Origins of life

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Lecture 2.

• Origins of life
• Microbial evolution
• Selection
• Mutation

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Early Earth

• Roughly 4.6 billion years old
• Crust became stable 3.9 billion years ago
• Oldest known rocks date back to 3.86 billion
  years ago

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Early cells

• Some ancient rock contain vaguely recognizable
  microfossils that appear bacterial shaped
• Early cells arranged in a filament

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Stromatolites

• In younger rock clear fossil evidence exists
• Many examples found in stromatolites (mounds of
  fossilized filamentous prokaryotes in sediment)
• From Warrawoona, 2.7 billion years old

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Living stromatolites

• Intact stromatolite present today in Sharks Bay
• Grow in shallow marine basins and in hot springs

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Conditions on early earth

• Atmosphere was slightly reducing
• Little free O2
• H2O was present
• Variety of gases, mainly CH4, CO2, N2, and NH3
• Much hotter than today

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Origins of life

• First biochemical compounds were made by abiotic
  syntheses
• Earliest life forms probably consisted of self
  replicating RNA
• With time, proteins replaced the catalytic
  functions of RNA and DNA replaced the coding
  function of RNA

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Evolution of cellular life forms from RNA life
forms

• This is a possible scenario
• Self replicating RNAs could have become cellular
  entities by being integrated into liposome
  vesicles
• With time, proteins replaced the catalytic
  functions of RNA and DNA replaced the coding
  function of RNA

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Primitive Energy Generation
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Phylogeny

• Phylogeny is the evolutionary history of
  organisms
• Fossil record of microbial evolution is
  incomplete
• Phylogeny determined by analyzing rRNA
• 3 lines of cellular descent were established
  leading to the Bacteria, Archae and Eukarya

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Tree of life

• Shows the three domains Bacteria, Archae and
  Eukarya
• Origin of life is within the bacterial domain
  (not on branch leading to Archaeal and Eukaryal
  domain)
• Archae diverged between the Bacteria and the
  Eukarya but closer to the eukaryotes

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Landmarks in biological evolution

• For most of earths history, only microbial life
  forms existed
• Note how oxygenation of atmosphere occurred over
  a period of 1.5 billion years

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Eukaryotes and organelles

• Primitive eukarya were structurally simple (no
  nucleus, mitochondria and chloroplast)
• Nucleus and mitotic apparatus probably arose to
  ensure orderly partitioning of DNA in
  large-genome organisms
• Mitochondria and chloroplasts ex prokaryotes
  became symbiotic within eukaryotic cells

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Progenitor cells

• Progenitor organisms established a basis for
  heredity, gene expression, material transfers and
  cellular energetics before divergence
• DNA as hereditary material
• Gene expression from DNA to RNA to protein
• ATP and proton motif force for cell energetics
• Central metabolic pathways (glycolysis and TCA
  cycle)
• All evolved prior to divergence of domains

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Genetic basis for evolution

• those organisms best adapted to survive in a
  given environment have a selective advantage
• Mutations introduce variability into genomes of
  organisms changes spread rapidly in microbial
  populations
• Many mutations are harmful
• Favorable mutation makes organism more fit

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Genetic exchange, recombination and evolution

• Mutation introduces variability into DNA
• genetic exchange and recombination are equally NB
• Recombination forms new allelic combinations
  which may be adaptive
• Exchange of genetic info may produce individuals
  with multiple attributes making them super fit

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Natural selection

• This determines which populations can
  successfully establish themselves in a community
• Some organisms possess features which makes them
  better adapted for survival in a particular
  ecosystem
• Less fit organisms are eliminated by natural
  selection
• With time, the interactions of genes and the
  environment through the process of natural
  selection led to continued diversification of
  living cells

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Evolution of physiological diversity (1)

• 3,6 BYA cells were anaerobic heterotrophs which
  degraded and derived energy from abiotically
  formed organic molecules
• next step anaerobic autotrophs able to fix CO2
  and turn CO2 and H into organic molecules
• Electron donors initially were H2 and H2S
• Were all thermophiles

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Physiological diversity (2)

• Energy sources for these autotrophs
• Chemoautotrophs used chemical energy from
  elements in surroundings
• As this E source was depleted, the ability to
  capture E from light evolved (phototrophs)
• Initially only anoxic photosynthesis (based on
  PSI)

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Physiological diversity (3)

• Earths 1st major crisis
• Electron donors became scarce(H2, H2S)
• Key innovation /- 2.5 BYA
• oxygenic photosynthesis by cyanobacteria (PSII)
• Uses H20 as electron donor and generates O2

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Physiological diversity (4)

• With 02 available, aerobes evolved
• Successful since oxidation of organic compounds
  generates more energy
• Higher population densities developed
• Increased chance for evolution of new types of
  organisms and metabolic schemes e.g. aerobic
  respiration

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The Ozone Shield

• Major consequence of the appearance of oxygen was
  the formation of ozone (03)
• 03 provides barrier against UV radiation
• In anoxic world only habitats shielded from
  direct radiation was habitable (oceans, under
  rocks)
• In oxic world, organisms could range over entire
  surface of earth