How did the first organisms evolve

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How did the first organisms evolve?

• Chapter 18

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When did life first evolve?

• Age of earth estimated to be 5 billion years
• Earliest fossils are of bacteria 3.5 billion
  years ago
• Stromatolites (deposits of cyanobacteria) are 3
  billion years old

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Origins of Life

• Creationist ideas (not scientifically testable)
• Non-creationist theories include
• It came from outer space
• Universe is older than earth, explains rare
  chemicals found in living organisms
• Some organic chemicals are found on space rocks
• Life originated on earth from non-living
  materials (Pre-biotic Evolution)

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Prebiotic Evolution of Complex Molecules

• Is not spontaneous generation as disproved by
  Pasteur's experiments
• May have taken 300 million years
• Earth atmosphere and conditions were very
  different than todays environment
• No oxygen was present

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Early earth environment

• Hot gases escaped due to weak gravity field
• Crust slowly solidified
• Hot inner core of nickel
• Pounded by meteors-vaporized all water

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Evolution of the Atmosphere

• Modern atmosphere 78 nitrogen, 21 oxygen, 1
  argon
• Small amounts of carbon dioxide, water vapor,
  etc.
• Early earth atmosphere had no oxygen
• No rust in sediments
• No ozone layer-very high ultraviolet radiation
• Lightning storms provided energy

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Where did the organic compounds come from?

• Three main hypotheses
• Formed in earth atmosphere
• Formed in undersea volcano vents
• Came on meteors from space

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Stanley Millers Experiment

• Combined early earth chemicals in lab
• hydrogen, methane, ammonia (no oxygen)
• Provided water vapor and energy (heat)
• Mixed them
• Found five amino acids and bases of nucleic acids
• Suggests that organic compounds formed and slowly
  built up in earth's oceans

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Another source of early compounds

• Hot volcanic vents in the oceans mix hydrogen and
  sulfide
• Provide energy that is not light dependent

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Meteor origin of organic compounds

• Organic compounds have been found on meteors
• Could have built up due to constant bombardment

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How did organic compound assemble outside of
cells?

• ATP (energy molecule) forms easily
• Nucleotides can form
• ATP can cause amino acids to form polypeptides
  (protenoids) similar to modern proteins
• Association of organic compounds concentrated
  into protobionts

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How can prebiotic cells form?

• Polypeptides, nucleic acids, and polysaccharides
  concentrate themselves into tiny droplets called
  coacervates
• Coacervates can contain enzymes
• Coacervates can take up, change, and release
  molecules
• Protenoid microspheres are assocations of
  protenoids that can absorb solutions, grow, and
  divide

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Early non-living cells evolve metabolism

• Coacervates selectively absorb compounds
• As these run out, natural selection favors
  alternate pathways
• New pathways evolve slowly

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First genetic system

• All true cells must show heredity (a genetic
  system)
• Modern cells store information in DNA, transcribe
  that into RNA, translate RNA into polypeptides
• DNA-RNA scheme too complex for first cells
• RNA is more likely candidate for first genetic
  system

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Why is RNA the likely candidate?

• RNA is simpler (single-strand vs. double-strand)
• RNA can assemble itself from component parts
• RNA can also act as an enzyme

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What about the evolution of translation?

• Translation is the process of going from
  RNA-polypeptides
• Genetic code is an accident of history
• Only first two letters have meaning, but reads
  all three

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Evolution of ribosomes and transfer RNA

• All ribosomal RNAs show similar anatomy,
  suggesting common ancestor
• Transfer RNA also show similarities indicating
  common ancestor

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Characteristics of first living cell

• Boundary to separate cell from environment
• Enzymes to extract energy,Energy storage in ATP
• RNA that controlled enzyme activity,RNA
  replication
• Specification of each amino acid by triplet codon
  (genetic code)
• Transfer RNA to attach amino acid to polypeptide
  chain

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Evolution of modern cells

• Bacteria evolved around 3.5 billion years
• Lasted nearly 2 billion years before eukaryotic
  cells evolved (1.7 billion)

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How did early life affect the environment?

• Early cells were heterotrophs (using energy from
  other organic molecules)
• No oxygen so did not use respiration pathway (all
  were anaerobic)
• Obtained energy by glycolysis or fermentation
• Autotrophs evolved to use light energy to build
  organic molecules from inorganic molecules

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Impact of photosynthesis

• Created oxygen as waste product
• Oxygen powerful oxidizer, breaks things apart
• Food of early heterotrophs fell apart
• Allowed evolution of new type of heterotrophic
  organism that uses oxygen to fuel respiration
• New food chain of heterotrophs feeding on
  autotrophs

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Evolution of mitochondria and chloroplasts

• Mitochondria and chloroplasts were likely
  free-living prokaryotes
• Based on the fact that they have unique DNA
• Became involved in endosymbiosis with eukaryotic
  cells
• Now totally dependent on eukaryotes

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Evolution of eukaryotic cells

• Eukaryotic cells are distinguished by internal
  cell membranes
• Creates specialized organelles (internal cell
  structures)