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
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
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.
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.
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.
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.
This section will focus on the four main branch points in this
phylogenetic tree.
1. Porifera (Sponges) – early branch  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  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)
3. Acoelomates – no body cavity
Example: flatworms
Separated from Pseudocoelomates (e.g. nematodes) and
Coelomates (e.g. humans) – have body cavities
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.
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.
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.