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History of Life on Earth (PowerPoint)

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					         History of Life on Earth
•   Chemical Evolution (prebiotic evolution) – most
    biologists believe that life developed from nonliving
    matter
•   Alexander Oparin (Russian) and John B. S. Haldane
    (England) were the first scientists (independently) to
    advance the idea that simple organic molecules could
    form spontaneously from more simple raw materials
    (1920’s)
•   they noted that the oxygen-rich atmosphere of today
    would not have permitted the spontaneous formation of
    organic molecules
•   they speculated that the Earth’s early atmosphere was
    very low in oxygen and rich in hydrogen in the form of
    hydrogen gas (H2), methane (CH4), and ammonia (NH3)
    – also contained carbon dioxide (CO2), water vapor
    (H2O), carbon monoxide (CO), and nitrogen (N2)
     Conditions on primordial Earth
•   Earth is about 4.6 billion years old
•   Earth was very hot when first formed
four requirements must have existed for chemical
    evolution:
1. little or no O2 – Earth’s early atmosphere was
    probably strongly reducing which would cause
    any free oxygen to react and form oxides and be
    removed from the atmosphere
2. a source of energy – early Earth was a place of
    high energy
  •   violent thunderstorms with torrential rainfall
  •   widespread volcanic activity
  •   bombardment from meteorites (caused cataclysmic
      changes in crust, ocean, and atmosphere)
  •   intense radiation (including UV radiation, since there
      was no ozone layer and younger suns emit more UV
      light)
3. presence of chemical building blocks –
   water, dissolved inorganic minerals
   (present as ions), and the gases present
   in the early atmosphere
4. time for molecules to accumulate and
   react with one another – Earth is
   approximately 4.6 billion years old, the
   earliest traces of life are approx 3.8
   billion years old
Oparin and Haldane’s hypothesis is tested by
   Stanley Miller and Harold Urey in the
   1950’s
•  they designed a closed apparatus that
   simulated conditions that presumably
   existed on early Earth
•  they exposed an atmosphere rich in H2,
   CH4, H2O, and NH3 to an electrical
   discharge to simulated lightening
•  analysis of the chemicals produced in a
   week revealed that amino acids and other
   organic molecules had formed
•   recent evidence indicates that organic polymers
    may have formed and accumulated on rock or
    clay surfaces (rather than in a “primordial soup” in
    the sea)
•   clay consists of microscopic particles of
    weathered rock and may have acted as a site for
    early polymerizations because it binds organic
    monomers and contains zinc and iron ions that
    might have served as catalysts
•   lab experiments using clay have confirmed that
    organic polymers form spontaneously from
    monomers on hot rock or clay surfaces
•   Protobionts – scientists have been able to
    synthesize several different protobionts
    (assemblages of abiotically produced organic
    polymers)
•   exhibit many characteristics of living cells –
    division after growth, maintaining an internal
    environment different from the external fluids
                  Microspheres
• protobionts formed by adding water to polypeptides
• microspheres show an electrical potential, may
  absorb materials from the surrounding environment
• microspheres may give clues as to the evolution of
  the cell membrane
• membranes are made of phospholipid bilayers with
  proteins
• scientists have heated amino acids without water
  and produced long protein chains – when water is
  added, stable microspheres (coacervates) are
  formed
• microspheres can accumulate compounds inside
  them and become more concentrated than outside,
  they also attracted lipids and formed a lipid-protein
  bilayer around them
       Protobionts




Microsphere     Liposome
The first cells probably assembled from organic
    molecules
•   Cells were evident in microfossils 3.5
    billion years old, perhaps even 3.8 billion
    years ago
•   The first cells were prokaryotic
•   Stromatolites offer more fossil evidence –
    rocklike columns composed of many
    minute layers of prokaryotic cells (usually
    cyanobacteria)
•   living stromatolite reefs are still found in
    hot springs and in warm, shallow pools of
    fresh and salt water
                Fossilized Stromatolites
                – 3.5 billion years old




Modern day
stromatolites
•   A crucial step in the origin of cells was
    molecular reproduction
•   both DNA and RNA can form
    spontaneously on clay, so… which came
    first?
•   RNA is self-catalytic and is believed to have
    appeared first (according to the proposed model
    of the “RNA World”)
•   chemistry of prebiotic Earth gave rise to self-
    replicating RNA that functioned both as enzyme
    and substrates for their own replication
•   RNA has catalytic properties – enzymatic RNAs
    are called ribozymes (in modern cells, ribozymes
    help catalyze the synthesis of RNA and process
    precursors into rRNA, tRNA, and mRNA)
•   ribozymes may have catalyzed the synthesis of
    RNA, and processed RNA molecules
•   RNA could also catalyze protein formation
    (catalyzes peptide bonds formation) – protein
    catalysis of RNA formation happen later
•   DNA probably evolved after RNA – it’s a
    more stable molecule
•   may have evolved from RNA making
    double stranded copies of itself
•   stability of DNA provides advantages as
    the information storage molecule
•   The first cells were probably heterotrophs
•   fermented organic molecules from the
    aqueous environment – appeared 3.1 –
    3.4 billion years ago
•   first cells were anaerobes, free O2 not
    available
•   as concentration of free organic
    molecules in environment declined,
    photosynthetic organisms had a selective
    advantage
•   first photosynthetic organism were autotrophs which split
    H2S as a hydrogen donor (purple and green sulfur
    bacteria)
•   the first photosynthetic organisms to use H2O as a
    hydrogen donor were the cyanobacteria (released O2 as
    by-product)
•   source of the first free oxygen in aquatic environment and
    atmosphere – O2 existed in significant quantities by 2
    billion years ago
•   Aerobes appeared after oxygen increased in
    atmosphere
•   aerobic respiration was “added” to glycolysis
    after free O2 became available
•   aerobic organisms are much more efficient in
    converting glucose to ATP
•   carbon dioxide produced helped to stabilize
    concentration of CO2 and O2 in atmosphere (by-
    product of each process – photosynthesis and
    aerobic respiration – are raw materials for other
    process)
•   O3 begins to accumulate in upper atmosphere to
    form ozone (protection from UV radiation) –
    allows organisms to live in more shallow water
    and ultimately on land
      Evolution of Eukaryotic cells
•   evolved from prokaryotes about 2 billion years
    ago
•   Endosymbiont Theory – first proposed by Lynn
    Margulis – suggests that mitochondria were
    originally independent prokaryotic aerobic
    organisms which developed a symbiotic
    relationship with another prokaryote
•   aerobic prokaryote was engulfed by endocytosis
    but not digested
•   aerobic prokaryote continued to function and
    formed a symbiotic relationship with host
•   similar process occurred later with the host cell
    and photosynthetic prokaryotes (which became
    chloroplasts)
other evidence:
• mitochondria and chloroplasts grow and divide
    like cells
• they have a naked loop of DNA like prokaryotes
• they synthesize some of their own proteins using
    70s ribosomes, like prokaryotes
• they have double membrane as expected since
    cells were taken into a vesicle by endocytosis
• cristae are similar to mesosomes of prokaryotes
• thylakoids are similar to structures containing
    chlorophyll in photosynthetic prokaryotes

				
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