Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 238

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Rise of the Animal Kingdom and
Epigenetic Mechanisms of Evolution

The Prelude to the Cambrian Burst of Animal Diversity

The mechanisms and driving forces of evolution are central to biological investiga-
tion, but a full discussion of these topics is out of the scope of this chapter. Instead,
I intend to focus on the major Cambrian diversification in the animal kingdom,
where effects of these mechanisms and forces manifest the best.
It is estimated that life emerged on Earth about 3.8 billion years ( Holland, 1997 )
(i.e., about 1 billion years after the Earth formed as a planet of the solar system).
The discovery of fossil stromatolites, colonies of extant cyanobacteria, indicates
that life started with prokaryote unicellular microorganisms. Transition to eukary-
otic unicellular algae occurred about 1.8-1.7 Mya ( Bengtson et al., 2009; Conway
Morris, 2003 ). Metazoans appeared not earlier than 650 Mya ( Conway Morris, 2003 )
and complex animal multicellulars appeared only about 635-543 Mya ( Knoll et al.,
2004 ).
Ignoring unconfirmed reports, Valentine et al. (1999) believe the earliest indica-
tions of metazoan forms are represented by a number of small disks and fossil creep-
ing traces from Canada about 610-590 Mya, which was immediately followed by
the sparely diversified Ediacaran fauna (named after a fossil Lagerstätte in Ediacara
Hills, southern Australia, discovered in 1946 by Reginald Sprigg) that lasted from
about 580 to 542 Mya. These soft-bodied animals are preserved as impressions in
rapid sediment depositions or volcanic ash ( Narbonne, 2005 ) ( Figure 5.1 ). The
Ediacaran period was dominated by unicellular organisms but, by the end of the
period, multicellular forms became more frequent. The Ediacaran forms have been
mostly sessile diploblastic-grade organisms of various forms, including bags, tubes,
and leaves, ranging from microscopic to a few millimeters/centimeters to 1 m in
length. Their position in the animal kingdom is still uncertain.
Including the Ediacaran organisms in the animal kingdom is questionable; some
paleontologists consider them related to bilaterian animals, some to prebilaterian ani-
mals, and others to plants, fungi, or even prokaryotes ( Erwin, 2009 ). Peterson et al.
(2003) believe Ediacaran forms may belong to quite different groups or kingdoms
( Peterson et al., 2003 ). These organisms lacked basic animal organs, such as mouth,
digestive, and other internal organs, eyes, and appendages (hence they could only
move by peristaltic movements). Most investigators believe they were membra-
nous organisms of two or more unicellular layers. In Seilacher's vivid expression,
Ediacaran forms are “air-mattress-like constructions”, mostly of discoidal morphol-
ogy and of considerable size.