Chapter 32
INTRODUCTION TO ANIMAL EVOLUTION
Life began in Precambrian seas.
CHARACTERISTICS OF THE ANIMAL KINGDOM
The following characteristics describe most animals:
1. Diploid multicellular eukaryotes.
2. Cells are specialized and organized into tissues, organs, etc. Their cells lack cell wall of any
kind. Tissues are held together by structural proteins, the most common being collagen.
3. Heterotrophs that inhabit the sea, fresh water and land. Most animals take their food by
ingestion.
4. Most are capable of locomotion at some stage of their lives.
5. Most can respond adaptively to external stimuli and have well-developed sense organs and
nervous system. They have muscular and nervous tissues.
6. Most reproduce sexually, with large non-motile eggs and small flagellated sperms.
7. The diploid zygote produced by fertilization divides by mitotic divisions, resulting in a ball of
cells that usually hollows out to become a blastula. Sponges are an exception.
 Zygote → eight-cell stage → blastula → gastrula → embryo develops...
 Blastula is a hollow ball of cells with a blastocoel or cavity in its center.
 Gastrulation is a rearrangement of the blastocoel cells, where on end folds inward, and
cells fill the blastocoel forming a three-layered structure: ectoderm, mesoderm and
endoderm.
8. Regulatory genes called Hox genes control and direct the transformation of the embryo into
an animal of a given form. These genes have sequences called homeoboxes.
 Genes that are both homeobox and homeotic in function (Hox genes) have been
discovered only in animals. See chapter 21 for further information on homeoboxes.
 Homeotic genes direct the identity of body parts, e. g. antennae on the head rather
legs on the head of flies.
About 35 phyla the majority of which are invertebrates.
The animal kingdom is probably monophyletic and probably evolved from colonial flagellate
protists related to the choanoflagellates at least 700 million years ago.
Phylogenetic trees are sets of hypotheses that are refined to accommodate new data.
CLASSIFICATION
Based on type of …
1. Presence or absence of true tissues.
2. Body symmetry: radial, bilateral.
3. Body cavity: acoelomate, pseudocoelomates, coelomates.
4. Developmental pattern: protostomes, deuterostomes.
1. Presence or absence of true tissues.
Sponges have a unique simplicity that separates them from all other animals. They lack true
tissues and they are called parazoans, meaning "beside the animals."
All other animal phyla have true tissues and are called eumetazoans.
2. Body symmetry (radial, bilateral) and germ layers.
The eumetazoans are divided into two major branches based in part on body symmetry.
a. The phyla Cnidaria and Ctenophora have radial symmetry and collectively called radiata.

These animals have a top and bottom ends or an oral and aboral side, but they don't
have a head or tail end or a right and left side.
b. All other eumetazoans posses bilateral symmetry and are collectively called bilateria.
Types of sectioning a specimen:



Sagittal section divides the body into right and left parts.
Cross or transverse section divides the body into anterior and posterior parts.
Frontal section divides the body into dorsal and ventral parts.
Bilaterians show a trend towards cephalization, the concentration of sensory organs at one
end, the anterior end or head.
The head end is an adaptation to movement.
Another difference in body plan separates the radiata from the bilateria.
a. During gastrulation, bilaterians develop three germ layers and are called triploblastic:



Ectoderm covers the surface of the embryo and gives rise to the outer covering of the
animal and, in some phyla, to the central nervous system.
Endoderm, the innermost germ layer, lines the developing digestive tube or
archenteron, and gives rise to the lining of the digestive tract and organs like the liver
and lungs.
Mesoderm is located between the previous two layers and forms the muscles and most
other organs.
b. The radiata have two germ layers. The central portion is not homologous with that of the
bilaterians.
The radiata are said to be diploblastic, having two germ layers.
1. Body cavity: acoelomate, pseudocoelomates, coelomates.
Coelom or body cavity is fluid-filled space located between the outer body wall and the
digestive tube.
Triploblastic animals may be...



Acoelomate: lack coelom or body cavity, e.g. flatworms.
Pseudocoelomate: coelom is partially lined with mesoderm, e.g. roundworms, rotifers.
Coelomate: coelom is completely lined with mesoderm, e. g. mammals.
2. Developmental pattern: protostomes, deuterostomes.
a. Animals can be classified as protostomes if the blastopore develops into the mouth
(mollusks, annelids, etc.) and deuterostomes if it develops into the anus (echinoderms,
chordates).
b. Deuterostomes and protostomes have different pattern of cleavage:


Radial cleavage is characteristic of deuterostomes;
Spiral cleavage is followed by protostomes.
c. Schizocoely method of coelom formation is characteristic of protostomes.
Deuterostomes follow the enterocoely pattern of coelom formation.
d. Protostomes also have a determinate cleavage in which the fate of the embryonic cells is
fixed very early in development.
Deuterostomes have an indeterminate cleavage in which each cell keeps longer the
capacity to develop into a full organism.
Protostomes
Blastopore becomes the mouth
Spiral cleavage
Determinate fate of early cells
Schizocoelous coelom formation
Deuterostomes
Blastopore becomes the anus.
Radial cleavage
Indeterminate fate of early cells.
Enterocoelous coelom formation
CONTRIBUTION OF MOLECULAR SYSTEMATICS
Molecular systematics is moving around several branches of the phylogenetic tree.
Based on molecular data, some zoologists split the protostomes into two groups, the
Lophotrochozoa and the Ecdysozoa.
Molecular systematists base their decision on the nucleotide sequences in the small subunit
ribosomal RNA, and the sequence of the Hox genes. It is based on very few genes.
Molecular systematics supports...


the Parazoa-Eumetazoa and Radiata-Bilateria basic divisions or dichotomies of the
phylogenetic tree.
the grouping of the echinoderms and chordates under the deuterostomes.
Molecular systematics differs from the traditional classification in...


the splitting of the protostomes in two distinct clades or branches: Lophotrochozoa and
Ecdysozoa. Based on comparative anatomy and embryology, it was uncertain the
lineage of these phyla.
the molecular data places the flatworms (acoelomates) and rotifers (pseudocoelomates)
in the Lophotrochozoa clade, and the nematodes in the Ecdysozoa.
Molecular data clarifies the position of the lophophorate phyla.

Phyla with a the larva having a lophophore (crown of feeding tentacles) share
characteristics with both the protostomes and deuterostomes and their position on the
phylogenetic tree was uncertain. Molecular systematics places them within the
protostomes in the lophotrochozoan clade.
ORIGINS OF ANIMAL DIVERSITY
Most animal phyla originated in relatively brief span of geologic time.
Life was flourishing in the Precambrian (630 - 590 m.y.a.)
Late Precambrian. Ediacaran Fauna (630 - 590 m.y.a.) from Australia's Ediacara hills, and
other parts of the world.

Metazoan fossils dating from 575 - 544 million years.
First described from the Ediacara Hills of southern Australia, but have since been found all over
the world.


Several body plans have been observed: animals with both radial and bilateral symmetry,
feather-like and worm or tube-like forms, etc.
Most of the animals in the Ediacaran fauna appear to be cnidarians and possibly mollusks.
Fossilized burrows of animals have been found in rocks that are 1.1 billion years old.
The ancestors of major groups of animal species began populating Earth more than 600 million
years earlier than indicated by their fossil remains, according to the largest study on the subject
using gene sequences. Animals, plants, and fungi first diverged from a common ancestor and
began evolving down separate evolutionary paths as early as about 1.6 billion years ago. This is
the earliest yet obtained by gene studies for this evolutionary event. (January 22, 1999,
Proceedings of the Royal Society of London (Series B) D. Y.-C. Wang, S. Kumar and S. B.
Hedges.)
Studies done prior to Hedges et al. yielded a wide range of dates for the origin of animals but all
of them agreed that it occurred before the Cambrian. Hedges et al. date of 1.6 billion years ago
is intermediate between those of earlier studies.
At the beginning of the Cambrian, about 545 m.y.a. there was an explosive radiation of
multicellular animals. This event is called the Cambrian explosion.


Rapid evolutionary changes occurred in the late Proterozoic and ended in the Early
Cambrian.
A change from mostly simple organisms to complex organisms comparable to present day
animals. This happened in 40 million years, 545 - 525 m.y.a.
It was a period of great evolutionary innovation. Almost all metazoa phyla appeared at this time.
Causes of the Cambrian explosion
These three hypotheses are not mutually exclusive.
Ecological causes:



Predators feed on the most abundantly prey species thus reducing their numbers and
letting others use the resources.
Cropping of a dominant species opens many niches, which become occupy by species
previously excluded.
Increase in prey species leads to an increase in predators.
Geologic causes:


Oxygen may have reached critical threshold at this time.
Oxygen supports a more active metabolism, which is required by fast moving animals.
Genetic causes:


Ohno claims that early Cambrian animals posses genes capable of forming oxygen crosslinks in collagen in ligaments and tendons; hemoglobin; and homeobox proteins that help
orient body plans along a directional axis.
The diversification of animals is associated with the evolution of Hox regulatory genes,
which lead to variation in morphology.
By the end of the Cambrian radiation, the animal phyla were locked into developmental patterns
that constrained evolution enough that no additional phyla evolved after that period.