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Chapter 32 Introduction to Animal Evolution. I.  What is an animal?             A.  Structure, nutrition, and life history define animals 1. Animals are multicellular, heterotrophic eukaryotes.

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Chapter 32 Introduction to Animal Evolution

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Chapter 32 introduction to animal evolution

Chapter 32

Introduction to

Animal Evolution


Chapter 32 introduction to animal evolution

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


Chapter 32 introduction to animal evolution

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


Chapter 32 introduction to animal evolution

      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.


Chapter 32 introduction to animal evolution

  •  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.


Chapter 32 introduction to animal evolution

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.


Chapter 32 introduction to animal evolution

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.


Chapter 32 introduction to animal evolution

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)


Chapter 32 introduction to animal evolution

3.  Acoelomates – no body cavity

                                    Example:  flatworms

Separated from Pseudocoelomates (e.g. nematodes) and Coelomates (e.g. humans) – have body cavities


Chapter 32 introduction to animal evolution

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.


Chapter 32 introduction to animal evolution

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 CambrianExplosion (543 to 524 million years ago), nearly all major body plans appeared.


Chapter 32 introduction to animal evolution

            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.


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