Chapter 34: Deuterostome Animals

An Overview of Deuterostome Animals

Four phyla of deuterostomes

  • Echinodermata
  • Hemichordate
  • Xenoturbellid
  • Chordates

All deuterostomes are considered bilaterians

But a remarkable event occurred early in the evolution of echinoderms

  • the origin of a unique type of type of radial symmetry
  • adult echinoderms have bodies with five-sided radial symmetry called pentaradial symmetry (STAR FISH)
  • even though both their larvae and their ancestors are bilaterally symmetric

the other remarkable event in echinoderm evolution was the origin of a unique morphological feature: a series of branching fluid-filled tubes and chambers called the water vascular system

  • tube feet
  • podia
  • endoskeleton

Hemichordates - half chordates that have an unusual opening into the throat called pharyngeal gill slits

Chordates are defined by the presence of four morphological features

  • pharyngeal gill slits
  • a still and supportive but flexible rod called a notochord – runs the length of the body
  • a bundle of nerve cells that runs the length of the body and forms the dorsal hollow nerve cord
  • a muscular post-anal tail – meaning a tail that contains muscle and extends past the anus

Three Major Lineages

  • Urochordates – tunicates or sea squirts
  • pharyngeal gill slits are present in both larvae and adults, but the notochord, dorsal hollow nerve cord, and tail are present only in the larvae
  • Cephalochordates – lancelets
  • are small, mobile suspension feeders that look a little like fish?
  • Vertebrates
  • dorsal hollow never cord is elaborated into the spinal cord.

the vertebrates are a morphically group distinguished by two traits

  • a column of cartilaginous or bony structures called vertebrae that form along the dorsal side of most species – protects spinal cord
  • cranium of skull – a bony cartilaginous or fibrous case that encloses the brain

The coordinated movements of vertebrates are possible in part because vertebrates have large brains that are divided into three distinct regions:

  • (1) a forebrain, housing the sense of smell
  • (2) a midbrain, associated with vision
  • (3) a hindbrain, responsible for balance and hearing.

vertebrate fossil record

  • cartilage and bone
  • jaw
  • tetrapod
  • amniotes and amniotic eggs - reproduce far away from water

In terms of the number of species and range of habitats occupied, the most successful lineages of deuterostomes are the echinoderms with 7000 names spices and the vertebrates with 54000 known species

  • echinoderms are widespread and abundant in marine habitats
  • in some deep-water environments, echinoderms represent 95% of the total mass of organisms
  • among vertebrates, the most species-rich and ecologically diverse lineages are the ray-finned fishes and the tetrapod

Echinoderms suspension feed, deposit feed, or harvest algae or other animals.

  • In most cases, the podia play a key role in obtaining food.

Vertebrates were not able to harvest food by biting until jaws evolved

The leading hypothesis for the origin of the jaw is that mutation and natural selection increased the size and modified the orientation of the gill arches

  • Three lines of evidence support the gill–arch hypothesis:
  • Both gill arches and jaws consist of flattened bars of bony or cartilaginous tissue that hinges and bends forward.
  • The muscles that move jaws and gill arches are derived from the same population of embryonic cells.
  • Both jaws and gill arches are derived from neural crest cells.

To explain why ray-finned fishes are so diverse in their feeding methods, biologists point to important modifications of the jaw - i.e. pharyngeal “throat” jaw

Fossils provide strong links between lungfish and the earliest land-dwelling vertebrates

  • Fossil evidence indicates a fin–to–limb transition

Wings and the ability to fly evolved independently in three lineages of tetrapod: the extinct flying reptiles called pterosaurs, birds, and insects.

To summarize, the evolution of the jaw gave tetrapod the potential of capture and process a wide array of foods and with limbs, they could move efficiently on land in search for food

Tetrapod were the first vertebrates that could breed on land.

Three major evolutionary innovations gave tetrapod this ability:

  • (1) the amniotic egg
  • (2) the placenta
  • (3) elaboration of parental care.

Besides having an outer shell or membrane that is largely watertight, an amniotic egg contains a membrane-bound supply of water in a protein-rich solution called ALBUMEN

  • the embryo itself is enveloped in a protective inner membrane known as the AMNION
  • inside an amniotic egg, the embryo is bathed in fluid
  • the egg itself is highly resistant to drying
  • The yolk sac contains nutrients, the allantois contains waste, and the chorion allows gas exchange.

Viviparous animals have an organ called the placenta that is rich in blood vessels and facilitates the flow of oxygen and nutrients from mother to offspring

  • after a development period called gestation, the embryo emerges from the mother’s body
  • placenta allows mother to nourish the child internally

The most extensive parental care observed among animals is provided by mammals and birds

With the combination of the placenta and lactation, placental mammals make the most extensive investment of time and energy in offspring known


lineage called Primates = two main groups

  • prosimians – before monkeys
  • lemurs, tarsiers, pottos lorises
  • live in trees
  • active at night
  • anthropoids – human like
  • new world monkeys, old world monkeys, gibbons,
  • HOMINIDAE – great apes
  • orangutans, gorillas, chimps, humans

Primates are distinguished by having eyes located on the front of either face

  • eyes that look forward provide better depth perception than to eyes on the side of the face

Primates also tend to have hands and feet that are efficient in grasping, flattened nails instead of claws on their fingers and toes, brains that are large relative to overall body size, complex social behaviour, and extensive parental care of offspring

  • hominids = lineage of great apes, humans, = HOMINIDAE
  • DNA – human are more closely related to chimps and then our next ancestor, gorillas
  • great apes = large body, long arms, short legs, no tail
  • orangutang = walk with their knuckles pressed to ground AND fist walk
  • gorillas + chimps = ONLY knuckle walk
  • humans = only great ape that can BIPDEAL – walk upright
  • bipedalism = shared derived character that defines the group called hominines

The Hominine or HOMONINS are a MONOPHYLETIC group comprising Homo Sapiens and more than a dozen extinct, bipedal relatives

Common ancestor of humans + chimps = AFRICA 7 MYA

  • rapidly increasing fossil record of hominin

Although naming the hominin species and interpreting their characteristics remain controversial, most researchers agree that they can be organized into four groups:


  • Four species of small apes called gracile australopithecines
  • slightly built organisms = slender/gracile
  • male = 1.5m and 36kg
  • South Africa = southern ape
  • several lines of evidence support the hypothesis that gracile australopithecines were BIPEDAL
  • i.e.. hole behind skull = orientated downwards like ours 

PARANTHROPUS - beside-human

  • three species
  • stocker – same height but 8-10kh heavier
  • skulls = much broader and robust
  • all 3 species had massive check teeth and jaws, very large cheekbones, and a sagittal crest – a flange of bone at the top of the skull
  • tremendous biting power
  • name = inspired by the hypothesis that that 3 known species = monophyletic group that was a side branch during human evolution – an independent lineage that went extinct


  • humans
  • flatter and narrower = faces
  • smaller teeth + jaws
  • larger brain cases than earlier hominin
  • appearance of early homo = parallel to appearance of tools and made of worked stone – handheld choppers or knives
  • extensive tool making = diagnostic trait of early homo


  • faces = even flatter
  • smaller teeth
  • larger brain cases than early homo
  • Cro-Magnons = accomplished painters and sculptors who buried their dead in carefully prepared graves
  • Neanderthal = made art and buried their dead in ceremonial fashion
  • H. florescence’s = found only on the island of Flores, Indonesia – also home to a species of dwarfed elephants (major source of food)
  • consisted of individuals that had brain cases smaller than australopithecines and were 1 m in length tall
  • they inhabited the island from 100 000-12 000 years before present

Although researchers do not have a solid understanding of the phylogenetic relationships among the hominin species, several points are clear from the availed data

  • 1) the shared, derived character that defines hominin is BIPEDALISM
  • 2) several species from the lineage were present simultaneously during most of the hominin evolution
  • i.e.. 1.8 MYA there may have been as many as 5 hominin species living in earth and southern Africa
  • because fossils from 1+ species have been found in the same geographic location in rock strata of the same age, it is almost certain that different hominin species lived in physical contact
  • 3) compared with the gracile and robust australopithecines and the great apes, species in the genus Homo have extremely large brain relative to their overall body size

leading hypothesis of why humans evolved large brains:

  • early Homo = began using symbolic spoken language along with initiating extensive tool use
  • increase tool use + language = triggered natural selection for the capacity to reason, communicate = LARGER BRAIN NEEDED
  • evidence 🡪 brain area responsible for language were enlarged in earliest Homo species

strong fossil evidence of speech in Neanderthals and early Homo sapiens:

hyoid bone = slender bone in voice box of us modern humans that holds muscles used in speech

  • Neanderthals and sapiens = have a hyoid that is dif shape from chimp

homo sapiens colonized Australia by boat between 60 000 – 40 000 YA

  • such an expedition cant be planned and carried out without symbolic speech


  • 1ST H. sapiens fossil = African rocks about 195 000 YA
  • 130 000 Y after = homo sapiens in Africa and Neanderthals in Europe and middle east
  • h. erectus in Asia at the time too

rocks from 60 000 – 30 000 YA – we in Europe, Asia, Africa and Australia

  • erectus extinct and Neanderthals as well (co exist with Sapiens in Europe)

in phylogenetic trees that show relationship of human pops today – 1st lineage to branch off lead to descendent populations that live in Africa today

  • logical to infer that the ancestral population of modern humans also lived in Africa

Three lineages are ought to descend from three major waves of migration that occurred as a Homo sapiens population disperses from east Africa to:

  • 1) north Africa, Europe and central Asia
  • 2) northeast Asia and the Americas
  • 3) southeast Asia and the south pacific

SUMMARIZE: the data suggests the out of Africa hypothesis:

  • 1) modern humans originated in Africa
  • 2) a population that left Africa split into 3 broad groups which then separated throughout the world
  • H. sapiens evolved independently – NO INTERBREEDING between H. sapiens + Neanderthals + h. erectus + h. floresiensis
  • H. sapiens = evolved its distinctive traits in Africa than dispersed throughout the world

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