Charles Darwin on the Origin of Life Letter to a friend 1871

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Charles Darwin on the Origin of Life- Letter to a
friend 1871

• It has often been said that all of the
  conditions for the first production of a living
  organism are now present which could ever have
  been present. But if (and oh! What a big if!) we
  could conceive in some warm little pond, with all
  sorts of ammonia and phosphoric salts, light,
  heat, electricity, etc. present, that a protein
  compound was chemically formed ready to undergo
  still more complex changes, at the present day
  such matter would be instantly devoured or
  absorbed, which would not have been the case
  before living creatures were formed.

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Evolution is not an explanation for the origin of
life
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• Why is this?

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How can life arise from nonlife?
?
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How can life arise from nonlife?

• Divine creation?

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How can life arise from nonlife?

• Divine creation?
• Spontaneous generation?
• Maggots from decaying meat

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Francisco Redi - 1600s

• Microorganisms result from spontaneous generation?

Conducted an experiment using meat and flies to
show that maggots developed from tiny eggs laid
by the flies, not by spontaneous generation
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Lazzaro Spallanzani - 1700s

• Microorganisms result from spontaneous generation?

Conducted an experiment with Needham's Gravy to
show that the microscopic organisms entered from
the air, not by means of spontaneous generation
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Pasteur, 1864

• Microorganisms result from spontaneous generation?

Conducted an experiment with nutrient broth in
curved-neck flasks to finally disprove
spontaneous generation
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Life generates more life

• How did it all start then?

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Four-stage hypothesis for the origin of life

• 1. Abiotic synthesis of organic monomers
• Amino acids and nucleotides

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Four-stage hypothesis for the origin of life

• 1. Abiotic synthesis of organic monomers
• Amino acids and nucleotides
• 2. Monomers joined into polymers
• Proteins and nucleotides

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Four-stage hypothesis for the origin of life

• 1. Abiotic synthesis of organic monomers
• Amino acids and nucleotides
• 2. Monomers joined into polymers
• Proteins and nucleotides
• 3. Origin of self-replication
• Eventually allow for inheritance

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Four-stage hypothesis for the origin of life

• 1. Abiotic synthesis of organic monomers
• Amino acids and nucleotides
• 2. Monomers joined into polymers
• Proteins and nucleotides
• 3. Origin of self-replication
• Eventually allow for inheritance
• 4. Formation of pre-cells
• Packaging molecules into protobionts droplets
  with membranes that can maintain different
  chemistry relative to outside environment

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1. Evidence for the abiotic synthesis of organic
monomers

• Oparin (Russia) and Haldane (Great Britain),
  1920s
• Oxidizing atmosphere of Earth today not
  conducive to spontaneous synthesis of organic
  compounds
• But before photosynthetic organisms evolved
  little atmospheric oxygen
• Atmosphere was derived mainly from volcanic
  vapors

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1. Evidence for the abiotic synthesis of organic
monomers

• Modern atmosphere- layer of ozone produced by
  oxygen
• Shields out most UV
• Not so in early atmosphere
• UV radiation provides one potential energy source
  needed for abiotic synthesis of organic molecules
• Lightning offers another energy source

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1. Evidence for the abiotic synthesis of organic
monomers
Stanley Miller (1953)- 23 yr old graduate
student, together with Harold Urey TESTED
OPARIN-HALDANE HYPOTHESIS Electrical discharge
simulated lightning Atmosphere contained gasses
such as ammonia, hydrogen, methane,
water Products include amino acids and other
organic molecules However, Miller-Urey
atmosphere- probably more reducing than early
atmosphere With various atmospheric recipes,
others have obtained organic compounds
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Alternative scenarios

• Early atmosphere was not important
• Submerged volcanoes and deep-sea vents
• Reached earth from space
• Meteorites containing organic molecules

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Regardless of where, same questions how?
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2. Evidence for the production of polymers from
monomers
Without enzymes and other cellular components,
how do organic monomers link together to form
polymers? What reaction links together
monomers into polymers???
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• Lab simulations
• Drip organic monomers onto hot sand, clay or
  rock
• Heat vaporizes water and concentrates monomers on
  substrate
• Some monomers spontaneously bond together to form
  polymers (including polypeptides)

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2. Evidence for the production of polymers from
monomers

• Raindrops or waves splash organic monomers onto
  fresh lava or other hot rocks
• Rinsed resulting polypeptides back into sea
• Deep sea vents as site for the origin of organic
  monomers and the site where the first polymers
  formed

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3. Evidence for the origin of self-replicating
molecules
Living organisms -- metabolic activity DNA? RNA?
Protein RNA world hypothesis- potential to
resolve chicken and egg paradox Solves the
problem of having to explain proteins before we
have genes to encode them
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3. Evidence for the origin of self-replicating
molecules

• RNA world hypothesis
• Short polymers of ribonucleotides can be produced
  abiotically in the lab
• In presence of monomers needed to make more RNA,
  5-10 nucleotide sequences are copied
• Add zinc to the mix (catalyst), sequences up to
  40 nucleotides long are copied

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3. Evidence for the origin of self-replicating
molecules

• Natural selection at the molecular level
• Observed in lab populations of RNA
• Molecule has genotype and phenotype
• Why?
• Single stranded RNA molecules- variety of shapes
  depending on sequence
• Depending on the environment, some RNA molecules
  are more stable and replicate faster
• Competition for monomers needed to replicate
  natural selection?

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4. Evidence for the formation ofcells through
self assembly

• Properties of life not contained by single
  molecule, but by interaction of molecules
• Protobionts (preceded living cells?)
• Aggregation of abiotically produced molecules
• Can maintain internal environment different from
  its surroundings

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4. Evidence for the formation ofcells through
self assembly

• Demonstrated in lab formation of protobionts
• Liposomes-
• Lipids organize into bilayer at surface of
  droplet
• Enzymes can also be incorporated into the
  droplets
• Can absorb substrates and release
  enzyme-catalyzed products (simple metabolism)

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Where now?