Chapter 15
Animal Evolution
Animal Origins
Animals are multicellular heterotrophs
that ingest their food.
 No cell walls
 Most reproduce sexually, some reproduce
asexually, and some can do both
 Are motile during part or all of their lives
 Have cells that specialize as the animal
deveops from embryo to adult
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Animal Origins
Probably evolved from heterotrophic,
colonial protists that could reproduce
 Scientists believe that some cells in the
colonies underwent mutations that
produced cells that could do some tasks and
not do other tasks at all
 These interdependent cells and the division
of labor that came with them put colonies
with these cells at a selective advantage
 New, specialized cell types were the result
and led to the evolution of animals
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Animal Origins
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Placozoans: early animals, very few genes,
simplest body plan of any animal, 2mm across
and has only 4 cell types
The first animal on Earth probably evolved in
the ocean about 1 billion years ago but most
animals came into being about 500 million years
ago
This is about the time that O2 concentrations in
the water increased dramatically and it allowed
larger, more active animals to evolve
Also, at about the same, continents were
breaking up, cutting off gene flow and leading to
speciation
Major Animal Characteristics
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All animals are descended from a common multicellular
ancestor
The earliest animals were only aggregations of cells
(like placozoans and sponges)
Most animals have tissues (one or more types of cells
that are organized in a specific pattern and that carry out
a particular task)
Early animal embryos had two tissue layers: ectoderm
and endoderm.
Later animal embryos had a middle tissue layer called
mesoderm
This embryo with three tissue layers allowed animals to
increase in complexity
Most animal groups have organs derived from the
mesoderm.
Embryonic Tissue Layers
Major Animal Characteristics
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The simplest animals have no symmetry
(asymmetrical); however later animals have
radial symmetry (body parts arranged
around a central axis with no front or back
end) and the most recent animals have
bilateral symmetry (two halves that are
mirror images of one another with a
distinctive head end and a concentration of
nerve tissue)
How would bilateral symmetry be
advantageous?
Symmetry
Sponges
Jellyfish and Sea Stars
Cnidaria
Most Animals
Major Animal Characteristics
Bilateral animals have a tubular gut with a mouth at
one end and an anus at the other
 This is called a complete digestive system and
allows parts of the tube to become specialized for
taking in food, digesting food, absorbing nutrients, and
compacting wastes, allowing all of these to be going on
at the same in the gut.
 Two lineages of bilateral animals differ in the
embryonic development of the tubular gut.
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◦ Protostomes are bilateral animals whose first opening
that appears in the gut of the embryo becomes the mouth
◦ In deuterostomes, the first opening in the embryo
becomes the anus and second opening becomes the
mouth.
Major Animal Characteristics
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Most animals have a fluid-filled body cavity that
surrounds the gut (rather than a solid body
cavity).
When the tissue that lines this cavity is derived
from the mesoderm, the body cavity is called a
coelom.
The advantage of this fluid-filled body cavity is
three-fold:
◦ Allows materials to diffuse through the fluid to body
cells
◦ Muscles can redistribute the fluid to alter body shape
and aid in movement
◦ Gives organs more room to grow larger and move
more freely
Major Animal Characteristics
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A closed circulatory system with a heart
distributes substances throughout the body much
more quickly than diffusion through the fluid-filled
body cavity alone.
Most bilateral animals have some degree of
segmentation, which is the division of a body into
interconnecting units that are repeated one after the
other along the main body axis.
Early human embryos are segmented.
This segmentation allowed evolutionary innovations in
body form since some segments could specialize in
certain functions without endangering the life of the
animal.
Most animals are invertebrates (do not have a
backbone).
Invertebrates: Sponges(Porifera)
Aquatic
 No symmetry, tissues, or organs
 Adults don’t move about
 Larval stage is a free-living, ciliated life
stage that moves about
 Filter-feeders
 Hermaphroditic (produces both eggs and
sperm)
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Invertebrates: Cnidarians
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Radial symmetry
Have nerve cells so that they can detect and respond
to stimuli; however, they have no central information
processing region that functions like a brain
Gastrovascular cavity with one opening that takes in
and digests food, expels wastes, and also exchanges
gases
Have specialized stinging cells called nematocysts
Two Cnidarian body plans:
◦ Medusae: bell or umbrella shape (jellyfish)
◦ Polyp: tubular with one end attached to a surface (sea
anemone)
Invertebrates: Flatworms
(Platyhelminthes)
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Are protostomes
Bilateral symmetry
No body cavity
Hermaphroditic
Flukes and tapeworms are flatworms that spend part
of their life cycle as parasites
Branching gut with a single opening
Pair of nerve cords that run the length of the body
Cluster of nerve cells in the head that serve as a
simple brain
Head also contains chemical receptors and light
detecting eyespots
Invertebrates: Segmented Worms
(Annelids)
Segmented body
 Coelom
 Complete digestive system
 Closed circulatory system
 Nerve cord extends the the length of the
body and connects to a simple brain
 Multiple hears that pump blood through
vessels
 Hermaphroditic
 Earthworms and leeches are Annelids
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Invertebrates: Mollusks
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Small coelom
Soft, unsegmented body
Have a mantle that forms a mantle cavity and
secretes a hard, calcium-rich shell
Very diverse group
◦ Gastropods: snails and slugs “belly foot”, only
mollusks that have lung and can breathe air, use a
radula (tongue-like organ) for eating
◦ Bivalves: clams, oysters, etc., hinged, two-part shell,
are filter feeders
◦ Cephalopods: squids, octopi, etc., are predators, eat
with a radula and biting mouthparts, move by jet
propulsion, closed circulatory system, have eyes with
lenses that focus light, are some of the fastest, biggest,
and smartest invertebrates
Invertebrates: Roundworms
(Nematodes)
More closely related to arthropod than to
other worms
 Unsegmented
 Bilateral symmetry
 Complete gut
 Reproductive organs
 Most are free-living decomposers
 Some are parasites (pinworms, dog
heartworms)
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Invertebrates: Arthropods
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There are more than a million arthropod species
Six evolutionary adaptations that have made these animals
so successful:
◦ Hard exoskeleton composed of chitin for support, protection,
and aids in movement
◦ Jointed appendages allow more freedom of movement
◦ Specialized segments (head, thorax, abdomen) and appendages
(claws, wings)
◦ Respiratory structures such as gills or air-conducting tubes
◦ Specialized sensory structures including one or more pairs of
compound eyes (that are highly sensitive to movement) and
one or two pairs of antennae (that can detect touch, odor, and
vibrations)
◦ Specialized developmental stages: metamorphosis(body plan
is dramatically remodeled as larvae develop into adults)- this
prevents juveniles and adults from competing for the same
resources
Arthropod Classes
Arachnids: spiders, scorpions-live on land,
four pair of walking legs, pair of touch
sensitive palps, no antennae
 Crustaceans: shrimps, crabs, lobstersmarine arthropods with two pairs of
antennae
 Insects: most diverse arthropods, three
part body (head, thorax, abdomen), only
winged invertebrates
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Invertebrates: Echinoderms
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The only invertebrates that are deuterostomes
Include, sea stars, sea urchins, and sea cucumbers
Spiny skin with interlocking plates made of
calcium carbonate embedded in their skin form
an endoskeleton
Radial symmetry but larvae have bilateral
symmetry
No brain but do have a nervous system
Eyespots detect light and movement
Water vascular system for movement
Sexes are separate
Chordates
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The majority of deuterostomes are
chordates.
Chordate embryos have:
◦ A notochord (a rod of stiff but flexible connective
tissue that extends the length of the body and
provides support)
◦ A dorsal, hollow nerve cord that runs parallel to
the notochord
◦ Gill slits that open across the wall of the pharnyx
◦ A muscular tail that extends beyond the anus
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Some, all, or none of these traits persist in
the adult, depending upon the chordate
group.
Chordates
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There are three groups of chordates:
1. Tunicates: invertebrates, attach to an undersea
surface and filter food
2. Lancelets: invertebrates, fish-shaped, no brain or
paired sensory organs but does have eyespots, are
probably the closest invertebrate relatives of
vertebrates
3. Vertebrates: have several major innovations
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Vertebral column: protects spinal cord
Jaws: opened up new feeding opportunities
Swim bladder/lungs: allows fish to adjust buoyancy/exchange gases
Four limbs: more mobility
Amniote (waterproof) egg: most successful land tetrapods
Chordates
Vertebrates
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All vertebrates have:
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A brain
Closed circulatory system with one heart
Urinary system with a pair of kidneys
Complete digestive system
Vertebrates: Fishes
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Jawless Lampreys: are ancient fishes, no jaws or
fins but do have a backbone
Jawed Fishes: have paired fins and scales
◦ Cartilaginous fishes: skeleton made of cartilage,
including sharks
◦ Bony fishes: embryonic skeleton made of cartilage
transforms to bone in adult, protective gill cover, swim
bladder
 Ray-finned fishes: flexible fins (salmon, sardines. tuna, etc.)
 Lobe-finned fishes: (coelacanths and lungfishes) fins are fleshy
body extension with bony elements inside of them, lungfishes
have lung-like sacs and many of them will drown if left
underwater
Vertebrates: Transition to Land
All land vertebrates evolved from lobefinned fishes.
 Bones inside of lobe-finned fish’s pelvic
and pectoral fins are homologous with
amphibian limb bones.
 Division of heart into three chambers
also preceded movement to land allowed
blood to flow to body and to LUNGS
 Eyelids and changes in inner ear (to
detect airborne sounds) also evolved
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Land Vertebrates: Amphibians
Tetrapods that spend time on land but
require water to breed
 Larvae have gills but adults lose gills and
develop lungs
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Land Vertebrates: The Amniotes
Branched off from amphibian ancestors
 Have amniotic eggs with four membranes
that enable embryos to develop away
from water
 Also covered in a keratin rich covering
that makes the egg waterproof to prevent
dessication
 Well-developed kidneys to conserve
water
 Internal fertilization is typical
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Land Vertebrate Amniotes: Reptiles
Are ectotherms (adjust their internal body
temperature by their behavior)
 Dry, thick skin that is waterproof
 Includes lizards, snakes, turtles, crocodilians
(such as crocodiles, alligators, caimans)
 Crocodilians have a four-chambered heart
like birds and mammals.
 They are the closest living relatives of birds
and, like birds, lay eggs, then protect and care
for their young
 All teeth are similarly shaped
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Land Vertebrate Amniotes: Birds
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Only modern amniotes with feathers
Feathers are modified scales that help adapt
bird for flight
Bones are hollow to make them lightweight
(another adaptation for flight)
Most efficient respiratory system of any
vertebrate
Four-chambered heart
Endotherms (produce their own heat
through metabolic processes)
Land Vertebrate Amniotes:
Mammals
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Only amniotes in which females nourish their
young with milk secreted from mammary glands
Only animals that have hair or fur
Endothermic
Most have more than one type of tooth
Three groups of mammals:
◦ Monotremes: egg-laying mammals (platypus)
◦ Marsupials: pouched mammals (koala, kangaroo)
◦ Placental mammals: organ called placenta provides
nourishment to developing offspring (humans)
Mammals
Koala
Platypus
Kangaroo
Placental mammals