The Origins of Life on
Earth Part Deux
Biology 2, College of the Atlantic
Spring 2002
Phylum Chordata
Evolution of the Vertebrates
Protistan Ancestor
(Choanoflagellate)
Asymmetrical
Parazoa
Porifera
Symmetrical
Eumetazoa
Radiata
Diploblastic
Cnidaria
Bilateria
Triploblastic
Acoelomate
Platyhelminthes
Coelomates
Pseudocoelomate
Nematoda
Eucoelomate
Protostome
Mollusca
Annelida
Arthropoda
Deuterostome
Echinodermata
Chordata
Deuterostomes
• Triploblastic, bilaterally symmetrical
• Eucoelomate (true body cavity fully isolated
by layers of mesoderm-derived tissue)
• During embryonic development, blastula/
gastrula exhibits
– Radial/Indeterminate cleavage
– Blastoporeanus (i.e. mouth develops second)
– Enterocoelous development
Chordata
• Chordates are eucoelomate deuterostomes,
exhibiting segmentation
• Phylum Chordata consists of three subphyla
– Urochordata (Tunicates)
– Cephalochordata (Lancelets)
– Vertebrata
• All chordates share four characteristics
exhibited at some point during development
• Single, hollow, dorsal nerve cord
• in vertebrates, differentiates into CNS [brain
and spinal cord]
• Notochord
• a flexible rod that runs the length of the body,
provides rudiments of an endoskeleton
• in vertebrates, becomes the vertebral column)
• Pharyngeal slits
• openings between intestinal tract and outside in
region of pharynx
• in many species, develop into gills
• Muscular post-anal tail
Urochordata - the Tunicates
• Marine, sessile filter feeders
• Larvae are free-swimming
• Adult form
– loses notochord and nerve cord
– secretes a cellulose* tunic
– Develops an incurrent/excurrent siphon
system, water flow by cilia
– Has pharyngeal slits that filter food onto an
endostyle
Cephalochordata - Lancelets
– Laterally compressed filter feeders that live
in sand
– Small sub-phylum, 23 species, most
belong to Brachiostoma (formerly
Amphioxus)
– Obvious segmentation in muscle groups
– Water in through oral hood, out through
pharyngeal slits (true intestinal tract)
– Molecular evidence suggest closest
ancestor to vertebrates
Vertebrata
• Vertebrates are characterized by
– Vertebral column (protects dorsal nerve
cord
– Skull (protects brain)
– Neural crest (responsible for development
of various organs)
– Internal organs (including heart and
associated closed circulatory system
– Endoskeleton (bone or cartilage)
Evolution of the vertebrates
Principally a marine sub-phylum (>50%)
– Two extant classes of jawless fish
(Ostracoderms)
• Agnatha (lampreys)
• Cephalaspidomorphi (hagfish)
– Two extant and two extinct classes of
jawed fish
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Acanthodii (spiny fish)
Placodermi (armored fish)
Chondrichthyes (cartilegenous fish)
Osteichthyes (bony fish)
A movement towards land
• Amphibia were the first vertebrates to
assume a terrestrial lifestyle
• Reptilia became further independent by
evolution of amniotic egg
• Aves and Mammalia both developed
from Reptilia independently
Basic characteristics of fish
• Aquatic, propulsion by post-anal tail
• Gills: pharyngeal slits develop into
respiratory surfaces
• Vertebral column, including cranium
• Single loop circulation using heart
• Inability to synthesize aromatic amino
acids (Phenylalanine, Tryptophan,
Tyrosine)
The jawless fish (Ostracoderms)
• Bottom-dwellers or parasitic, dominant
in Silurian (438-360 mya)
• Specialized mouth into a sucker with
teeth
• Finless, move through sinusoidal
motion (aided by vertebral column and
post-anal tail muscular development)
• Extant representation by lampreys and
hagfish
JAWS!!
• Developed approx. 410 mya from frontmost gill arch (supports first pharyngeal
slit). Teeth developed dermally
– First seen in Acanthodii (extinct), that also
developed paired fins
– Succeeded by the Placodermii (extinct) more successful predators, some 30’ long
– Chondrichthyes is the first extant jawed group
Chondrichthyes Cartilegenous fish
– Evolved 400 mya; more efficient jaw
– Superior design for swimming
– Fully developed caudal tail, high
maneuverability
– Endoskeleton made of cartilage (not a
regressive trait)
– Ovoviviparous
– Extant chondrichthyian species includes sharks
and rays. Extinct species include Carcharadon
Osteichthyes - the bony fish
– Parallel evolution with cartilegenous fish (400 mya)
– Key difference is ossification of endoskeleton
– Other developments exclusive to bony fish include
• Swim bladder
• Fully developed lateral line system
• Gill cover (operculum), improving ventilation
– Two super-orders include ray-finned and lobefinned fish
• Lobe-finned fish developed precursors to lungs and
limbs
Amphibia - the move to land
• Evolved from lobe-finned fish (probably
closely related to lungfish) about 390
mya, although most species are now
extinct.
• Three orders:
– Anura (“tail-less”—frogs)
– Urodela (“with tail”—salamanders, newts)
– Apoda (“legless”—caecilians)
• Characterized by modifications for
movement on land
– Legs (unless burrowers [caelilians])
– Improved metabolism through
• Cutaneous and lung respiration (by positive
pressure ventilation)
• Partially divided heart (more efficient for circulation
of oxygenated blood)
Reproduction is still aquatic-based
“Amphibia” (two lives) refers to metamorphosis
Reptilia - the first amniotes
• Amniotic egg was an adaptation that
provided first true independence from
water
– Extra-embryonic membranes, including
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Chorion (gas-permeable, waterproof)
Amnion (protection of embryo in fluid-filled sac)
Yolk sac (food for development of embryo
Allantois (storage of waste products)
Amniotic diversity
Ancestral amniot e
Eosuchian
Sauropsids
Anapsids
(t urtles)
Ichthyosaurs/
Pleiosaurs
Lizards
Synapsids
Diapsids
Therapsids
Squamates
Snakes
Crocodiles
Pterosaurs
Archosaurs
(Thecodont s)
Mammalia
Saurischians
Ornithischians
Aves
Reptile facts
• Three main extant orders
– Chelonia (turtles)
– Squamata (lizards and snakes)
– Crocodilia (crocodiles/alligators)
• Evolution towards a four-chambered
heart and endothermy
• Dermal scales (preadaptation for
feathers?)
Aves: birds -flight revisited
• Birds are essentially dinosaurs with
feathers
• Closest ancestor Archaeopteryx,
endothermic, probably developed
feathers for insulation, and was a
bipedal glider
• Adaptations for flight include
– Feathers
– Hollow bones
– Highly developed pectoral muscles/keel
– Efficient respiration (evolution of lungs to
include air sacs to maximize absorption of O2)
– Efficient circulation (true 4-chambered heart,
systemic and pulmonary circuits)
– Endothermy
Mammalia
• Evolved from Reptiles independently
from class Aves
• Independent (convergent) evolution of
endothermy, in addition to evolution of
keratinous fur and lactation
• Includes the orders:
– Monotremata
– Marsupialia
– Eutheria
Monotremes
• Includes spiny anteater
and duck billed platypus
• The only example of an
oviparous mammal, laying
a reptilian-like (amniotic)
egg
• Monotremes lactate, but
have no nipples
• Found only in Australia
and New Guinea
Marsupials
• Found predominantly in Australia, with
some occurrences in South Africa (a
good example of biogeographic theory)
• Marsupials give birth to a premature
fetus that completes its development in
a maternal pouch
• Marsupials and placentals convergently
evolve along many phylogenetic lines
Eutheria (placental mammals)
• Embryonic development is completed in
the womb
• Four branches
– Chiroptera and Insectivora
– Lagomorpha, Perissodactyla, Artiodactyla,
Sirenia, Proboscidea, Cetacea
– Carnivora
– Primates, Rodentia
The Evolution of Man
The Archonta
• Approximately 65 mya, mammalian
ancestors known as Archonta diversified
into three orders
– Bats (Chiroptera)
– Tree shrews
– Primates
• Archonta were insectivorous, nocturnal,
and arboreal
“Mankind stood up first and
got smart later”
• Stephen Jay Gould
The evolution of order Primates
The phylogeny of Primates has five
distinct trends:
– Shift towards daytime vision and primary
sensory system (away from olfaction)
– Skeletal changes leading to bipedalism
– Skeletal changes leading to prehensile and
opposable hand-grips
– A shift in dentition to omnivorous diet
– Increase in skull, and brain size
– Onset of cultural evolution
• Improved vision
– migration of eye sockets to front of skull, leading to
binocular vision
– physiological and cellular improvemnets to refine
detection of color and contrast
• Palaentology provides clues to the
onset of bipedalism
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location of the foramen magnum at base of skull
lack of splayed toe
shorter, s-shaped backbone, with loss of tail
Other changes to pelvis (bowl shape), femur
angle, and forelimbs
• Improvement of grip was an adaptation
to arboreal lifestyle
– thought to have evolved to increase speed
– bipedal movement freed hands for other
tasks (not immediately for tool use)
– Prehensile and opposable grips were
preadaptations for tool use
• A change in diet...
– Dentition shifts from carnivorous (insects)
to fruits and leaves, to omnivory
– Shift from pointed jaw with emphasized
canines (monkeys) to bow-shaped jaw with
smaller teeth
• Increased brain size selected for by
need to
– coordinate movements in arboreal setting
(brachiated movement)
– operate binocular (depth of field) vision that
operates in low-level light
– cordinate complex digit movements
– develop highly altricial parental care
– increased social behaviours
• Increase in brain size led to the
development of cultural evolution
Primate taxonomy
• Two suborders, with split believed to
have occurred >50 mya
• Prosimians (“before monkeys”)
– Lemurs and Lorises
– Tarsiers
• Arthropoids
– monkeys, apes and hominids
Sub-order Arthropoida
• Oldest representative may have lived 45
mya (Eosimias, Catopithecus)
• Arthropoid characteristics include bony
eye socket, shovel-shaped incisors, and
fused forehead bones
• Families include
– New World monkeys - arboreal
cappuchins, howlers, spider monkeys
– Old World Monkeys - some-ground
dwellers, toughened pads for sitting
– Hominoids (Apes and Hominids)
The Hominoids
• Apes include Gibbons (Hylobates),
Orangutan (Pongo), Gorillas (Gorilla)
and Chipanzees (Pan)
• Most are ground dwelling and lack tails
• Some are closely related to Homo
sapiens in terms of nuclear DNA:
– Gorilla (97.7%) diverged 8 mya
– Pan (98.4%) diverged 6 mya
• Hominids include Ardipithecus,
Australopithecus, and Homo
Hominids
• Earliest hominids diverged ~5 mya
(Ardipithecus)
• Distinguished by bipedal stance
• Most fossil specimens of early hominids
are Australopithecus
• Appearance in fossil record coincident
with cooling of Africa to convert rain
forests to savannah plain, resulting in a
rapid adaptive radiation of at least 6
species
Australopithecus - forerunner
or evolutionary dead end?
• Apelike - large face, small skull/brain
size (400 cm3). Bipedal, developed grip
• Well developed teeth, indicating
movement to tougher foods that
required more physical digestion
• Systematics provide little information on
relationships between various species
of Australopithecus
– e.g. “Lucy”, 1974: A. afarensis
The first humans, early Homo
• Genus Homo distinguished by larger
brain size ≤ 700 cm3, evolving 2 mya
• First association with tools: Homo
habilis, “handy man”. Short, long arms intiially thought to be a species of
Australopithecus
• Mystery species H. rudolfensis had
larger brain, appeared to co-exist with
H. habilis
Late Homo
• Homo erectus, the first true human?
• Very large brain: 1000cm3
• Probably not the direct ancestor of
modern man
• May have had speech ability
• Cave-dwelling hunters, used fire, clothed
in animal skins
• Nomadic, responsible for rapid spreading
into Asia and Europe by 0.5 mya
The final phase...
• Homo neanderthalensis orginated in
Africa, but rapidly spread to other
continents. Very common 100,000 years
ago, but not the ancestor of modern
man. Disappeared 35,000 ya
• Examination of burial grounds suggests
social ritualism - the first evidence of
abstract thought
Homo sapiens
• probably evolved from H. heidelbergensis,
a close relative of H. neaderthalensis
• Oldest known H. sapiens fossil is 130,000
years old, cranial capacity of 1500 cm3
• Controversy over origin
– Monogenesis model*
– Multiregional model
Postscript
• “From 40,000 years ago to today, human
evolution has been cultural, not biological”
• Cambell suggests three stages to that
cultural evolution:
– hunter-gatherer phase
– agriculture phase
– Industrial revolution
• For the first time in the history of the planet
comes a species capable of defying natural
selection