Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 240

Ma

490

500

510

Upper
Middle

Burgess shale faunas

520

Sirius passet fauna
Chengjiang fauna
First trilobites
Appearance of small shelly fossils

530

540

550

Expansion of trace fossils

560

Ediacaran fauna

570

580

590

Varangian glaciation
(Worldwide)

Figure 5.1 The location of the Cambrian period in geological time with some important
fossil occurrences and events.
Source : Levinton (2001) .

incompleteness of the fossil record but instead was a sudden event that took place
more than half a billion years ago ( Levinton, 2001, pp. 46-48 ). For gradualist edu-
cated biologists, the exceptionally rapid tempo of evolution during the Cambrian
represents an unresolved puzzle that since the early 1970s has been called the
Cambrian explosion .

Is the Cambrian Explosion a Fact or a Paleontological
Artifact?

No consensus exists on whether the Cambrian explosion is a sudden evolutionary
or Simpsonian event. Some authors believe that the divergence of metazoan clades
occurred much earlier in the Earth's history; hence, the Cambrian explosion may be
a normal, gradual Darwinian process of evolution ( Ayala et al., 1998; Fortey et al.,
1996; Lieberman, 2003 ). But there are two serious arguments against these conjec-
tural conclusions. Firstly, their estimations are based on molecular clock estimates
alone; that is on base substitutions that have taken place in particular genes, which