Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 242

emergence and evolutionary success of the Cambrian fauna stand in stark contrast to
the preceding Ediacaran biota. The sudden upsurge in the diversification of animal
forms and the rapidly increasing complexity of their structure is still waiting for a
scientific explanation.
Darwin was also puzzled:
To the question why we do not find rich fossiliferous deposits belonging to these
assumed earliest periods prior to the Cambrian system, I can give no satisfactory
answer.
Darwin (1859)
To solve the enigma, Darwin assumed that the sudden appearance of animal
diversity and the absence of transitional forms in the Cambrian fossil record might
be related to the incompleteness of the record. However, the discovery of the
Ediacaran biota of the Pre-Cambrian eon failed to provide any transitional forms that
would connect them to animal diversification and the appearance of about 40 body
plans that evolved within the petite window of the Cambrian:
the appearance of the remains and traces of bilaterian animals in the Cambrian
remains abrupt.
Erwin et al. (2011)
Attempts are made to explain the scarcity of Ediacaran fossils lacking skeletons
or exoskeletons of Ediacaran forms. This is rejected by the relative abundance of fos-
sils of soft-bodied Cambrian cnidarians, ctenophores, echinoderms, and so on.
Even a glimpse at the Cambrian explosion and the blossoming evolution of the
animal kingdom to the present leads to two important observations. First, the sud-
den emergence of all animal body plans at the phylum level, immediately after the
extinction of the Ediacaran fauna, unequivocally implies the absence of transitional
forms. Second, that during the Post-Cambrian evolution, no new phyla evolved even
as the rate of evolution of new species and higher taxa was relatively rapid, implying
that diversification of species and other taxa did not follow a “bottom-up” pattern as
would be conventionally expected.
There is no question that since it emerged about 3.8 billion years ago, life evolved
significantly. Paleontology and other biological disciplines provide a rational out-
line of that evolution until the Cambrian, vindicating the gradualist view of evolu-
tion. The troubling question in the current debate on the Cambrian explosion is what
caused the unprecedented and unrivalled burst of diversification of animal forms in
only about 10 million years. This is a mandatory question.
The emergence of about 40 phyla, the highest rank of animal groups in the
Linnaean system, within the narrow Cambrian time window, is also perplexing for
the neoDarwinian prediction that higher taxa result from the evolution of lower ones
(species→genus→family→order→class→phylum). The Cambrian fossil evidence
clearly shows that the direction of evolution is the other way around: the Bauplans
of the phyla preceded those of lower taxonomic ranks (i.e., classes, families, orders,