Vertebrae Evolution

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Vertebrae Evolution Diversity
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Invertebrate Chordates The Origin of Vertebrates

• The vertebrates are part of a phylogenetic branch
  of the animal kingdom consisting of
  deuterostomes.
• The deuterostome branch has two main modern
  phyla the chordates and the enchinoderms.

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Four anatomical features characterize the phylum
Chordata
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• Chordates are named for a skeletal structure, the
  notochord, present in all chordate embyros. The
  notochord is a longitudinal, flexible rod located
  between the digestive tube and the nerve cord.
  Composed of large, fluid-filled cells encased in
  fairly stiff, fibrous tissue, it provides
  skeletal support through most of the length of
  the animal.
• In most vertebrates a more complex, jointed
  skeleton develops, and the adult only retains
  remnants of the embryonic notochord, the
  gelatinous material of the disks between the
  vertebrae of humans.

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• The nerve cord of a chordate embryo develops from
  a plate of ectoderm that rolls into a tube
  located dorsal to the notochord. The result is a
  dorsal, hollow nerve cord unique to chordates.
  The nerve cord of a chordate embryo develops into
  the central nervous system the brain and spinal
  cord.
• Other animal phyla have solid nerve cords,
  usually ventrally located.

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• In the embryos of all chordates, a tube that
  eventually develops into the digestive tract
  extends between the future mouth and the future
  anus. The region posterior to the mouth is the
  pharynx, which has pairs of pouches in chordate
  embryos.
• The pharyngeal pouches of various vertebrates
  develop into a diversity of other structures,
  including gills and components of auditory organs.

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• Most chordates have a tail extending posterior to
  the anus. The chordate tail contains skeletal
  elements and muscles and provides much of the
  propulsive force in many aquatic species.
• Most nonchordates have a digestive tract that
  extends nearly the whole length of the body.

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Invertebrate chordates provide clues to the
origin of vertebrates
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Subphylum Urochordata

• Urochordatesare commonly called tunicates. Most
  tunicates are sessile marine animals, but others
  are planktonic or colonial. Adult tunicates
  scarcely resembles a chordate, it doesnt display
  a trace of notochord, nerve cord or tail. All
  four chordate trademarks are manifest in the
  larval form of some groups of tunicates.

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Subphylum Cephalochordata

• Known as lancelets because of their bladelike
  shape, cephalochordates closely resemble the
  idealized chordate. The notochord dorsal, hollow
  nerve cord numerous gill slits and postanal
  tail all persist into the adult stage. Lancelets
  are small animals that live in the sand at the
  bottom of the sea in coastal regions.

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The Relationship Between Invertebrate Chordates
Vertebrates

• Molecular evidence suggests that cephalochordates
  are the vertebrates closest relatives, and
  urochordates are their next closest relatives.

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• According to one hypothesis, there are two stages
  in the evolution of vertebrates from
  invertebrates In the first stage, an ancestral
  cephalochordate evolved from a form resembling a
  modern urochordate larva in the second stage, a
  vertebrate evolved from a cephalochordate.
• The first stage may have been preceded by
  paedogenesis, the precocious development of
  sexual maturity in a larva.

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Introduction To The Vertebrates
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Neural crest, pronounced cephalization, a
vertebral column, and a closed circulatory system
characterize the subphylum Vertebrata

• In vertebrates, a group of embryonic cells called
  the neural crest forms near the dorsal margins of
  the closing neural tube. The neural crest
  contributes to the formation of certain skeletal
  elements such as some of the bones and cartilage
  of the cranium, and many other structures that
  distinguish vertebrates from other chordates.

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An overview of vertebrate diversity

• Gnathostomes have jaws and also have two sets of
  paired appendages. Among these jawed vertebrates,
  there are various classes of aquatic animals that
  we generally call fishes the cartilaginous
  fishes and three classes of bony fishes. In the
  case of fishes, the two sets of paired appendages
  are fins that function in swimming.

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• All the other gnathostomes are tetrapods, in
  which the two sets of paired appendages are
  modified as legs that can support the animal on
  land. The tetrapods include the amphibians along
  with the clade identified as amniotes. The
  amniotes are named for the amniotic egg.

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Jawless Vertebrates
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Some extinct jawless vertebrates had ossified
teeth bony armor

• A diversity of taxa informally called
  ostracoderms thrived from about 450 to 375
  million years ago. Most species were small, less
  than 50 cm in length. Most lacked paired fins and
  apparently were bottom dwellers, but there were
  also some more active species with paired fins.

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• Ostracoderm means shelled skin, a reference to
  the armor of bony plates that encased these
  animals. The plates may represent an early
  evolutionary stage of ossification, the hardening
  of connective tissue that occurs when specialized
  cells secrete calcium and phosphate, which
  precipitate as calcium phosphate, a hard mineral
  salt.
• Even earlier evidence of ossification is found in
  fossils of ancient vertebrates called conodonts,
  which date back as far as 510 million years.
  These vertebrates are named for their cone-shaped
  toothlike structures, which are ossified.

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Fishes and Amphibians
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Vertebrate jaws evolved from skeletal supports of
the pharyngeal slits

• The remaining gill slits, no longer required for
  suspension feeding, remained as the major site of
  respiratory gas exchange with the external
  environment.
• New adaptations usually evolve by the
  modification of existing structures. Evolution is
  limited by the raw material with which it must
  work.

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Class Chondrichthyes Sharks rays have
cartilaginous skeletons

• The vertebrates of the class Chondrichthyes are
  called cartilaginous fishes because they have
  relatively flexible endoskeletons made of
  cartilage rather than bone. Jaws and paired fins
  are well developed in the cartilaginous fishes.
  The cartilaginous skeleton is a derived
  characteristic because it evolved secondarily.

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• The largest sharks and rays are suspension
  feeders that feed on plankton. Most sharks,
  however, are carnivores that swallow their prey
  whole or use their powerful jaws and sharp teeth
  to tear flesh from animals too large to swallow
  in one piece. Shark teeth probably evolved from
  the jagged scales that cover the abrasive skin.
• The digestive tract of many sharks is
  proportionately shorter than the digestive tube
  of other vertebrates. Within the shark intestine
  is a spiral valve, a corkscrew-shaped ridge that
  increases the surface area and prolongs the
  passage of food along the short digestive tract.

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• Sharks have sharp vision but cannot distinguish
  colors. The nostrils function only for olfaction
  (smelling), not breathing. Along with eyes and
  nostrils, the shark head also has a pair of
  regions in the skin that can detect electrical
  fields generated by the muscle contractions of
  nearby animals.
• Shark eggs are fertilized internally. Some
  species of sharks are oviparous, which means they
  lay eggs that hatch outside the mothers body.
  Other species are ovoviviparous, which means they
  retain the fertilized eggs in the oviduct they
  are nourished by the egg yolk and then they hatch
  within the uterus. A few species are viviparous
  the young develop within the uterus, nourished
  prior to birth by nutrients received from the
  mothers blood through a placenta.

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Osteichthyes The extant classes of bony fishes
are the ray-finned fishes, the lobe-finned
fishes, the lungfishes

• Nearly all bony fishes have an ossified
  endoskeleton with a hard matrix of calcium
  phosphate. Glands in the skin of a bony fish
  secrete a mucus that gives the animal its
  characteristic sliminess, and adaptation that
  reduces drag during swimming. Bony fishes breath
  by drawing water over 4 or 5 pairs of gills
  located in chambers covered by the operculum.
  Another adaption of most bony fishes is the swim
  bladder. Most species are oviparous, however,
  internal fertilization and birthing characterize
  other species.

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• Nearly all the families of fishes familiar to us
  are ray-finned fishes. Examples are bass, trout,
  perch, tuna and herring. The fins, supported
  mainly by long flexible rays, are modified for
  maneuvering, defense, and other functions.

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• Lobe-finned fishes have muscular pectoral and
  pelvic fins supported by extensions of the bony
  skeleton. Many lobe-fins were large, apparently
  bottom dwellers.

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• Three genera of lungfishes live today in the
  Southern Hemisphere. They generally inhabit
  stagnant ponds and swamps, surfacing to gulp air
  into lungs connected to the pharynx of the
  digestive tract. Lungfishes also have gills,
  which are the main organs for gas exchange. When
  ponds shrink during the dry season, some
  lungfishes can burrow in the mud and aestivate.

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Tetrapods evolved from specialized fishes that
inhabited shallow water

• Amphibians were the first tetrapods to spend a
  substantial portion of their time on land. As the
  earliest tetrapods, amphibians benefited from an
  abundance of food and relatively little
  competition.

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Class Amphibia Salamanders, frogs, caecilians
are the 3 extant amphibian orders

• Today the amphibians are represented by a total
  of about 4800 species of salamanders (order
  Urodela), frogs (order Anura), and caecilians
  (order Apoda).
• Some urodeles are entirely aquatic, but others
  live on land as adults or throughout life.

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• Anurans are more specialized than urodeles for
  moving on land. Frogs display a great variety of
  adaptations that help them avoid being eaten by
  larger predators. They exhibit color patterns
  that camouflage. The skin glands of frogs secrete
  distasteful, or even poisonous, mucus. Many
  poisonous species have bright coloration that
  apparently warns predators.

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• Apodans, the caecilians, are legless and nearly
  blind, and superficially resemble earthworms. The
  reduction of legs evolved secondarily from a
  legged ancestor.

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• Amphibian means two lives, a reference to the
  metamorphosis of many frogs. In spite of the name
  amphibian, however, many frogs do not go through
  the aquatic tadpole stage, and many amphibians do
  not live in a dualistic life. There are some
  strictly aquatic and strictly terrestrial frogs,
  salamanders, and caecilians. Most amphibians
  maintain close ties with water and are most
  abundant in damp habitats such as swamps and rain
  forests. They reply heavily on their moist skin
  to carry out gas exchange with the environments
  some terrestrial species lack lungs and breathe
  exclusively through their skin and oral cavity.

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Amniotes
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Evolution of amniotic egg expanded the success of
vertebrates on land

• The amnioteclade consists of the mammals, birds,
  and vertebrates we commonly call reptiles. The
  amniotic egg, a reproductive adaptation that
  enabled terrestrial vertebrates to complete their
  life cycles on land and sever their last ties
  with their aquatic origins. Amniotic eggs have a
  shell that retains water and can hen be laid in a
  dry place.

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• The shells of bird eggs are calcareous and
  inflexible, while shells of many reptile eggs are
  leathery and flexible. Most mammals have
  dispensed with the shell instead, the embryo
  implants in the wall of the uterus and obtains
  nutrients from the mother.

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• Reptiles, birds, and mammals all have specialized
  membranes within the amniotic call called the
  extra-embryonic membranes. The extra-embryonic
  membranes arent part of the body of the
  developing animal, these structures function in
  gas exchange, waste storage, and the transfer of
  stored nutrients to the embryo.
• Amniotes also show other adaptations to
  terrestrial life, including waterproof skin and
  increasing use of the rib cage to ventilate the
  lungs.

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A reptilian heritage is evident in all amniotes

• Reptilian Characteristics
• Scales containing the protein keratin waterproof
  the skin of a reptile, helping prevent
  dehydration in dry air. Reptiles cant breathe
  through their keratinized dry skin, and they
  obtain all their oxygen with lungs.

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• Most reptiles lay shelled amniotic eggs on land.
  Fertilization must occur internally, before the
  shell is secreted as the egg passes through the
  reproductive tract.
• They are sometimes labeled cold-blooded animals
  because they do not use their metabolism
  extensively to control body temperature. Because
  they absorb external heat rather than generating
  much of their own, reptiles are said to be
  ectotherms.

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Birds began as feathered reptiles

• Characteristic of Birds
• Almost every part of a typical birds anatomy is
  modified in some way that enhances flight. The
  bones have an internal structure that is
  honeycombed. Another adaptations reducing the
  weight of birds is the absence of some organs.
  Birds are endothermic. Senses must be acute.

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Mammals diversified extensively in the wake of
the Cretaceous extinctions

• Mammalian Characteristics
• All mammalian mother nourish their babies with
  milk. Hair is another mammalian characteristic.
  Mammals are endothermic and most have an active
  metabolism. Most are born rather than hatched.
  Have generally larger brains and many species are
  capable learners. Differentiation of teeth is
  another important mammalian trait.

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• Monotremes
• The platypuses and the echidnas are the only
  mammals that lay eggs. Monotremes have hair and
  produce milk. On the belly of the mother are
  specialized glands that secrete glands. After
  hatching, the baby sucks the milk from the fur of
  the mother, who has no nipples.

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• Marsupials
• Examples are opossums, kangaroos, bandicoots, and
  koalas. A marsupial is born very early in its
  development and completes its embryonic
  development while nursing. In most species, the
  nursing young are held within a maternal pouch
  called a marsupium.

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• Eutherian (Placental) Mammals
• Young eutherians complete their embryonic
  development within the uterus, joined to the
  mother by the placenta.

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Primates The Evolution of Homo sapiens
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Primate evolution provides a context for
understanding human origins

• Some General Primate Characteristics
• Most primates have hands and feet adapted for
  grasping, and have large brains and short jaws,
  giving them a short face. Have forward-looking
  eyes and flat nails.
• The opposable thumb is only found in monkeys,
  apes, and humans.

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• Modern Primates
• Examples of prosimians (premonkeys) are lemurs
  of Madagascar and the lorises, pottos, and
  tarsiers that live in tropical Africa and
  southern Asia.
• The antropoids include monkeys, apes, and humans.
  

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Humanity is one very young twig on the vertebrate
tree

• Paleonanthropology is the study of human origins
  and evolution.
• Hominoid refers to great apes and humans fossils
  are more closely related to chimpanzees,
  gorillas, or orangutans.
• Hominid refers to the twigs of evolutionary tree
  that are more related to us than others living
  species.

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• Some Major Features of Human Evolution
• Brain size- Hominoids of about 6 million years
  ago had brains with volumes of about 400-450 cm3
  modern human brains average 1300 cm3.
• Jaw Shape- Hominoids had prognathic (longer)
  jaws. During human evolution, shortening of jaws
  resulted in flatter faces with more pronounced
  chins.

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• Bipedal Posture- Hominoid ancestors walked on all
  four limbs when on the ground like modern apes.
• Reduced Size Difference Between the Sexes- In
  hominoids the size difference between males and
  females is a major feature of dimorphism. Modern
  ape males weigh about twice as much as females.
  Monkey males are about 1.35 times heavier than
  females and human males average 1.2 times the
  weigh of females.

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• Some Key Changes in Family Structure- To identify
  evolutionary changes in social behavior,
  researcher rely more on comparisons between
  humans and other extant hominoids. In contrast to
  the social organization of most ape species (as
  well as most monkeys), monogamy prevails in most
  human cultures. Newborn human infants are
  dependent on their mothers and the duration of
  parental care for offspring is much longer in
  humans than in other hominoids. The extended
  parental car, couples with the large brain size,
  enhanced learning and is associated with the
  behavioral complexity of humans.