The Origins of Life on Earth (or, a History of our planet in a week or less)

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The Origins of Life on Earth(or, a History of
our planet in a week or less)

• Biology 2, College of the Atlantic
• Spring 2002

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• How old is this planet anyway?
• Theories of Origin
• Geological and Biological timescales
• Phylogeny (and an awful lot of it)

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How old is this planet anyway?

• The Universe is probably 13 billion years old
  (Big Bang Theory/Doppler Shift)
• Earth is 4.5 billion years old (begins with
  cooling of crust/solidification)
• Earliest records of life 3.5 billion years ago
• First humans (Australopithecus), 0.005 billion
  years ago
• Discovery of Australopithecus fossils ,
  0.0000000002 billion years ago

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The Fragility of Life - Coincidence 1

• Life can only exist within temperatures
  corresponding to the boiling and freezing point
  of water
• This range is a fraction of the range between
  absolute zero (-273C) and the temperature of the
  sun (106C)

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How did life evolve?

• Three theories
• Creationism
• Extraterrestrial origin (Panspermia)
• Spontaneous Origin (Coincidence 2)

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Physical conditions of early Earth - Coincidence
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• Temperatures in correct range (in general, water
  in fluid state, carbon compounds non-brittle)
• Size of planet retains an atmosphere
• Early atmosphere lacked oxygen, therefore highly
  reductive
• High energy bombardment from sun
• ??promotes generation of organics

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Spontaneous origins of life - 4 steps

• Abiotic synthesis and accumulation of organic
  compounds
• Polymerization
• Aggregation of polymers into nonliving structures
  (Protobionts)
• Origin of heredity

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Experimental evidence of Spontaneous Origin

• Theories of Oparin and Haldanetested by Miller
  and Ureydemonstrate formation of organics under
  conditions typical of early Earth
• Polymerization can occur with appropriate
  substrate
• Abiotically produced proteins (proteinoids)
  self-assemble into Protobionts (selectively
  permeable membrane)

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The final key - Heredity

• First passage of genetic information probably
  occurred through short strands of RNA (also
  autocatalyst, e.g ribozymes)
• Mutations cause variation
• Natural selection of molecular combinations
• Origin of DNA

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Biological time scales

• Biological timescales by necessity follow
  geological timescales
• Often, geological events marked by key biological
  events (mass extinctions/diversifications)
• First fossil record of life 3.5 billion years ago
  (prokaryote), in the Precambrian
• Earliest eukaryote 1.5 billion years ago
  (endosymbiotic theory)

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Earth - The Early Years

• Late Precambrian saw the first eukaryotic
  multicellular life
• Boundary between Precambrian and Cambrian (580
  mya) marked by a rapid adaptive
  radiation/diversification of marine life
  (Cambrian explosion)
• By the middle of the Cambrian, all of the animal
  phyla existing today had evolved

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The drive behind Macro-Evolution

• Biological forces natural selection working in
  general, but particularly effectively on genes
  controlling
• allometric growth
• paedomorphosis
• Physical forces
• Plate tectonics, leading to formation and
  splitting of supercontinents

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The study of evolutionary history Phylogeny

• Modern Darwinian synthesis suggests adaptive
  radiation from a common ancestor
• Concept of phylogeny supported through studies of
  homology
• Traditional classification systems (Linnaeus) are
  monophyletic, based on homology ? parallel or
  divergent evolution
• Some groupings are polyphyletic, with analogous
  structure ? convergent evolution

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The Kingdom System

• Scientists follow various taxonomic systems
  Campbell uses the 5 kingdom classification scheme
• Monera
• Protista
• Plantae
• Fungi
• Animalia

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Phylogeny recounts the natural selection of
species (Earth the Middle Years)

• First major extinction at end of the Paleozoic
  era (the Permian Extinction), probably caused by
  collision of tectonic plates to form the
  supercontinent, Pangaea
• Pangaea marks the birth of a new era, the
  Mesozoic (Triassic, Jurassic, Cretaceous)
• Mesozoic ends with second mass extinctionthe
  Cretaceous Extinction (impact hypothesis)

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And now...

• Currently in the Recent epoch of the Quarternary
  period of the Cenozoic era
• History may tell of a third mass extinction?
• Radically changing planet will continue to apply
  selective pressure to species

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Monera the Pioneers of Life on Earth

• The most successful group of organisms on the
  planet
• 3.5 billion year history
• Although only 4000 species known, the number of
  extant species is thought to be 4,000 4 x106
• Found in all ecological niches, including some
  where other forms of life cannot exist

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The current importance of Monera

• In some cases at base of food chain
• Vital roles in various elemental cycles
• Carbon cycle
• Nitrogen cycle
• Interactions with human life
• Symbiosis (E.coli)
• Pathogenic bacteria (physical, exo/endotoxins)
• Commercial/Industrial/Scientific uses

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The phylogeny of Prokaryotes
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Archaebacteria

• Treated either as Domain, or subphylum
• Cell plan similar to most primitive prokaryotic
  fossils
• Tend to exist in extreme environments
• Smaller group of species
• Methanogens (mmm-mmmm!)
• Extreme Halophiles
• Extreme Thermophiles (Sulfolobus)

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Eubacteria

• More diverse group
• Spirochetes (Treponema)
• Clamydias (Clamydia trachomatis)
• Gram-positive eubacteria (Bacillus)
• Cyanobacteria (blue-green alga)
• Proteobacteria (E.coli, Salmonella)

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Structure

• Small (1-5 µm)
• Of three general shapes
• Coccus (pl. cocci), e.g. Streptococcus
• Bacillus (pl. bacilli) e.g. Bacillus
• Spirillum (pl. spirilla) e.g. Treponema
• Cell wall made of peptidoglycan
• Leads to gram /-ve distinction
• Some have a capsule, and/or pili, and/or flagella

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Physiology

• Various forms of nutrition
• Autotrophs (obtain carbon from inorganic CO2
• Photoautotrophs (energy from sunlight)
• Chemoautotrophs (energy from inorganics)
• Heterotrophs (carbon from organics)
• Photoheterotrophs
• Chemoheterotrophs
• Origins of glycolysis, chemiosmosis and cellular
  respiration

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Reproduction

• Single strand of DNA
• No mitosis/meiosis
• Only Binary fission
• Some sexual recombination through
• Transformation
• Conjugation
• Transduction
• Some form endospores

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Kingdom Animalia

• Invertebrata

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• What is an animal?
• Anatomy, Embryology and Ontogeny
• Parazoa
• Eumetazoa
• Radiata/Bilateria
• Acoelomate/Pseudocoelomate/Eucoelomate
• Protostomes/Deuterostomes

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What is an animal?

• Likely ancestor is a protist a Precambrian
  choanoflagellate
• Multicellular, heterotrophic eukaryotes (usually
  exhibit ingestion)
• Storage of energy-rich reserves as glycogen
• Lack of cell walls. Unique cell junctions
• Unique tissues muscle, nervous tissue
• Unique embryology

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Embryology

• Diploid zygote divides by the mitotic process of
  cleavage
• Formation of blastula followed by gastrulation
  (creation of gastrula)
• Mode of embryological development provides a
  taxonomic key to invertebrates

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First taxonomic dichotomy

• Asymmetry versus Symmetry divides Kingdom
  Animalia into sub-kingdoms
• Parazoa (lacks tissues, asymmetrical, like
  animals). Represented by only one phylum
  Porifera (sponges)
• Eumetazoa (true animals, exhibit symmetry,
  represented by remainder of Kingdom Animalia

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Porifera (Sponges)

• General form Two layered cup (separated by
  mesohyl), with porocytes entering into
  spongocoel, exiting through osculum
• Outer layer (epidermis) reinforced by spicules
  (Si)
• Filter feeding by Choanocytes that line
  endodermis
• Transport of materials by Amoebocytes

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Second taxonomic dichotomy

• Radial versus bilateral symmetry
• Super-phylum Radiata exhibits radial symmetry.
  Two phyla include
• Cnidaria (jellyfish, anemones, corals and hydra)
• Ctenophora (combjellies)
• Super-phylum Bilateria exhibits bilateral
  symmetry (rest of invertebrata)

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Bilateral symmetry leads to... Cephalization

• Bilateral symmetry implies a directionality to
  the animal
• With movement in a specific direction comes
  development of sensory equipment at end that
  encounters environment first
• Collection of sensory nervous tissue at anterior
  end of animal cephalization

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• Type of symmetry also reflected by embryonic germ
  layers seen in blastula/gastrula

Triploblastic Bilateria (endoderm/mesoderm /ec
toderm)
Diploblastic Radiata (endoderm/ectoderm)
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Cnidaria (Jellyfish, etc.)

• Diploblastic, radially symmetrical (i.e. no
  cephalization)
• Phylum characterized by cnidocytes that eject
  nematocysts
• Gastrovascular cavity (GVC), with one opening
  (mouth/anus simultaneously)
• Tentacles to pull prey into GVC
• Some species exhibit alternation of sexual and
  asexual forms (e.g. Obelia)

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• Classes within Cnidaria include
• Hydrozoa (e.g. Obelia, Hydra)
• Scyphozoa (jellyfish, e.g. Sea wasp, Lionsmane,
  Portuguese Man-o-war)
• Anthozoa calcareous secretions build an
  exoskeleton (e.g. coral, anenomes, Metridia)

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Third taxonomic dichotomy

• Design of body cavity (or lack thereof)
  characterizes Bilateria
• Acoelomates lack a body cavity, e.g.
  Platyhelminthes (flatworms)
• Body cavities
• Pseudocoelomates have a body cavity lined by
  mesoderm-derived tissue on one side only, e.g.
  Nematoda (roundworms), Rotifera
• Eucoelomates (coelomates) body cavity is lined on
  both sides by mesodermally-derived tissue
  (everything else upwards)

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Tissue derived from Ectoderm Mesoderm Endoderm
GVC Gastrovascular cavity DT Digestive
tract PC Pseudocoelom euC Coelom
GVC
DT
DT
PC
euC
Acoelomate Pseudocoelomate Eucoelomate
note true digestive tract first seen in
pseudocoloemates
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Platyhelminthes (flatworms)

• Triploblastic, bilateral, cephalized acoelomates
  that have been flattened dorso-ventrally
• No internal transport system
• (Use of GVC, with muscular pharynx)
• Some species exclusively parasitic

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• Classes of Platyhelminthes include
• Turbellaria (planarians, e.g. Dugesia, Planaria).
  Free-living, aquatic. Movement by cilia, eased by
  secretion of mucus
• Trematoda (parasitic flukes, e.g. Schistosoma)
• Monogenea (parasitic flukes)
• Cestoda (parasitic tapeworms)
• n.b. Parasitic forms do not possess GVCs

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Body cavities have various functions

• Cushions internal organs
• Independence of movement
• Primitive circulatory system
• Transport of nutrients and metabolic wastes,
  gaseous exchange
• Basis of hydrostatic skeleton
• Helped development of a true digestive tract
  (phylum Nemertea)

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Nematoda (Roundworms)

• Cylindrical triploblastic pseudo-coelomates
• Some are freeliving saprobes - important role in
  decomposition of dead organic matter
• Other are parasitic (e.g. hookworm, Trichinella)
• hmmm, pork chop

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Fourth taxonomic dichotomy

• Protostomes
• Spiral/Determinate cleavage
• Blastopore?mouth
• Schizocoelous development
• e.g. Mollusca thru to Arthropods

• Deuterostomes
• Radial/Indeterminate cleavage
• Blastopore?anus
• Enterocoelous development
• e.g. Echinodermata and Chordata

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Mollusca

• Triploblastic coelomates body plan divided into
  three (foot, visceral mass, and mantle)
• Mantle may secrete calcareous shell for
  protection against dessication and predators
• Gaseous exchange by gills. In some cases, gills
  are modified to filter feed
• Open circulatory system

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• Classes of Mollusca include
• Polyplacophora (Chitons, herbivorous grazers)
• Gastropoda (snails and slugs mostly grazers, but
  some predatorse.g. cone shells)
• Bivalvia (Bivalves clams, oysters,
  musselsfilter feeders)
• Cephalopoda (shellednautilus, or
  unshelledsquid, octopus)

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Segmentation

• Defined as a system of similar units
• Allows specialization of regions along body
  length
• Evolved separately in protostomes (Annelida,
  Arthropoda) and deuterostomes (all Phyla)

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Annelida (segmented worms)

• Triploblastic segmented eucoelomates
• Specialization of body regions (e.g. see
  digestive tract)
• Closed circulatory system
• Three classes include
• Oligochaeta (earthwormLumbricus)
• Polychaeta (marine segmented worms)
• Hirudinea (leeches)

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Arthropoda

• The most successful animal phylum ever
• Characterized by highly developed cephalization,
  exoskeleton (made from armor-tough chitin),
  division of body into head, thorax and abdomen
• Open circulatory system, including haemocoel as
  well as coelom
• Modified appendages per segment first
  evolutionary development of flight

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• Classification of arthropods is complex, but
  subphylums include
• Trilobitomorpha (extinct trilobites)
• Cheliceriformes (spiders, mites and ticks,
  scorpions)
• Uniramia (insects)
• Crustacea (crabs, lobsters, shrimp, copepods)

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• Chelicerates includes the class Arachnida
• Simple eyes
• Modified appendages into
• walking legs (4 pairs)
• feeding mouthparts, including pedipalps and fangs
  (chelicerae), not mandibles
• Spinnerets

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Uniramians

• Insects (Insecta), Millipedes (Diplopoda) and
  Centipedes (Chilopoda). Have compound eyes,
  mandibles, and sensory antennae
• Most insects have developed flight and occupy
  most ecological niches
• Gaseous exchange through spiracles and tracheae
• Waxy cuticle to prevent dessication
• Other dessication defenses through reabsorption
  of water from faeces
• Some species undergo metamorphosis

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Crustacea

• Mainly marine
• Extensively specialized, jointed appendages
• Classes include
• Decapoda (crabs, lobsters, shrimp, prawns)
• Copepoda (copepods)
• Amphipoda (amphipods)
• Isopoda (isopods)

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Deuterostomes Echinodermata

• Triploblastic coelomates
• Pentaradially symmetrical as adult, but
  bilaterally symmetrical as larva
• Unique water vascular system
• Classes include
• Asteroidea (sea stars)
• Echinoidea (sea urchins and sand dollars)
• Holothuroidea (sea cucumbers)

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The invertebrate chordates

• Phylum Chordata traditionally thought of as a
  vertebrate group, but two of the three subphyla
  are invertebrate
• Urochordata (sea squirts, tunicates)
• Cephalochordata (lancelets, e.g. Amphioxus)
• All chordates (including vertebrates) share the
  common features of pharyngeal slits, muscular
  post-anal tail, notochord and dorsal hollow nerve
  cord

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