Chapter 10 – Life’s Big Bang: Origins of Multicellular Animals
10.1 Engima of Multicellular Organisms
 Precambrian was dominated by bacteria (prokaryotes) and protists (unicellular
eukaryotes)
o Eukaryotes arose no later than 2 billion years ago (perhaps earlier)
o Organic films representing seaweeds appeared 1.5-1.0 billion years ago
o Multicellular animals (metazoans) did not appear for another 0.5-1.0
billion years
o Fossil hard parts began to be preserved in the Phanerozoic
 Many questions remain about the appearance of metazoans
10.2 Stages of Life’s Big Bang
 Ediacara Fauna
o Discovered in the Pound Quartzite, found in the Ediacara Hills of South
Australia
o Initially thought to be Cambrian
o Later found worldwide, thus representing a global fauna
o The biological affinity of many Ediacarans is unclear
 Trace Fossils and Early Hard Parts
o Burrows, tracks and trails of metazoans (ichnofossils) started to appear in
the fossil record about the time of the Ediacara fauna
o The earliest tracks and trails are thought to have been made by a
bilaterally symmetrical animal (probably a worm of some kind)
o Formed near the surface of the sediment, suggesting that burrowers could
not burrow deeply into sediment
o The small shelly fossils also appear in the Cambrian; may represent
mineralized parts of animals rather than the shells of tiny animals
o Some small shelly fossils may be ancestors of modern chiton (mollusks)
amd sponges
 Burgess Shale and the “Cambrian Explosion”
o The Burgess Shale is of Middle Cambrian age; it is a lagerstatte in which
soft-bodies animals are preserved by carbon films
o Some members of the Burgess Shale fauna appear to be combinations of
animals considered as separate groups later in the Phanerozoic
o The phylum Chordata (which would later evolve to produce the
vertebrates) occurs in the Burgess Shale and in slightly older deposits
10.3 What Do These Faunas Tell Us?
 Fossil assemblages such as the Ediacara and Burgess shale faunas help geologists
evaluate the completeness of the fossil record
 Also helps establish the timing for the appearance of animal phyla (distinctive
body plans)
 Study of these faunas has helped paleontologists determine how much evolution
and experimentation occurred during the late Proterozoic and the early Phanerozoic in
response to fluctuating environments
10.4 Why Did Metazoans Appear?
 Snowball Earths
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o Hydrothermal vents may have provided refugia for creatures during
Snowball Earths
o Survivors would disperse and evolve when favorable conditions returned
 Oxygen
o Rising atmospheric and oceanic oxygen levels may have allowed
metazoans to evolve and grow larger
o The appearance of hard parts (skeletons) in animals and an increase in the
complexity of burrowing may reflect rising oxygen demand
 Predation
o Predation is another possible cause of the appearance and diversification
of metazoans
o Several additional lines of evidence point to an increase of predation in the
Cambrian, which may have stimulated some groups to develop skeletons
o Animals without skeletons would have to escape predators through
behavioral responses (such as swimming or burrowing)
 Food
o The increasing complexity of burrows, increase in body size, formation of
skeletons, and predation may all be related to increasing amounts of food
available to animals
o Phosphorites from Proterozoic continental shelf deposits suggests that
increasing amounts of phosphorous (a marine nutrient) were available
o The net effect of increasing nutrient availability would be to stimulate
marine photosynthesis on the continental shelves
 Changes in Biogeochemical Cycles
o In effect, metazoans may have diversified because of changes in Earth’s
biogeochemical systems
o Positive feedback may have occurred between evolution and food
availability
 Ecological and Genetic Mechanisms
o According to the ecologic hypothesis, primitive animals radiated into
ecologic space made suitable by changing physical environmental
conditions
o The genomic hypothesis states that the origin of metazoans was based on
genetic “experimentation” during life’s Big Bang
o At no time since the late Neoproterozoic and Cambrian has the ecologic
world been as vacant (only the Late Permian comes close); thus Earth
never again witnessed anything like the evolutionary explosions of that
early time
 Extinction
o Changes in animal biodiversity in the late Precambrian and Cambrian may
be related to the extinction of the Ediacaran fauna.
10.5 Molecular Clocks and the Fossil Record of Early Metazoans
 Molecular clocks indicate an earlier origin for metazoans than does the
fossil record
o If so, this implies that much early metazoan evolution would have
occurred among very small, soft-bodied creatures
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