A2.1

• What plausible hypothesis could account for the origin of life?
• What intermediate stages could there have been between non-living matter and the first living cells?
• Cells have always worked using the same basic principles
• Conditions on early Earth and the pre-biotic formation of carbon compounds

  Lack of free oxygen and therefore ozone, higher concentrations of carbon dioxide and methane, resulting in higher temperatures and ultraviolet light penetration. The conditions may have caused a variety of carbon compounds to form spontaneously by chemical processes that do not now occur.
• Cells


  The smallest units of self-sustaining life
• Cells are highly complex structures that can currently only be produced by division of pre-existing cells
• Catalysis, self-replication of molecules, self-assembly and the emergence of compartmentalization were necessary requirements for the evolution of the first cells
• Claims in science, including hypotheses and theories, must be testable
• The exact conditions on pre-biotic Earth cannot be replicated and the first protocells did not fossilize
• Miller-Urey experiment


  1. Simulated conditions on early Earth

  2. Produced amino acids and other organic compounds
• Spontaneous formation of vesicles by coalescence of fatty acids into spherical bilayers

  Formation of a membrane-bound compartment is needed to allow internal chemistry to become different from that outside the compartment
• RNA


  • May have acted initially as both the genetic material and the enzymes of the earliest cells
  • Ribozymes in the ribosome are still used to catalyse peptide bond formation during protein synthesis
• Evidence for a last universal common ancestor includes the universal genetic code and shared genes across all organisms
• Other forms of life may have evolved but become extinct due to competition from the last universal common ancestor (LUCA) and descendants of LUCA
• Approaches used to estimate dates of the first living cells and the last universal common ancestor
• Evidence for the evolution of the last universal common ancestor in the vicinity of hydrothermal vents includes fossilized evidence of life from ancient seafloor hydrothermal vent precipitates and evidence of conserved sequences from genomic analysis
• Formation of the Earth

  4.6 billion years ago
• First water formed


  4.4 billion years ago
• Hadean Eon


  Prebiotic earth - no evidence of any life forms
• Evidence of conditions on early Earth


  • Frequent volcanic eruptions
  • Meteorites
  • Lightning
  • High temperatures and UV radiation
• Composition of early Earth's atmosphere

  • Carbon dioxide
  • Hydrogen
  • Nitrogen
  • Water vapour
  • Methane
  • Ammonia
  • Hydrogen sulfide
• Implications of early Earth's atmosphere

  • No oxygen
  • No ozone (O3 - a protective gas blocks UV radiation)
  • High levels of Ultraviolet radiation - mutates DNA
  • High surface temperatures
• May have resulted in the formation of carbon compounds such as amino acids, fatty acids, nucleic acids
• Exact mechanism of prebiotic formation of carbon compounds is unknown
• Inorganic molecules


  Don't have carbon and hydrogen within the same molecule, usually small
• Organic molecules


  Contain carbon and hydrogen, can be large with many atoms and associated with living organisms
• Converting simple inorganic molecules into complex organic molecules requires energy
• Functions of living things

  • Metabolism
  • Response
  • Homeostasis
  • Growth
  • Excretion
  • Reproduction
  • Nutrition
• Cell theory


  • All living things are composed of cells (or cell products)
  • The cell is the smallest unit of life
  • Cells only arise from pre-existing cells
• All living things are composed of cells (or cell products)
• All cells have an outer membrane, genetic material, and metabolic processes
• Specialized structures within cells (organelles) carry out different functions, and cannot survive alone
• Viruses are not considered living because they lack the organelles to carry out metabolism and can only reproduce inside host cells
• To be classified as living, life forms must evolve by natural selection, be separate from but in communication with their surroundings, and be chemical and physical machines that receive and respond to stimuli
• Cells are highly complex structures and no mechanism has been found for producing cells from simpler subunits
• The first cells must have arisen from non-living material, as it is extremely difficult to generate cells from anything but other cells
• Key problems in explaining the spontaneous origin of cells

  • Non-living synthesis of simple organic molecules
  • Assembly of organic molecules into polymers
  • Formation of polymers that can self-replicate
  • Formation of membranes to package the organic molecules
• Miller and Urey experiment

  Tried to recreate conditions of prebiotic Earth in a closed system, including a reducing atmosphere, high radiation, high temperatures, and electrical storms. After one week, they found amino acids and complex organic compounds had formed.
• Once formed, small organic molecules may have been able to spontaneously join to form polymers such as proteins, fatty acids, and RNA
• Deep-sea hydrothermal vents


  Fissures in a planet's surface from which geothermally heated water issues, providing the right conditions and chemicals to allow organic polymers to arise