Origins of life

Useful references

• W. F. Loomis, 1988. Four billion years. Sinauer. --- A biochemical viewpoint
• W. Day. 1984. Genesis on planet earth, 2nd ed. Yale. --- A more general treatment
• R. Cowen. 1990. History of life. Blackwell.

Part 1: Origins of Earth

• Origins of the Universe
  • Big Bang roughly 10-18 billion years ago
  • Formulation of Carbon and higher elements in the first generation of stars
    • Hydrogen, Helium main elements in the early universe

• Formation of the Earth and Solar system
  • Earth roughly 4.7 billion years old
  • Earth' s crust becomes stable by 3.9 billion years ago
  • Life appears around 3.6-3.7 billion years ago

• Reducing versus oxidizing atmosphere
  • Current atmosphere is oxygen rich (Oxidizing)
    • Breaks down Organic molecules
    • One manifestation of this is fire
  • Early Earth' s atmosphere was slightly reducing
    • Organic molecules are much more stable
    • little free O2

Part 2: The appearance of Life

• The timetable
  • 3.6-3.7 billion years ago: appearance of life
  • 2.5 billion years ago oxygen-forming photosynthesis
  • ~2.2 billion years ago: aerobic respiration
  • ~1.5 billion years ago: first evidence of fossil eukaryotes

• The appearance of Life: anaerobic heterotrophes
  • 3.6-3.7 billion years ago: appearance of life
  • Most likely first cells were anaerobic, heterotrophic bacteria
    • anaerobic = does not require free oxygen
    • heterotrophic = does not make its own food

• The next step: anaerobic autotrophs
  • Were able to fix CO2
    • turning CO2 + H into organic molecules
  • Hydrogen donors initially were H2, H2S

• Energy sources for autotrophics
  • First used chemical energy from elements in surrounding medium
    • chemoautotrophs (deep-sea vents)
  • As this energy ran low, evolved ability to capture energy from light
    • Photoautotrophs

• Life' s first major crisis
  • Easy hydrogen donors (H2, H2S) used up quickly
  • Key innovation around 2.5 billion years
    • oxygen-forming photosynthesis (cyanobacteria)
    • Use of H2O as a hydrogen donor

• Life' s second major crisis
  • Huge amounts of toxic O2 released
  • Most of the initial O2 was locked up by iron in the oceans and soils (Banded iron formations) = rust
  • More O2 from water keep coming, leading to an O2 rich atmosphere

• Life' s next major innovation
  • Aerobic respiration
    • much more efficient than anaerobic respiration
    • Allowed larger cells and the future potential of multicellular organisms

Part 3: The fossil record relating to the origins of life

• Map of important fossil locations

  

• Fossil bacterial series showing evidence of cell division

  

• Early cells arranged in a filament (Warawoona, 3.5 BYA)

  

• Stromatolites (mounds of photosynthetic cyanobacteria) 2.7 billion years old.
  • Stromatolites fossil from Warrawoona

    

  • Intact stromatolites present today in Shark's Bay

    

Part 4: Experimental studies of the origins of life

• Early thinking on the origin of life (i.e., the first cell)
  • Spontaneous generation
    • Pasteur' s experiments (1860' s)
  • Oparin, Haldane (1920' s)
  • Notion of a primeval soup

• Key steps in the origins of life
  • Formation of complex organic molecules
  • Self-replicating systems
  • Protein synthesis
    • DNA is the genetic material, but it requires proteins to replicate
  • Compartmentalization: the first cell

• Origins of complex organic molecules
  • nucleosynthesis in stars to form complex molecules
  • molecular clouds
  • A very significant fraction of the Earth' s carbon came from extensive cometary bombardment on the primitive Earth

• Model systems for prebiotic evolution
  • Miller-Urey experiment
  • Fox' s microspheres
  • Cech' s Catalytic RNA

• The Miller-Urey experiment (1953)
  • Showed that complex organic molecules (amino acids) can be built up from very simple organic molecules (such as methane)

    

• Compartmentalization: Fox' s microspheres
  • In the 1970' s, Fox showed that by heating certain proteins, microspheres form spontaneously

    

• Catalytic RNAs
  • Self-cleaving rRNA
  • RNA can both cleave itself as well as polymerase itself
  • the solution to the chicken versus egg problem
    • don' t need proteins as RNA can act as an enzyme

• The first cells may have had RNA genomes
  • DNA synthesis requires RNA primer
  • RNA, not DNA used in protein synthesis
  • Reverse transcriptase RNA --> DNA

Summary

• Formation of complex organic molecules
  • These are expected to be common on early earth
• Self-replicating systems
• Protein synthesis
  • RNA can be both genetic material and serve the role of proteins in replication
• Compartmentalization: the first cell
  • microspheres offer a solution

• Food
  • use what' s there -- heterotrophs
  • make your own -- autotrophs
• Making food: energy source
  • chemicals
  • light
• Making food: hydrogen source:
  • H2, H2S are not that abundant
  • H2O -> abundant, but using releases O2
• Dealing with O2