian Period. He noted this could be used as an argument against his controversial new theory, which predicted a more gradual appearance of simpler organisms. At the time, Darwin pointed to the imperfection of the fossil record as his only defence, arguing complex animal life must have lived long before the Cambrian, but traces of that life had not yet been found. The presence of large, soft-bodied, putative animals (problematic as they may be) in Ediacaran seas does indeed make the "explosion" appear less abrupt. But the fact remains that the Early Cambrian was a time of major change in marine animal communities and environments, with the rapid and unprecedented advent of disparate new body plans and novel ecological niches. By the end of the period, every major animal phylum was firmly established, and life after the Cambrian was radically different from what had gone before. So it is safe to call this event an "explosion" - it was crucial to the evolution of life on Earth as we know it. Back to top