THE ORIGIN OF LIFE

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THE ORIGIN OF LIFE

• In the beginning, there was methane, and there
  was ammonia, and there was no free oxygen

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WHAT DO WE KNOW?
Living organisms are incredibly diverse 1.5
million species identified so far Many more
remain unidentified
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WHAT DO WE KNOW?

• All living organisms share common ancestry
• Populations of organisms change through time
• Change (evolution) may be slow or relatively
  rapid
• Given life, evolution is inevitable

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ORIGIN OF LIFE

• Still, how did life originate?
• Evolution simply changes living populations
• New species are descended from preexisting
  species
• Can life arise from non-life?
• Can this question be addressed scientifically?

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ORIGIN OF LIFE

• Can we identify the physical and chemical
  conditions that prevailed on the Earth when life
  originated?
• Do the known principles of physics, chemistry,
  and evolution support or disprove the hypothesis
  that organic molecules formed spontaneously and
  evolved into molecular systems with the
  fundamental properties of life?
• Can we design experiments to test the hypothesis
  that living systems emerged through chemical
  evolution?

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HISTORY OF THE UNIVERSE

• 12 15 billion years ago
• Time zero
• Everything compressed into volume of sun
• Incredibly dense, incredibly hot
• Big bang
• Matter and energy very rapidly distributed
  throughout universe
• Temperatures dropped
• Fusion reactions created light elements
• Resulting background radiation is still detectable

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HISTORY OF THE UNIVERSE

• First billion post-big bang years
• Gaseous particles collide, condense under force
  of gravity
• First stars are formed
• As stars grew, nuclear reactions ignited
• Heat and light liberated
• Heavier elements formed
• Explosive deaths of stars released these heavy
  elements
• Released elements incorporated into newly forming
  stars and orbiting planets
• Still heavier elements formed
• New star formation currently visible in dust
  clouds of Orion, etc.

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ORIGIN OF THE EARTH

• Contracting cloud formed our solar system
• H2, H2O, Fe, Silicates, HCN, NH3, CH4, H2CO, and
  other small inorganic and organic molecules
  present
• Planets formed 4.6 4.5 billion years ago
• Earth was hot
• Asteroid impacts, internal compression,
  radioactive decay of minerals
• Much of rocky interior melted
• Many heavier elements moved toward interior
• Lighter elements floated toward surface

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EARTH

• Crust
• Surface zone
• Basalt, granite, and other low-density rocks
• Mantle
• Interior to crust
• Intermediate-density rocks
• Core
• High-density, partially molten nickel and iron

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EARTH

• Earth 4 billion years ago
• Thin-crusted inferno
• Earth 3.8 billion years ago
• Life arose, but how did this happen?

Fossil Cyanobacteria
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CHEMICAL EVOLUTION

• If life originally arose from non-life, how might
  this have happened?
• Consider the following scenario
• Synthesis and accumulation of small organic
  molecules
• Joining of these monomers into polymers
• Aggregation of these molecules into droplets to
  form localized microenvironments
• Origin of heredity

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CHEMICAL EVOLUTION

• Sounds unlikely?
• Well, it was
• The existence of life is testament to a series of
  unlikely events
• Can we test this scenario in laboratory
  experiments?

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EARLY ATMOSPHERE

• 1920s Scientists postulate that conditions of
  the early earths atmosphere must have favored
  the synthesis of organic compounds from inorganic
  precursors
• A. I. Oparin (Russia)
• J. B. S. Haldane (Great Britain)
• What was the composition of this early atmosphere?

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EARLY ATMOSPHERE

• Earths early atmosphere had a composition very
  different than todays atmosphere
• No free O2
• More reducing than present atmosphere
• Initially thought to contain H2O, H2, CH4, NH3
• Can we recreate this environment?

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EARLY ATMOSPHERE

• 1950s Stanley Miller Harold Urey recreated the
  assumed early atmosphere
• Contained H2O, H2, CH4, NH3
• Lacked free O2
• Energy input in forms of heat and electrical
  sparks
• Mimic geothermal heat and lightning

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EARLY ATMOSPHERE

• The initial Miller-Urey experiment and various
  similar experiments succeeded in producing
• All 20 amino acids
• Several sugars
• Lipids
• Purines and pyrimidines
• ATP (when phosphate was added)

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EARLY ATMOSPHERE

• The environment produced by Miller was more
  reducing than we now believe the earths early
  atmosphere to have been
• H2O, CO, CO2, and N2 emitted by modern volcanoes
• Relatively little O2
• This seems to more accurately represent the
  earths early atmosphere
• Still, the Miller-Urey experiment did demonstrate
  that key organic molecules critical to life could
  be produced abiotically from inorganic precursors

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ABIOTIC SYNTHESIS

• Were these key organic molecules formed elsewhere
  in the early earth?
• Submerged volcanoes?
• Mineral-rich deep sea vents?
• There is still significant debate as to where the
  abiotic synthesis of organic molecules that
  ultimately gave rise to life occurred

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POLYMER FORMATION

• Once these small organic molecules accumulated,
  polymers were formed
• e.g., Proteins are polymers of amino acids
• This polymerization in living cells is catalyzed
  by enzymes
• Early polymerizations must have occurred without
  the aid of enzymes
• Is this possible?

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POLYMER FORMATION

• Sidney Fox (University of Miami) demonstrated the
  abiotic polymerization of organic monomers
• Polymers were formed when dilute solutions of
  organic molecules were dripped onto hot sand,
  clay, or rock
• Proteinoids
• Clay can serve to concentrate these molecules
• Monomers bind to charged sites on clay particles
• Metal ions in clay have catalytic function

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PROTOBIONTS

• Aggregations of abiotically produced molecules
• Preceded living cells
• Laboratory experiments have demonstrated their
  formation from organic compounds

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PROTOBIONTS

• Maintain a localized environment separate from
  the surroundings
• Incapable of precise reproduction
• Exhibit some properties associated with life
• Metabolism
• Some polymers possessed catalytic activity
• Excitability
• Membrane potential (voltage across surface)

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ORIGINS OF HEREDITY

• RNA was probably the first genetic material
• Similar in structure to DNA
• Short RNA polymers have been produced abiotically
  in the laboratory
• RNA possesses some catalytic activity
• Ribozymes discovered in 1980s
• Catalyze RNA splicing synthesis of new RNA
• Natural selection at the molecular level has been
  observed operating on RNA populations in the
  laboratory

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PROTO-CELLS

• Chemical evolution ultimately led to the
  formation of proto-cells
• Membrane-surrounded sacs containing genetic
  material and metabolically-active molecules
• Such structures have been experimentally produced
• From these proto-cells, cells ultimately arose

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ORIGIN OF LIFE

• Can we identify the physical and chemical
  conditions that prevailed on the Earth when life
  originated?
• Do the known principles of physics, chemistry,
  and evolution support or disprove the hypothesis
  that organic molecules formed spontaneously and
  evolved into molecular systems with the
  fundamental properties of life?
• Can we design experiments to test the hypothesis
  that living systems emerged through chemical
  evolution?

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ORIGIN OF LIFE

• Unfortunately, our understanding of the origin of
  life is incomplete
• Many laboratory experiments lend support to an
  abiotic origin of life through chemical evolution

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EARLIEST LIFE

• Life arose 3.8 billion years ago
• The earliest cells were prokaryotic
• Lack a membrane-bound nucleus
• Early in the history of life, populations
  diverged into two major lineages
• ? bacteria
• ? archaea eukaryotes

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EARLIEST LIFE

• How do we know that domain Eukarya is more
  closely related to domain Archaea than to domain
  Bacteria?
• Analysis of rRNAs and other highly conserved
  genes and proteins provide the strongest
  evidence

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PHOTOSYNTHESIS

• The cyclic pathway of photosynthesis evolved
  early
• 3.5 3.2 billion year ago
• Sunlight served as a source of energy
• Does the cyclic pathway generate O2?
• Dominated the living world for 2 billion years
• Stromatolites are hardened remnants of the large
  mats of these autotrophs

Stromatolites
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PHOTOSYNTHESIS

• The non-cyclic pathway of photosynthesis arose
  2.5 billion years ago
• O2 was produced, and began to accumulate
• O2 can be very damaging to molecules and cells
• Abiotic formation and accumulation of complex
  organic molecules ceased
• Populations living in this environment must
  evolve a defense against the damaging effects of
  O2
• Oxygen can be helpful to cells
• Remember how?

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AEROBIC RESPIRATION

• Aerobic respiration arose 2.5 billion years ago
• Allowed the use of the accumulating O2
• O2 neutralized by use as an electron acceptor
• (Other methods to neutralize O2 also exist)
• Became dominant energy-releasing pathway

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PREDATION

• Predation arose gt 2 billion years ago
• Bacteria are an abundant food source
• Increased size afforded some protection
• More difficult to digest
• Move faster
• Increased size has some drawbacks
• Lower surface area-to-volume ratio
• Less efficient nutrient delivery waste removal

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EUKARYOTES

• Eukaryotes evolved 2.1 billion years ago
• Possess several membrane-bound organelles
• Some originated from infoldings of the plasma
  membrane
• Some originated through endosymbiosis

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ENDOSYMBIOSIS

• Mitochondria Chloroplasts
• Surrounded by two membranes
• Possess highly infolded inner membranes
• Inner membrane is involved in energy
  transformation
• Possess their own DNA
• Organized in prokaryotic (bacterial) fashion
• Possess ribosomes
• 70S (bacterial), not 80S (eukaryotic or archaean)
• Reproduce separate from rest of cell

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ENDOSYMBIOSIS

• Both mitochondria and chloroplasts were once free
  living organisms
• Engulfed by early eukaryotic cells
• Maintained rather than being digested
• Endosymbionts
• Most genes have been lost or transferred to the
  nucleus
• A few genes are retained
• At present, host cells cannot live without their
  endosymbionts, and these endosymbionts cannot
  live without their host cell
• Other organellles are likely endosymbionts also

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EUKARYOTES

• Eukaryotes evolved 2.1 billion years ago
• Possess several membrane-bound organelles
• Some originated from infoldings of the plasma
  membrane
• Some originated through endosymbiosis

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MULTICELLULARITY

• Increased size evolved in response to predation
• Increased size has drawbacks
• Lower surface area-to-volume ratio
• The evolution of multicellularity allowed
  increased size without these limitations
• Multicellular algae arose 1 billion years ago
• Multicellular animals arose 800 million years
  ago
• Specialization of cells was also possible
• e.g., Roots to anchor

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TIMELINE

• 12 - 15 b.y.a.
• 4.5 b.y.a.
• 3.8 b.y.a.
• 3.5 3.2 b.y.a.
• 2.5 b.y.a.
• 2.5 b.y.a.
• 2.1 b.y.a.
• 1 b.y.a.

• Big bang
• Origin of the earth
• Origin of life
• Electron transport, photosynthesis
• Non-cyclic photosynthesis, O2
• Cellular respiration
• Origin of eukaryotes
• Origin of multicellularity

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TIMELINE