The Cambrian Explosion

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. 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 plansĀ  appeared during this time, changing the biosphere forever. The rapid appearance of a wide variety of animals led to the development of radical new ecological interactions such as predation. Consequently, ecosystems became much more complex. 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. 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. –