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    Understanding the Origin of Life

    The term biodiversity refers to the entire array
    of living organisms found on earth.

    Bidiversity is in constant flux.
    Species arise and become extinct over time.

    Global biodiversity on earth waxes and wanes over time,
    punctuated by mass extinctions.

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    Evolution is the process by which life changes over time.

    The initial Origin of Life was an entirely different phenomenon.

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    What Makes Something "Alive"?

    To be considered alive, an object must meet specific criteria.

    A living thing must have

    • an organized structure (anatomy)
    • chemical reactions coordinated to perform vital functions (metabolism)
    • ability to maintain constant internal environment (homeostasis)
    • reactions internal and external stimuli (responsiveness)
    • ability to develop and grow (growth)
    • ability to adapt to environmental changes (adaptability)
    • ability to reproduce/pass on genetic information (reproduction)

    How did organic and inorganic molecules combine and organize
    to meet all of these criteria?

    Not all aspects of the Origin of Life are fully understood.
    But many pieces of the puzzle are coming together.

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Early Ideas About the Origin of Life

Inspired by common observations, ancient Romans believed that living organisms could spring,
fully formed, from non-living matter.

This process was known as spontaneous generation.

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    Maggots were believed to arise, fully formed, from rotting meat.

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    Head lice were thought to be generated
    from the sweat on a person's head.

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    Turtles and other animals found in sediments
    were believed to form from the sea mud itself.

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    An Ancient Recipe for Making Mice (ca. 1650)

    1. Place sweaty underwear and husks of wheat in an open-mouthed jar.
    2. Wait 21 days.
    3. Sweat from the underwear will penetrate the wheat husks.
    4. Presto! Fresh mice for all your fresh mouse needs.

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    This is where bunnies come from.

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It took the work of more than one scientist to lay the idea of spontaneous generation to rest.

    Francesco Redi (1626 - 1697)

    In 1668 this Italian physician experimented by placing rotting meat
    in covered and uncovered jars.

    He noted that maggots formed only in uncovered jars.
    Enlightenment: Maggots are juvenile flies. They come from fly eggs.

    Redi is considered by some to be the "Father of Experimental Biology".

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    Anton van Leewenhoek (1632 - 1723)

    In 1676, this Dutch scientist invented the microscope,
    revealing a world teeming with microscopic life.

    Interest in spontaneous generation was revived, albeit on a smaller scale.

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    Georges Buffon (1707 - 1788)

    Curator of the King's Garden in Paris, this intellectual
    was the first European to propose that the earth and solar system
    had arisen due to natural processes.

    He noted that the earth and living things were all composed
    of the same types of "particles" (elements).

    Although he did not propose a mechanism for life's emergence,
    he was among the first to propose an idea other than divine creation.

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    John Needham (1713 - 1781)

    By 1745, it was well known that boiling killed microorganisms.

    To test whether they would reappear spontaneously after boiling,
    Needham boiled broth, then covered it.

    A few days later, microorganisms grew in his flasks.

    Some believed this was evidence of a "life force"
    residing in the broth that allowed spontaneous generation.

    But Needham's methods were imperfect:

    • He didn't boil long enough to kill bacterial resting stages (endospores)
    • He allowed the broth to cool, uncovered,
      to room temperature before covering it allowing
      plenty of time for bacterial colonization.

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    Lazaro Spallanzani (1729 - 1799)

    This Italian priest suspected that microorganisms had entered
    Needham's flask after boiling.

    In 1768, Spallanzani

    • filled two flasks with broth
    • sealed one, but not the other
    • removed the air from the sealed flask, creating a partial vacuum
    • boiled broth in both flasks
    • Only the unsealed flask produced microorganisms.

    However, his experiments were discounted because

    • Others with less meticulous techniques could not replicate his result.
    • Critics claimed he showed only that air was necessary for spontaneous generation.

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    Louis Pasteur (1822 - 1895)

    In 1862 his elegant Swan-necked flask experiments convinced
    the scientific community that microorganisms did not arise via spontaneous generation.

    • Broth boiled in an open swan-necked flask would stay clear indefinitely.
    • But break off the neck (to remove the curved bacteria trap),
      and the broth became cloudy with bacteria in a few days.

      Pasteur's experiments put the controversy to rest.
      Omne vivum e vivo ("all life from life") became the accepted aphorism.

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    The Origin of Life: Modern Methods

    We now know that life originated via four major phases

      1. Generation of small organic molecules from
      xxx abiotic (non-living) precursors

      2. Joining of these smaller subunits into biological macromolecules

        • proteins - polymers of amino acids
        • nucleic acids - polymers of nucleotides
        • carbohydrates - polymers of sugars
        • fats - polymers of fatty acids

      3. Packaging of these macromolecules into protocells:

        • a membrane surrounding an internal space
        • internal space containing fluid different from its surrounding medium

      4. Origin of self-replicating molecules that made inheritance possible

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Abiogenesis: Life from Abiotic Matter

Abiogenesis is the origin of life from inorganic precursors.

Many investigators have contributed to our understanding of how this could have occurred.
We will meet only a few of them today.

    Aleksandr Oparin (1894 - 1980)

    This Russian biologist published The Origin of Life (1924)
    in which he proposed that chemical reactions in the ancient oceans
    could have generated life.

    Later, he studied physical ways that organic molecules
    might have formed the precursors of life.

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    Coacervates and Colloids and Life, Oh My.

    Depending on their chemical properties, organic molecules in aqueous solutions can

    • spread out into sheets
    • form small clusters
    • disperse very uniformly in the water, forming a colloid

    • form microscopic, spherical droplets called coacervates

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    Add Lipids, Shake Well

    Oparin combined
    • gelatin (a protein)
    • gum arabic (a polysaccharide/carbohydrate)

    ...and mixed them with a wavelike motion, mimicking that of the early ocean.

    Stable, globular coacervates formed.

    When lipids were added to the mixture, coacervates

    • formed an even more stable membrane-like sheet.
    • readily took up enzymes and substrates added to the aqueous medium.
    • These began to function normally inside the coacervates.

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    Protocells

    Oparin considered a coacervate containing
    working enzyme systems to be a protocell/protobiont.

    A protobiont has some--but not all--properties of life.

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    Hell's Kitchen: The Miller-Urey Apparatus

    Stanley Miller (1930 - 2007) and Harold Urey (1893 - 1981)
    were among the first to simulate the chemical conditions
    of the primordial oceans in the laboratory.

    Stanley Miller, then a graduate student in Urey's lab,
    constructed the now-iconic Miller-Urey Apparatus.

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    Millery Urey: Inorganic to Organic

    They introduced into the apparatus compounds they believed
    to have comprised the earth's early atmosphere and oceans.

    • carbon dioxide (CO2)
    • water vapor (H20)
    • hydrogen (H2)
    • nitrogen (N2)

    • ammonia (NH3)
    • hydrogen sulfide (H2S)
    • carbon monoxide (CO)
    • methane (CH4)
    Samples of water in the apparatus yielded organic molecules
    that had been formed from the introduced inorganic components.

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    RNA World Hypothesis

    Ribonucleic acid (RNA), not deoxyribonucleic acid (DNA),
    was most likely the original genetic material.

    • RNA is simpler than DNA to construct from raw materials
    • RNA has enzymatic properties (ribozyme activity)
    • RNA can modify its own structure
    • RNA can self-replicate
    • RNA can be reverse transcribed into its more stable cousin, DNA
    • Extant retroviruses demonstrate this process.

    Evolution of comparatively stable DNA from an RNA precursor
    would have conferred a selective advantage on the organisms
    lucky enough to have sythesized those first DNA molecules.

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    Once DNA showed up, it was off to the races.

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    A Warm Little Pond

    John Sutherland and his team at the University of Manchester, UK
    demonstrated that RNA nucleotides could have arisen from
    inorganic precursors by means of a previously unsuspected mechanism.

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(click on pic for source)

    Warm Little Pond, or Smoking Hot Volcano?

    You can't have life without membranes.

    But models of ancient membranes are unstable at the high temperatures
    and salinities near deep sea hydrothermal vents, where many suspect life first arose.

    But a research team led by Nick Lane at University College London discovered that modification of membrane components conferred membrane stability at extreme temperature, pH, and salinity, as are found near hydrothermal vents.

    Once a stable membrane could form around reacting molecules...
    the situation was ripe for natural selection.

    And here we are.

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