Introduction to

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Chapter 32 Introduction to Animal Evolution
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I.  What is an animal?             A.  Structure,
nutrition, and life history define animals 1.
Animals are multicellular, heterotrophic
eukaryotes.                                    
- Animals must take in organic molecules by
ingestion they eat other organisms or organic
material that is decomposing.   2.  Animal cells
have no cell walls.  - Bodies are held together
by proteins, especially collagen.   3.  Animals
have two unique types of tissues                 
                    a.  Nervous tissue
and                                     b. 
Muscle tissue
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4.  Most animals reproduce sexually.              
                       a.  Dominant stage is
typically diploid. b.  Motile sperm fertilizes a
larger, non-motile egg (both 1n). c.  The
resulting zygote (2n) goes through embryonic
development as follows       i.  The zygote
undergoes cleavage, a series of mitotic cell
divisions.   ii.  Cleavage results in the
formation of a blastula, a hollow ball of
cells.   iii.  Further development results
in the formation of a gastrula, a two-layered,
cup-shaped cluster of cells.                      
                                       - This is
the stage where tissue differentiation
occurs     - Endoderm digestive tract     -
Ectoderm skin, nerves
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      iv.  Many embryos develop directly into
adults (sexually mature organisms)                
                                       v.  Some
develop into larvae (sexually immature organisms)
and later undergo metamorphosis to the adult
stage    Figure 32.1 (p. 634, ed. 6 Fig 32.2,
p.627, ed. 7) Early embryonic development.
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•  5.  Growth from embryo to adult is modulated
  (controlled and organized) by Hox genes.  Thus,
  the time sequence of development is controlled by
  Hox genes.
• Many genes are the same or similar in all
  animals.  The sequence in which they are turned
  on and off during development causes embryos to
  develop into different animals. 
• Thus, for example, the same genes that give rise
  to dolphins and humans are in both organisms, but
  the sequence and time in which they are turned on
  creates either the dolphin or human. This is
  controlled by the Hox genes.
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B.  Animals probably evolved from colonial,
flagellated protists  Figure 32.3 (p. 635, ed. 6
Fig. 32.4, p. 628, ed. 7) One hypothesis for
the origin of animals from a flagellated protist.
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II.  Two views of animal diversity             -
The phylogenetic tree of animals has about 35
phyla. (The continuous remodeling of the
phylogenetic tree illustrates the process of
scientific inquiry.)                         1. 
The traditional view of animal diversity is based
on anatomy and embryology.   2.  Newer trees are
based on molecular evidence. You should remember
the tree that is based on anatomical features
(body plans) for this course.                     
    Figure 32.4 (p. 636) A traditional view of
animal diversity based on body-plan grades.
Closest version in Edition 7 Fig. 32.10, p. 634.
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This section will focus on the four main branch
points in this phylogenetic tree.   1.  Porifera
(Sponges) early branch à Parazoa structural
simplicity no true tissue differentiation        
                 ? Separated from Eumetazoa all
have true tissue differentiation. The next phyla
are all Eumetazoa   2.  Radiata radial
symmetry in body structure (no left/right side)
Example  jellyfish Separated from Bilateria
two-sided symmetry à left/right, dorsal/ventral,
anterior/posterior cephalization sensory
organs concentrated on anterior end Figure 32.5
(p. 637, ed. 6) Body symmetry. (Fig. 32.7, p.
630, ed. 7)
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3.  Acoelomates no body cavity                  
                   Example  flatworms            
             Separated from Pseudocoelomates
(e.g. nematodes) and Coelomates (e.g. humans)
have body cavities                               
     
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  4.  Protostomia body cavity forms from cell
masses, blastopore becomes mouth                  
                   Examples  snails, worms,
insects                         Separated from
Deuterostomia body cavity develops from
digestive tube, blastopore becomes
anus                                    
Examples  starfish, chordates (humans)
  Figure 32.7 (p. 639, ed. 6 Fig. 32.9, p. 632,
ed. 7) A comparison of early development in
protostomes and deuterostomes. Finally,
Lophophorates uncertain phylogeny with traits
from both deuterostomes and protostomes.
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III.  Origins of animal diversity             A. 
Most animal phyla originated in a relatively
brief span of geologic time                       
  1.  Modern phyla developed in about 40 million
years total.   2.  During the Cambrian Explosion
(543 to 524 million years ago), nearly all major
body plans appeared.
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            B.  What caused the Cambrian
explosion?                         1. 
Development of predators and evolution toward
prey escaping/predator hunting. Increased need
for speed and better sensory equipment.   2. 
Oxygen levels reached present levels that allow
for rapid metabolism exhibited by animals.   3. 
Hox genes evolved at that time and allowed for
differential development.