Animal Evolution The Vertebrates

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Animal Evolution The Vertebrates

• Chapter 23

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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

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Lancelet Body Plan
DORSAL, TUBULAR NERVE CORD
TAIL EXTENDING PAST ANUS
NOTOCHORD
PHARYNX WITH GILL SLITS
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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
notochord
nerve cord
gut
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Adult Tunicate
atrial opening (water out)
oral opening (water in)
pharynx with gill slits
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Cephalochordates

• 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
tentacles
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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Craniates

• 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
tunicates
lancelets
hagfishes
lampreys
amphibians
reptiles
birds
mammals
lungfishes
amniotes
tetrapods
jawed vertebrates
lungs or swim bladder
vertebrates
craniates
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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Evolution of Amphibians
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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
• Crocodilians
• Turtles
• Tuataras
• Snakes and lizards

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Crocodile Body Plan
stomach
brain
kidney
lung
heart
liver
intestine
cloaca
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Turtles and Tortoises

• Armorlike shell
• Horny plates instead of teeth
• Lay eggs on land

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Tuataras

• 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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Birds

• 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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Amniote Egg
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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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Mammals

• 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 mothers 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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Hominoids

• 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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Australopiths

• 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

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Primate Family Tree