Origins of cells

• Conditions on early Earth

  Not able to support life, but instrumental in the origin of biological compounds that made life possible
• Early atmosphere

  • Higher levels of carbon dioxide and methane than present atmosphere
  • Methane and carbon dioxide are powerful greenhouse gases
  • Trap infrared radiation, increasing surface temperatures (greenhouse effect)
• UV radiation

  • Penetrated to the surface of Earth due to lack of ozone
  • Causes DNA damage and increases mutation rate, harmful to living organisms
• These conditions on early Earth may have resulted in the spontaneous formation of carbon compounds by chemical processes that do not currently occur
• Formation of organic molecules

  1. Adding energy (heat or UV) to mixture of gases in early atmosphere

  2. Led to formation of amino acids, simple sugars, nucleotides, fatty acids
• Primordial soup hypothesis


  Proposed by Oparin and Haldane to explain origin of biological molecules
• High levels of UV radiation on early Earth could have catalysed formation of larger polymers (proteins, complex sugars, mRNA, phospholipids) from simpler molecules
• Miller-Urey experiment

  • Recreated conditions thought to have existed on Earth prior to life
  • Produced simple organic molecules including amino acids
• Miller-Urey apparatus

  • Boiled water to produce steam
  • Mixed steam with gases (methane, hydrogen, ammonia)
  • Added electrical discharges to simulate lightning
  • Cooled the mixture
• Limitations of Miller-Urey experiment

  • Methane availability may have been low
  • Used electrical discharge instead of UV light
  • Amino acids tend to remain as monomers in watery environment
  • Unable to generate nucleotides
• Cells

  • Smallest units of life
  • Enclosed by plasma membrane
  • Store genetic information in DNA
  • Express genetic information during protein synthesis
• Characteristics of life

  • Metabolic reactions
  • Need for nutrition
  • Production of metabolic waste
  • Ability to reproduce and pass on genetic information
  • Ability to receive and respond to stimuli
  • Ability to grow
• Viruses

  • Considered non-living
  • Lack cell structure and organelles
  • Unable to perform most characteristics of life
• Spontaneous origin of cells

  1. Synthesis of simple organic compounds from inorganic molecules

  2. Assembly of organic compounds into polymers

  3. Development of self-replicating polymers

  4. Formation of membranes to create compartments
• Theories on origin of cells

  • Protocell-first theory
  • Gene-first theory
  • Metabolism-first theory
• Formation of vesicles

  1. Lipid molecules form monolayers, then bilayers

  2. Bilayers spontaneously form microspheres/vesicles

  3. Fatty acids combine with glycerol to form triglycerides

  4. Triglycerides undergo phosphorylation to form phospholipids
• RNA world hypothesis

  • RNA may have performed functions of replication and catalysis in early cells
  • As life evolved, DNA took over genetic storage and proteins became catalysts
• Evidence for RNA as first genetic material includes: RNA can assemble spontaneously, replicate itself, and catalyse chemical reactions
• Evidence that RNA may have been around before DNA includes: Ribose can be formed from methanal, and deoxyribose in DNA is produced from ribose
• Last Universal Common Ancestor (LUCA)


  Ancient common ancestor of all life on Earth, existed ~4 billion years ago
• Evidence for common ancestry

  • Same biochemistry
  • Same DNA bases and genetic code
  • Same amino acids in proteins
• Researchers found shared genes between eubacteria and archaea, indicating inheritance from LUCA
• Molecular clock


  Used to determine date of life's origin based on rate of DNA/protein changes over time
• Fossil evidence and genetic analysis indicate LUCA may have been an autotrophic extremophile that lived in hydrothermal vents