University of Bristol


The appearance of such a large range of body plans and life strategies at the base of the Cambrian in an apparently short space of geological time has intrigued palaeontologists for many years. There is still a great deal of speculation as to what caused or triggered the metazoan 'explosion', and why it happened when it did after 3 billion years of potential evolutionary time. It seems obvious that something must have changed or reached a critical level favorable for the building of large, complex bodies and the construction of hard skeletal material. The theories of what such evolutionary triggers may have been can be split into extrinsic or external environment factors, and intrinsic or internal biotic factors.

Cambrian Seas

A Cambrian scene. Why did all these forms of life appear in such a short space of time? From The Crucible of Creation. Conway Morris S. 1998, plate 4.

Many of these theories are linked together and inter-dependent. It could be that a change in the Precambrian environment initiated the radiation event seen at the base of the Cambrian, but for this to happen there must have been an internal response within the physiology of the Precambrian organisms.

With the Precambrian fossil record providing more and more evidence for a long history of metazoan life before the start of the Palaeozoic (see recent discoveries), and cladistic analysis indicating metazoan evolution extending back into the Precambrian (see references Fortey et al. 1997), it is possible that there was no real 'explosion', and therefore no requirement for a special evolutionary mechanism or environmental pressure operating at this time at all.

timing triggers fossils controversies

This report was written by Abby Lane and was last updated on 20th January 1999