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1. Describe the evidence that suggests animals may have first evolved about a billion years ago
2. Explain the significance of the Cambrian explosion. Describe three hypotheses for the cause of the Cambrian explosion
3. Compare the developmental
differences between protostomes and deuterostomes, including:
a. pattern of cleavage
b. fate of the blastopore
c. coelom formation
3. According to this phylogenetic tree, annelids are most closely related to _____.
4. According to this phylogenetic tree, the clade that includes echinoderms and chordates is most closely related to the clade that includes _____.
5. According to this phylogenetic tree, cnidarians are most closely related to _____.
6. According to this phylogenetic tree, chordates are most closely related to _____.
7. Cnidarians are in the _____ clade
8. Platyhelminths are _____.
9. Ectoderm can give rise to _______; mesoderm can give rise to ______; endoderm can give rise to _________.
Mechanisms of multicellular development developed independently in plants and animals.
The last ancestor of plants and animals was a unicellular eucaryote. Gene comparisons show there is not much homology between the genes that make up the body plan of plants and animals
Although homeobox- as well as MADS box genes existed in the last common ancestor, the MADS box gene family plays an important part in regulation of plant development, but not in animal development, where homeobox genes are important.
Animal cells are motile.
Animal tissues may be folded and moved against each other easily. At metazoan gastrulation this way a triple layered system is built (entoderm, mesoderm and ectoderm). Some animal cells may even move to other sites autonomously.
Plant cells are positionally fixed.
Plant cells are trapped in rigid cell walls made of cellulose, which prevents movement of cells and tissues. Plants form three basic tissue systems as well (dermal, ground and vascular), yet without gastrulation.
Rigidity of the body shape
The animal body plan is in most parts clearly determined.
The basic body plan of an animal during its different life stages is mostly clearly determined by its genes. If the environment changes they may react e.g. by moving to another place or changing their short and long term behavior.
Plant development is highly regulated by the environment.
As in most cases it may not choose or change its environment, it has to adapt to it. The body plan is variable and characterised by multiple times occurring, often iterative structures. Proportions and frequency of organs may vary
During the animal life cycle there is just one continuously multicellular stage
It is what we refer to as "the animal". Yet many animals undergo one or more transformation, when their body plan changes dramatically
The life cycle of land plants (and many other plants) has haploid and diploid stages.
This kind of life cycle is called alternation of generations and leads to two different body plans during the life cycle of the plant (sporophyte and gametophyte).
In animals gametes are formed directly through meiosis.
There is nothing that could be compared to the gametophyte in plants.
Plants undergo no gametic meiosis, but a sporoic meiosis.
In plants the meiosis produces spores and not gametes. First the gametophyte is formed by mitotic divisions, which then forms the gametes.
Many animal species set aside reproductory stem cells early in development.
This decreases mutation accumulation.
No reproductory stem cells are set aside early in development in plants.
Still some plants leave certain meristems or meristem parts more inactive till the gametophyte is to be formed.
Animals develop to a distinct, complete body shape.
During their life stages still some reorganisation may take place, yet only in seldom cases new structures will develop. Some animals develop stepwise into different shapes.
Plants go through a longer period of morphogenesis.
During their development plants do not head for a distinct body plan. Many plants just grow and develop on and on till they die. Areas of actively dividing, undifferentiated cells, called meristems, allow for iterative growth and the formation of more and more new organs and structures during a plants life. They resemble embryonal stem cells in animals, yet they continue existing during adult life stages.
What is an animal? How do we define an animal?
While there are exceptions to nearly every criterion for distinguishing an animal from other life forms, the following criteria, when taken together, create a reasonable definition
1. Animals are multicellular eukaryotes.
2. Animals are Heterotrophes:
Cannot make their own food - cannot convert simple inorganic molecules into complex organic molecules. They must take in preformed organic molecules through ingestion, eating other organisms or organic material that is decomposing.
Plants are Autotrophs they can synthesize their nutrition from simple inorganic molecules into complex organic molecules.
3. Animal cells lack cell walls that provide structural supports for plants and fungi.
The multicellular bodies of animals are held together with the extracellular proteins, especially collagen.
In addition, other structural proteins create several types of intercellular junctions, including tight junctions, desmosomes, and gap junctions, that hold tissues together.
Animals have skeleton endoskeleton (bones) or exoskeleton (shells) that provides rigidity of form.
Animals are capable of moving from one place to another. This complex function is achieved by coordinated functioning of two unique tissue types.
i. Nervous tissue for impulse conduction
ii. Muscle tissue for movement.
Exception: Some animals do not have locomotor ability sedentary.
Origin of animal body form diversity
Most systematists now agree that the animal kingdom is monophyletic.
If we could trace all the animals lineages back to their origin, they would converge on a common ancestor.
That ancestor was most likely a colonial flagellated protist that lived over 700 million years ago in the Precambrian era.
One hypothesis for origin of animals from a flagellated protist suggests that a colony of identical cells evolved into a hollow sphere.
The cells of this sphere then specialized, creating two or more layers of cells.
Neoproterozoic Era (1 Billion524 Million Years Ago)
Early members of the animal fossil record
Include the Ediacaran fauna
Paleozoic Era (542251 Million Years Ago)
The Cambrian explosion
Marks the earliest fossil appearance of many major groups of living animals
Is described by several current hypotheses
Mesozoic Era (25165.5 Million Years Ago)
During the Mesozoic era
Dinosaurs were the dominant terrestrial vertebrates
Coral reefs emerged, becoming important marine ecological niches for other organisms
Cenozoic Era (65.5 Million Years Ago to the Present)
The beginning of this era
Followed mass extinctions of both terrestrial and marine animals
Modern mammal orders and insects
Diversified during the Cenozoic
Animals can be categorized
According to the symmetry of their bodies, or lack of it
Some animals have radial symmetry
Like in a flower pot
Some animals exhibit bilateral symmetry
Or two-sided symmetry
Bilaterally symmetrical animals have
A dorsal (top) side and a ventral (bottom) side
A right and left side
Anterior (head) and posterior (tail) ends
Cephalization, the development of a head
Animal body plans
Also vary according to the organization of the animals tissues
Are collections of specialized cells isolated from other tissues by membranous layers
Form germ layers, embryonic tissues, including ectoderm, endoderm, and mesoderm
Have two germ layers
Have three germ layers
In triploblastic animals
A body cavity may be present or absent
2. Germ Layers:
Three germ layers:
Ectoderm: forms the outer covering and, the central nervous system (if present).
Endoderm: lines the digestive tract and the organs derived from it, such as the liver and lungs of vertebrates.
Mesoderm: lies between the endoderm and ectoderm and develops into muscles and other organs e.g., kidneys and gonads.
i. Diploblastic: have two germ layers ectoderm and endoderm. e. g., the radiata.
ii. Triploblastic: have three germ layers ectoderm, endoderm and mesoderm. e. g., the bilateria
3. Coelom: The germ layers form a cavity around the internal organs Coelom.
Coelom has many functions.
Its fluid cushions the internal organs, helping to prevent internal injury.
The noncompressible fluid of the body cavity can function as a hydrostatic skeleton against which muscles can work.
The present of the cavity enables the internal organs to grow and move independently of the outer body wall.
Coelomates are organisms with a true coelom, a fluid-filled body cavity completely lined by mesoderm. The inner and outer layers of tissue that surround the cavity connect dorsally and ventrally to form mesenteries, which suspend the internal organs.
. A coelom the body cavity - could be a true coelom or a false coelom.
Based on the type of coelom and presence or absence of a coelom and animal could be classified as:
Pseudocoelomate (false coelom)
Acoelomate (no coelom)
Pseudocoelomates, have a body cavity, but it is not completely lined by mesoderm, e.g., rotifers (phylum Rotifera) and the roundworms (phylum Nematoda).
Is a body cavity derived from the blastocoel, rather than from mesoderm
What Is an Animal?
An Overview of Animal Phylogeny and Diversity
Parazoans lack true tissues
Radiata and bilateria are the major branches of eumetazoans
Evolution of body cavities led to more complex animals
Coelomates branched into protostomes and deuterostomes
The Origins of Animal Diversity
Most animal phyla originated in a relatively brief span of geological time
Developmental genetics may clarify our understanding of the Cambrian diversification
Phylum Porifera: Sponges are sessile with porous bodies and choanocytes
Phylum Cnidaria: Cnidarians have radial symmetry, a gastrovascular cavity, and cnidocytes
Phylum Ctenophora: Comb jellies possess rows of ciliary plates and adhesive colloblasts
Phylum Platyhelminthes: Flatworms are dorsoventrally flattened acoelomates
Phylum Rotifera: Rotifers have jaws and a crown of cilia
Phylum Nematoda: Roundworms are unsegmented and cylindrical with tapered ends
The Coelomates: Protostomes
Phylum Nemertea: The phylogenetic position of proboscis worms is uncertain
The lophophorate phyla: Bryozoans, phoronids, and brachiopods have ciliated tentacles around their mouths
Phylum Mollusca: Mollusks have a muscular foot, a visceral mass, and a mantle
Phylum Annelida: Annelids are segmented worms
Phylum Arthropoda: Arthropods have regional segmentation, jointed appendages, and an exoskeleton
Phylum Echinodermata: Echinoderms have a water vascular system and secondary radial symmetry
Phylum Chordata: The chordates include two invertebrate subphyla and all vertebrates
All animals, and only animals
Have Hox genes that regulate the development of body form
Although the Hox family of genes has been highly conserved
It can produce a wide diversity of animal morphology
Reproduction and Development
Most animals reproduce sexually
With the diploid stage usually dominating the life cycle
After a sperm fertilizes an egg
The zygote undergoes cleavage, leading to the formation of a blastula
The blastula undergoes gastrulation
Resulting in the formation of embryonic tissue layers and a gastrula
: The history of animals may span more than a billion years
The animal kingdom includes not only great diversity of living species
But the even greater diversity of extinct ones as well
: Animals can be characterized by body plans
One way in which zoologists categorize the diversity of animals
Is according to general features of morphology and development
A group of animal species
That share the same level of organizational complexity is known as a grade
The set of morphological and developmental traits that define a grade
Are generally integrated into a functional whole referred to as a body plan
Protostome and Deuterostome Development
Based on certain features seen in early development
Many animals can be categorized as having one of two developmental modes: protostome development or deuterostome development
In protostome development
Cleavage is spiral and determinate
In deuterostome development
Cleavage is radial and indeterminate
In protostome development
The splitting of the initially solid masses of mesoderm to form the coelomic cavity is called schizocoelous development
In deuterostome development
Formation of the body cavity is described as enterocoelous development
Fate of the Blastopore
In protostome development
The blastopore becomes the mouth
In deuterostome development
The blastopore becomes the anus
: Leading hypotheses agree on major features of the animal phylogenetic tree
Zoologists currently recognize about 35 animal phyla
The current debate in animal systematics
Has led to the development of two phylogenetic hypotheses, but others exist as well
One hypothesis of animal phylogeny based mainly on morphological and developmental comparisons
One hypothesis of animal phylogeny based mainly on molecular data
Points of Agreement
All animals share a common ancestor
Sponges are basal animals
Eumetazoa is a clade of animals with true tissues
Most animal phyla belong to the clade Bilateria
Vertebrates and some other phyla belong to the clade Deuterostomia
Disagreement over the Bilaterians
The morphology-based tree
Divides the bilaterians into two clades: deuterostomes and protostomes
In contrast, several recent molecular studies
Generally assign two sister taxa to the protostomes rather than one: the ecdysozoans and the lophotrochozoans
Ecdysozoans share a common characteristic
They shed their exoskeletons through a process called ecdysis
Lophotrochozoans share a common characteristic
Called the lophophore, a feeding structure
Go through a distinct larval stage called a trochophore larva
Future Directions in Animal Systematics
Phylogenetic studies based on larger databases
Will likely provide further insights into animal evolutionary history
Next upAnimal 2!