The Precambrian-Cambrian fossil record has been the basis for palaeontologist's ideas about
metazoan origins for a long time, before genetics, molecular clocks and cladistic methods added to
the picture. Indeed it was the lack of fossil evidence from the Precambrian which Darwin saw as the
biggest stumbling block to his theory of gradualistic evolution through natural selection. He envisaged
a Precambrian swarming with life as yet undiscovered. The 'explosion' scenario of course has no prerequisite
for such a well stocked Precambrian, and in fact reads the origin of metazoan life directly from
the fossil record as it stands.
There are several important groups of fossils contributing to the Cambrian story, three of them are
described here.
Lines of evidence other than the fossil record have more recently been used to change the
timing of the metazoan radiation, and question the existence of the explosion at all (see timing of metazoan origins). A clade divergence
time of up to 1000 million years ago has been suggested despite the lack of fossil evidence (see
references Wray et al.1997).
