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									The Origin and Early History of Life

            Chapter 4




                                       1
                     Outline

•   Fundamental Properties of Life
•   Origin of Life Hypotheses
•   Chemical Evolution
•   Cell Origin Theories
    – Bubble Theories

•   Prokaryotic Cells
    – Archaebacteria

    – Bacteria

•   Eukaryotic Cells
•   Extraterrestrial Life
                                     2
          Fundamental Properties of Life

•   cellular organization
•   sensitivity
•   growth
•   development
•   reproduction
•   regulation
•   homeostasis
•   heredity

                                           3
            Origin of Life Hypotheses

•   Special creation
    – supernatural or divine origin: This cannot

      be tested, and falls outside of science
•   Extraterrestrial origin
    – Panspermia: origination from beyond

      Earth, and leaves question unanswered
•   Spontaneous origin
    – life originated from inanimate materials;

      supported by chemistry and geology
                                                   4
                Reducing Atmosphere

•   Life most likely emerged under
    high-temperature conditions.
     – Early atmosphere is often

       referred to as a reducing
       atmosphere.
          exact conditions unknown
             ample availability of
             hydrogen atoms
             rich in CO2, N2, H2O, H2,
             H2S, NH3, CH4
             very little oxygen

                                         5
                 Origin of Life - Location
•   All feature some restrictive space and catalytic agents
•   Ocean’s edge
     – bubble hypotheses; membranes from foam

•   Under frozen seas
     – problematic due to necessary conditions

•   Deep in Earth’s crust
     – byproduct of volcanic activity; metal catalysts

•   Within clay
     – positively-charged clay

•   Deep-sea vents
     – conditions suitable for Archaea; metal sulfides as

        catalysts for prebiotic molecules
                                                              6
              Miller-Urey Experiment

•   attempted to reproduce early reducing atmosphere
    and produce organic compounds from inorganic
    materials
     – hydrogen-rich in N , H O, H , H S, NH ; also,
                          2  2    2   2     3
       CO2
     – electrical discharge

          formed CH4, CH2O (formaldehyde), HCN,
          combining into HCOOH (formic acid) and
          NH2CONH2 (urea)
          succeeded in producing amino acids

                                                       7
Miller-Urey Experiment




                         8
               Chemical Evolution

•   Ongoing debate concerning actual path
    – RNA World - Molecules could not have

      consistently formed without a mechanism
      of heredity. Thus, “ribozymes”. Unstable.
    – Protein World - Replication would be

      impossible without enzymes. No heredity.
    – Peptide-Nucleic Acid World - RNA is too

      unstable, thus a precursor must have
      existed.

                                                  9
              Cell Origin Theories

•   Bubble theories
    – Certain molecules spontaneously form

      bubbles.
    – Formation of a membrane would isolate

      prebiotic molecules from environment;
      limited space increases chances of
      reaction
         would serve to shield hydrophobic
         regions from contact with water
            supports ocean’s edge scenario
                                              10
                Cell Origin Theories

•   Oparin’s Bubble Hypothesis
    – Primary abiogenesis - protobionts allowed

      chemical complexity to develop
    – Membranes concentrated materials, forcing

      reactions, and protected new materials from
      environmental degradation
    – Many different bubble scenarios have been

      proposed: proteinoid microspherules,
      coacervate droplets. Key: hollow lipid or
      phospholipid spheres. Coacervates actually
      undergo limited glucose decomposition
    – no agreement about composition or how the

      process occurred                              11
Bubble Hypothesis




                    12
                     Earliest Cells

•   Microfossils have been found in
    rocks as old as 2.5 billion years
    old.
     – resemble prokaryotes:

       “before nucleus”
         lack nucleus of more
         complex eukaryotes

•   Eukaryotes (“all nucleus”)
    appear as fossils 1 billion years
    ago, more or less.
                                        13
                  Archaebacteria

•   extreme-condition prokaryotes
     – lack peptidoglycan in cell walls

         methanogens: methane-
         producing obligate
         anaerobes
         extreme halophiles
         extreme thermophiles: life in
         boiling water
     – thought to have split from

       ancestral bacteria 2 bya.
                                          14
                        Bacteria

•   second major group of prokaryotes
     – strong peptidoglycan cell walls

     – contains most modern prokaryotes,

       including anaerobes, aerobes, and
       facultative anaerobes
     – includes photosynthetic bacteria and

       cyanobacteria, leading to first major
       pollution event: oxygen
       concentrations rose from 1% to 21%
     – some fix nitrogen into nitrate in soil

       and water, key for protein synthesis
                                                15
                First Eukaryotic Cells
•   Origin of the Nucleus
     Infolding                                                     Nuclear
     of the         Plasma                        Endoplasmic      envelope
     plasma         membrane                      reticulum (ER)
     membrane




    DNA
                        Cell wall                 Nucleus
                                                  Plasma
                                                  membrane
                                    Prokaryotic
          Prokaryotic               ancestor of              Eukaryotic
          cell                      eukaryotic               cell
                                    cells
                                                                              16
             First Eukaryotic Cells

•   Eukaryotes probably arose about 1.5 bya.
    – Internal membrane-bound structures such

      as mitochondria and chloroplasts are
      thought to have evolved via
      endosymbiosis.
         Energy-producing bacteria were
         engulfed by larger bacteria.
           beneficial symbiotic relationship


                                                17
Endosymbiosis




                18
              First Eukaryotic Cells

•   Sexual reproduction
    – Eukaryotic cells can reproduce sexually,

      thus allowing for genetic recombination.
        Genetic variation is the raw material
        necessary for evolution.
        Prokaryotes can use a similar process
        for exchanging genetic material
•   Multicellularity
    – arisen many times among eukaryotes

        fosters specialization
                                                 19
Fig. 4.16(TE Art)




                               PHANEROZOC
                                                                                    Bacteria Archae- Protista Plantae Fungi   Animalia
                          0             Cenozoic                                             bacteria
                                        Mesozoic  Colonization of land
                        500             Paleozoic by animals
                                                  Appearance of animals
                                                  and land plants
                                   PROTEROZOIC

                        1000                    First multicellular
                                                organisms
Millions of years ago




                        1500                                   Oldest definite fossils
                                                 PRECAMBRIAN



                                                               of eukaryotes
                        2000                                   Appearance of oxygen
                                                               in atmosphere
                        2500                                   Oldest definite fossils
                                                               of prokaryotes
                                   ARCHEAN




                        3000

                        3500
                                                               Molten- hot surface of
                        4000                                   earth becomes somewhat
                                                               cooler
                        4500                                   Formation of earth
                                                                                                                                         20
Fig. 4.15




            21
                Extraterrestrial Life

•   Universe has 1020 stars similar to our sun.
    – Conditions may be such that life has

      evolved on other worlds in addition to our
      own.
        ancient bacteria on Mars?
        largest moon of Jupiter, Europa,
        covered with ice, and liquid water may
        be underneath. Spectral analysis
        indicates possible hydrocarbons.

                                                   22
                    Summary

•   Fundamental Properties of Life
•   Origin of Life Hypotheses
•   Chemical Evolution
•   Cell Origin Theories
    – Bubble Theories

•   Prokaryotic Cells
    – Archaebacteria

    – Bacteria

•   Eukaryotic Cells
•   Extraterrestrial Life
                                     23
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