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Animal Evolution – The Vertebrates. Chapter 23. Chordates. Most are coelomate, bilateral animals All share four features: Notochord supports body Nervous system develops from dorsal nerve cord Embryos have pharynx with slits Embryos have tail that extends past anus. Lancelet Body Plan.

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Animal evolution the vertebrates l.jpg

Animal Evolution – The Vertebrates

Chapter 23

Chordates l.jpg

  • Most are coelomate, bilateral animals

  • All share four features:

    • Notochord supports body

    • Nervous system develops from dorsal nerve cord

    • Embryos have pharynx with slits

    • Embryos have tail that extends past anus

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Lancelet Body Plan





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Invertebrate Chordates

  • Many of the animals that preceded vertebrates were like the simplest chordates – the urochordates

    • Sea squirts

    • Other tunicates

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Larval Form of a Sea Squirt


nerve cord


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atrial opening (water out)

oral opening (water in)

Adult Tunicate

pharynx with gill slits

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  • Lancelets

  • Fish-shaped filter feeders that lie buried in sediments

  • Chordate characteristics of adult:

    • Notochord lies under dorsal nerve cord

    • Pharynx has gill slits

    • Tail extends past anus

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Hagfish Body Plan


gill slits (twelve pairs)

mucus glands

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Trends in the Evolution of Vertebrates

  • Shift from notochord to vertebral column

  • Nerve cord expanded into brain

  • Evolution of jaws

  • Paired fins evolved, gave rise to limbs

  • Gills evolved, gave rise to lungs

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  • Cranium is a chamber of cartilage or bone that encloses all or part of a brain

  • First craniates evolved by 530 million years ago

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Evolution of Jaws

  • First fishes lacked jaws

  • Jaws are modifications of the anterior gill supports

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Evolution of Fishes

cartilaginous fishes

ray-finned fishes

lobe-finned fishes












jawed vertebrates

lungs or swim bladder



ancestral chordates

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Jawed Fishes

  • Most diverse and numerous group of vertebrates

  • Two classes:

    • Cartilaginous fishes

    • Bony fishes

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Cartilaginous Fishes

  • Most are marine predators

  • Cartilaginous skeleton

  • Main groups:

    • Skates and rays

    • Sharks

    • Chimaeras (ratfishes)

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Bony Fishes

  • Includes 96 percent of living fish species

  • Three subclasses:

    • Ray-finned fishes

    • Lobe-finned fishes

    • Lung fishes

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Evolution of Amphibians

  • Lobe-finned fishes arose during the early Devonian

  • Used their fins to travel over land from pool to pool

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Early Amphibians

  • Lungs became more effective

  • Chambers of the heart became partially separated, making circulation more efficient

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Modern Amphibians

  • All require water at some stage in the life cycle; most lay eggs in water

  • Lungs are less efficient than those of other vertebrates

  • Skin serves as respiratory organ

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Living Amphibian Groups

  • Frogs and toads

  • Salamanders

  • Caecilians

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Evolution of Reptiles

  • Reptiles arose from amphibians in the Carboniferous

  • Adaptations to life on land

    • Tough, scaly skin

    • Internal fertilization

    • Amniote eggs

    • Water-conserving kidneys

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Reptilian Radiation

  • Adaptive radiation produced numerous lineages

  • Extinct groups include:

    • Therapsids (ancestors of mammals)

    • Marine plesiosaurs & ichthyosaurs

    • Dinosaurs and pterosaurs

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Living Reptiles

Four orders made it to the present day:




Snakes and lizards

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Crocodile Body Plan









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Turtles and Tortoises

  • Armorlike shell

  • Horny plates instead of teeth

  • Lay eggs on land

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  • Only two living species

  • Live on islands off the coast of New Zealand

  • Look like lizards, but resemble amphibians in some aspects of their brain and in their way of walking

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Lizards and Snakes

  • Largest order (95 percent of living reptiles)

  • Most lizards are insectivores with small peglike teeth

  • All snakes are carnivores with highly movable jaws

venom gland

hollow fang

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  • Only birds have feathers

  • Arose from reptilian ancestors

    • Feathers are highly modified reptilian scales

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Amniote Eggs

  • Like reptiles, birds produce amniote eggs

  • Inside the egg, the embryo is enclosed in a membrane called the amnion

  • Amnion protects the embryo from drying out

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Adapted for Flight

  • Four-chambered heart

  • Highly efficient respiratory system

  • Lightweight bones with air spaces

  • Powerful muscles attach to the keel

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  • Hair

  • Mammary glands

  • Distinctive teeth

  • Highly developed brain

  • Extended care for the young

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Mammalian Origins

  • 200 million years ago, during the Triassic, synapsids gave rise to therapsids

  • Therapsids were the reptilian ancestors of mammals

  • The first mammals had evolved by the Jurassic

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Three Mammalian Lineages

  • Monotremes

    • Egg-laying mammals

  • Marsupials

    • Pouched mammals

  • Eutherians

    • Placental mammals

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Living Monotremes

  • Three species

    • Duck-billed platypus

    • Two kinds of spiny anteater

  • All lay eggs

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Living Marsupials

  • Most of the 260 species are native to Australia and nearby islands

  • Only the opossums are found in North America

  • Young are born in an undeveloped state and complete development in a permanent pouch on mother

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Living Placental Mammals

  • Most diverse mammalian group

  • Young develop in mother’s uterus

  • Placenta composed of maternal and fetal tissues; nourishes fetus, delivers oxygen, and removes wastes

  • Placental mammals develop more quickly than marsupials

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Earliest Primates

  • Primates evolved more than 60 million years ago during the Paleocene

  • First primates resemble tree shrews

    • Long snouts

    • Poor daytime vision

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  • Apes, humans, and extinct species of their lineages

  • In biochemistry and body form, humans are closer to apes than to monkeys

  • Hominids

    • Subgroup that includes humans and extinct humanlike species

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From Primates to Humans

“Uniquely” human traits evolved through modification of traits that evolved earlier, in ancestral forms

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Trends in Lineage Leading to Humans

  • Less reliance on smell, more on vision

  • Skeletal changes to allow bipedalism

  • Modifications of hand to allow refined hand movements

  • Bow-shaped jaw and smaller teeth

  • Longer lifespan and longer period of dependency

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Adaptations to anArboreal Lifestyle

  • During the Eocene, certain primates became adapted to life in trees

    • Better daytime vision

    • Shorter snout

    • Larger brain

    • Forward-directed eyes

    • Capacity for grasping motions

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  • Earliest known is A. anamensis

  • A. afarensis and A. africanus arose next

  • All three were slightly built (gracile)

  • Species that arose later, A. boisei and A. robustus, had heavier builds

  • Exact family tree is not known

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Humans Arise

  • First member of the genus Homo is H. habilis

  • Lived in woodlands during late Miocene

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Homo erectus

  • Evolved in Africa

  • Migrated into Europe and Asia about 1.5 million - 2 million years ago

  • Had a larger brain than H. habilis

  • Was a creative toolmaker

  • Built fires and used furs for clothing

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Homo sapiens

  • Modern man evolved by 160,000 years ago

  • Had smaller teeth and jaws than H. erectus

  • Facial bones were smaller, skull was larger

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Colonization by H. sapiens

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Homo Neanderthalensis

  • Early humans that lived in Europe and Near East

  • Massively built, with large brains

  • Disappeared when H. sapiens appeared

  • DNA evidence suggests that they did not contribute to modern European populations

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Where Did H. sapiens Arise?

  • Two hypotheses:

    • Multiregional model

    • African emergence model

  • Both attempt to address both biochemical and fossil evidence

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Multiregional Model

  • Argues that H. erectus migrated to many locations by about 1 million years ago

  • Geographically separated populations gave rise to phenotypically different races of H. sapiens in different locations

  • Gene flow prevented races from becoming species

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African Emergence Model

  • Argues that H. sapiens arose in sub-Saharan Africa

  • H. sapiens migrated out of Africa and into regions where H. erectus had preceded them

  • Only after leaving Africa did phenotypic differences between races arise

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Earliest Fossils Are African

  • Africa appears to be the cradle of human evolution

  • No human fossils older than 1.8 million years exist anywhere but Africa

  • Homo erectus left Africa in waves from 2 million to 500,000 years ago