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Animal Evolution –
The Invertebrates
Comparing Key Groups
 Evolutionary trends toward
• Greater size
• Compartmentalization (division of labor among
cells, tissues, and organs)
• Integration of specialized activities that keep the
organism alive
Body Symmetry: Radial and Bilateral
 Most animals are bilateral
Body Organization
 Cephalization
• Sensory cells concentrated at the head
 Segmentation
• Repetition of body units, front-to-back
Body Cavities
 Most bilateral animals
have a coelom and a
complete gut
Key Concepts:
INTRODUCING THE ANIMALS
 Animals
• Multicelled heterotrophs (ingest other organisms)
• Grow and develop through a series of stages
• Actively move about during all or part of life cycle
 Cells of most animals form tissues and
extracellular matrixes
Sponges (Phylum Porifera)
 Sponges
• No symmetry, tissues, or organs
• Flattened cells line the body wall (many pores;
spikes of silica and/or proteins)
• Filter feeders (flagellated collar cells absorb food;
amoeboid cells digest and distribute it)
• Zygote develops into free-living larva
Sponges
Cnidarians (Phylum Cnidaria)
 Jellyfishes, corals, and sea anemones
• Radial, tentacled carnivores
• Gastrovascular cavity (respiration and digestion)
• True epithelial tissues with a jellylike matrix in
between
• Simple nervous system
Cnidarian Body Plans
Unique Cnidarian Weapons
 Nematocysts
• Used to capture prey and for defense
Cnidarian Predators
Cnidarian Life Cycles
Flatworms (Phylum Platyhelminthes)
 Free-living turbellarians (planarians), parasitic
tapeworms and flukes
• Simplest animals with organ systems
• Paired nerve cords
Parasite Life Cycle: Blood Fluke
Parasite Life Cycle: Tapeworm
Annelids
 Segmented worms (earthworms, polychaetes)
and leeches
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Closed circulatory system
Digestive and excretory systems
Nervous system, ganglia in each segment
Muscles and fluid in chambers act as a
hydrostatic skeleton
head
end
secretory organ
Polychaete
Mollusks (Phylum Mollusca)
 Including gastropods (snails), bivalves
(scallops), chitons, nudibranchs, cephalopods
• 100,000 named species
Aquatic Snail Body Plan
Cephalopods
 The fastest (squids), largest (giant squids), and
smartest (octopuses) invertebrates
 Have a mantle
• Sheetlike part of the body mass, draped back on
itself
Cephalopods
Cuttlefish Body Plan
Roundworms (Phylum Nematoda)
 More than 22,000 kinds of roundworms
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Free-living decomposers or parasites
Some agricultural pests and human parasites
Cylindrical body with bilateral features
A complete gut
Organ systems in a false coelom
Parasitic Roundworms
Key Concepts:
BILATERAL INVERTEBRATES
 Most animals show bilateral symmetry
 Bilateral animals have tissues, organs, and
organ systems
 All adult tissues arise from two or three simple
layers that form in early embryos
Simple Arthropods
 Trilobite, millipede, centipede
Arthropod Characteristics
 Key arthropod adaptations
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Hardened exoskeleton
Jointed appendages
Specialized and fused segments (wings)
Efficient respiratory and sensory structures (eyes,
antennae)
Chelicerates
 Horseshoe crabs and arachnids (spiders,
scorpions, ticks, and mites)
• Predators, parasites, or scavengers
Crustaceans
 Mostly marine crustaceans (crabs, lobsters,
barnacles, krill, and copepods)
Insect Diversity
Unwelcome Arthropods
 Spiders, ticks, and mosquitoes
Echinoderms
 Sea stars, sea urchins, sea cucumbers, etc.
• Exoskeleton with spines, spicules, or plates of
calcium carbonate
• Water-vascular system with tube feet
• Adults are radial, but bilateral traits appear in
larval stages
Echinoderms: “Spiny-Skinned”
Fig. 23.34, p.381
Animal Evolution –
The Vertebrates
Chordate Heritage
 Four features define chordates
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A notochord
A dorsal hollow nerve cord
A pharynx with gill slits
A tail extending past the anus
 All features form in embryos
• May or may not persist in adults
Invertebrate Chordates
 Tunicates and lancelets (marine filter-feeders)
Lancelet Body Plan
Craniates
 Chordates with a braincase of cartilage or bone
• Hagfish (jawless fish): Simplest modern craniate
Vertebrate Evolution
 Key innovations laid the foundation for adaptive
radiations of vertebrates
• Vertebral column of cartilaginous or bony
segments
• Jaws evolved in predatory fishes
• Gills evolved in water, then lungs for dry land
• Paired fins were a starting point for other limbs
 Gill-Supporting Structures
Key Concepts:
TRENDS AMONG VERTEBRATES
 In some vertebrate lineages, a backbone replaced the notochord as
the partner of muscles used in motion
 Jaws evolved, sparking the evolution of novel sensory organs and
brain expansions
 On land, lungs replaced gills, and more efficient blood circulation
enhanced gas exchange
 Fleshy fins with skeletal supports evolved into limbs, now typical of
vertebrates on land
Jawed Fishes and Tetrapods
 Jawed fishes
• Cartilaginous fishes (sharks and rays)
• Bony fishes
 Body plans adapted to life in water
• Streamlined shape reduces drag
• Swim bladder (in bony fishes) adjusts buoyancy
Cartilaginous Fishes
Bony Fishes
 The most diverse vertebrates
• Lungfishes
• Lobe-finned fishes (coelacanth)
• Ray-finned fishes
Early Lineages
 Coelacanth, lungfish, and Devonian tetrapod
Amphibians
 Frogs, toads, and salamanders
• Carnivorous vertebrates
• Adapted to life on land (lungs, 3-chambered
heart)
• Nearly all return to the water to reproduce
Amphibian Evolution
Amniotes
 First vertebrates able to complete their life cycle
on dry land
• Water-conserving skin and kidneys
• Amniote eggs (four membranes)
• Active life-styles
Dinosaur Extinctions
 K-T asteroid impact hypothesis
• A huge asteroid impact caused extinction of last
dinosaurs; spared earliest birds and mammals
Modern Reptiles
 Major Groups
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Turtles (shell attached to skeleton)
Lizards (the most diverse reptiles)
Snakes (limbless)
Crocodilians (closest relatives of birds)
Fig. 24.17, p.397
Fig. 24.17, p.397
Fig. 24.17, p.397
Reptile Characteristics
 General characteristics
• Live on land or in water
• Cold-blooded
• Have a cloaca (opening for wastes and
reproduction)
• Eggs are fertilized in the body, usually laid on
land
hindbrain, midbrain,
forebrain
olfactory lobe
(sense of smell)
spinal vertebral
column
cord
gonad
kidney (control of water,
solute levels in internal
environment)
snout
unmatched rows
of teeth on upper
and lower jaws esophagus lung heart liver stomach
intestine
cloaca
Fig. 24.16, p.396
Birds
 Birds are the only modern animals with feathers
Bird Eggs
 Birds are warm-blooded amniotes
Adaptations for Flight and Migration
 Feathers, lightweight bones, and highly efficient
respiratory and circulatory systems
Mammals
 Animals with hair, females that nourish young
with milk from mammary glands, a single lower
jawbone and four kinds of teeth
Modern Mammals
 Three major lineages
• Egg-laying mammals (monotremes)
• Pouched mammals (marsupials)
• Placental mammals, the most diverse and
widespread mammals
Three Major Lineages
Placental Mammals
Primates
Primate Evolution
 Key trends
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Better daytime vision
Upright walking (bipedalism)
More refined hand movements
Smaller teeth
Bigger brains
Social complexity (extended parental care;
culture evolved in some lineages)
The Foramen Magnum
 Four-legged walkers versus upright walkers
Emergence of Early Humans
 Hominoids and hominids originated in Africa
Australopiths: Upright Walking
Early Humans
 Humans (Homo) arose 2 million years ago
• H. habilis was an early toolmaking species
• H. erectus dispersed into Europe and Asia
Emergence of Modern Humans
 Extinct Neandertals and modern humans are
close relatives with distinct gene pools
 Modern H. sapiens evolved 195,000 years ago
Dispersal of Homo sapiens
 Based on fossils and studies of genetic markers