INTRODUCTION TO ANIMAL DIVERSITY Reading: Chapter 18, p. 383 (homeotic genes); Chapter
21, pp. 457-459
(homeoboxes and Hox genes); Chapter 25, pp. 535-536; Chapter 32 A.
WHAT ARE ANIMALS?
1. Animals are multicellular, heterotrophic eukaryotes.
2. Animals ingest other organisms or organic debris.
3. Animal cells are not surrounded by cell walls.
4. Animals have intercellular junctions found in no other group of organisms.
5. Animals have nervous tissue and muscle tissue.
6. Animals have common features of early
Fig. 32.2 Note the evident polarity in the gastrula stage – one end is different from the other. How
might the polarity in the multicellular gastrula have its origin in the unicellular zygote?
7. Almost all animals have Hox genes, which contain homeoboxes and which are homeotic in
function. Hox genes function during development to specify features of the animal’s body plan.
Hox genes encode regulators of the transcription of other genes, thereby turning on or off genes
whose products build body parts during development.
Variation in Hox gene activity can therefore lead to variation in animal body plan. Hox genes are of
great evolutionary significance when we try to understand animal diversification at the levels of the
phylum and the class. B. FUNDAMENTAL DIFFERENCES in ANIMAL FORM and DEVELOPMENT
1.Some of these differences define clades, while others define grades.
A grade is a group of
organisms that share an important adaptation, but not through common descent.
A grade is a group of organisms that share an important adaptation, but not through common
descent. E.g. possession of chloroplasts
2.Presence of tissues – integrated groups of cells that perform specialized functions
No tissues – Parazoa (sponges, placozoans), some cell specialization, but no tissues.
Tissues – Eumetazoa (all other animals)
Fig. 32.11 Animal phylogeny based on molecular data. You should compare this phylogeny with the
one in Figure 32.10 (which is based on morphological arguments) and note the similarities and
differences. Why, generally speaking, might a morphological tree differ from a molecular tree?
3. Body symmetry and the number of germ layers in the embryo
radial symmetry and two germ layers (ectoderm and endoderm; diploblastic)
polarity of the adult animal. oral – anal
Radially symmetric animals are often sessile (attached to a substrate) or planktonic (drifting).
Encounters threats and opportunities from all directions. E.g. jelly fish, anemone
Bilateral symmetry and three germ layers (ectoderm - nervous system, mesoderm – muscles and
endoderm – gut/organs; triploblastic) e.g. lobster
Polarities of the adult animal. Anterior (oral) – posterior (anal), dorsal (back) – ventral (front), right -
Bilateral symmetry is associated with cephalization – the anterior concentration of neural processing
power, sensory apparatus, and sometimes weapons. (e.g. fangs)
Suited to more active animals – encounters the world from one direction.
4. Early development: protostomes and deuterostomes
Cleavage division – cleavage products have a specific fate; they cannot turn into a fully developed
animal if separated from other cells. (determinate)
determinate cleavage – early cleavage products lose the ability to form complete embryos by
indeterminate cleavage – early cleavage products can form complete embryos by themselves
ability to develop in fully functional animals if cells are separated. (each one would develop into an
individual animal) Fig. 32.9 Consider the associations of traits pictured here. Do you think these traits are associated for
functional reasons or for historical reasons? Evolutionary reasons.
Humans relate closely in embryo development with echinoderms.
5. Presence of a body cavity (a coelom) between the gut and the body wall.
coelom is lined with mesoderm only
pseudocoelom is lined with endoderm and mesoderm Acoelomate:
no coelom; mesoderm forms a solid mass of cells (no blood circulation) i
Functions of the Coelom (devrived from mesoderm)
- space for complex organ development
- blood vascular system (ability to grow large)
- independent movement of organs
- hydrostatic skeleton
- allows organs to be suspended in particular order while still being able to move freely. Fig. 32.8 Where in your own body is your coelom?
Different compartments within animals.
C. ORIGINS of ANIMALS and EARLY ANIMAL EVOLUTION
1. The animal kingdom is monophyletic, with the divergence of animals from a colonial protist
occurring long before any fossil traces of animals appear.
LCA – 675 – 875 million yrs ago. (when animals began to diverge)
2. Animal near-relations alive today: choanoflagellates
Perhaps resemble the earliest animals.
Collar cells (choanocyte)
Signaling molecules, adhesion molecules
e.g. tumors could be our oldest ancestors – restraint of excessive cell multiplication led to complex
Mesomycetozoans – could be earliest animal divergence
3. Early animal evolution might have been stimulated by the end of the Snowball Earth phase
835-635 million years ago; Earth was encased in ice.
Reduction in ice resulted in increase level of oxygen on earth which aided animals in being active
(oxygen helps produce ATP)
4. The Doushantuo fossils (approx. 600 – 580 million years ago) look like animal embryos.
Earliest stages of cleavage are not accompanied by any increase in size regardless of number of
cells in embryos.
5. The Ediacarans (585 – 542 million years ago) – the first large multicellular organisms.
Looked like: sponges, cnidarians (jellyfish), annelids, mollusks, arthropods, echinoderms, nothing
6. The Cambrian explosion (542 – 525 million years ago) – half of extant animal phyla were
Many basic body plans appeared and many diversified during the Cambrian.
key fossil formation: the Burgess shale
505 million years ago, soft-bodied animals,