Earth History and Evolution

Earth History and Evolution

• Key ingredients for life: essential elements, water, energy
• Oldest rocks: 4.3 billion years ago (bya)
• Significant events in Earth's history
  • 4.5 bya: Earth formation (using meteor data)
  • 4.3 bya: Oldest rocks in Quebec, evidence for liquid water
  • 3.5-3.8 bya: Evidence of microbial life (stromatolites, carbon isotopes, microfossils)
  • 2.7 bya: Oxygenic photosynthesis by cyanobacteria
  • 2.4 bya: Great Oxidation Event (O2 = 1 ppm)
  • 2 bya: Oldest known eukaryotic microfossil
  • 1.9-1.4 bya: Multicellular microfossils
  • 0.6 bya: O2 at present levels, large multicellular organisms
• Stromatolites: Layered structures formed by sediment trapping, binding, and cementation by microbial films
• Carbon isotopes:
  • Organisms prefer "light" carbon (12C) over 13C and 14C, influencing rock composition.
• Microfossils
  • Presence of 12C in Greenland rocks (3.85 bya) marks early life
• Origin of life series
  • Panspermia: Life originated elsewhere.
  • Abiotic formation of components (amino acids and nucleotides)
  • Surface catalysis: Using clay minerals as catalysts.
• RNA world: RNA could act as information and catalyst.
• Implications of oxygenated environment:
  • Toxic atmosphere conditions.
  • New metabolic pathways evolving (presence of SO4²⁻, Fe³⁺, NO₃⁻, oxygen respiration)
  • Energetic benefits of oxygen respiration (rapid diversification)
  • Creation of ozone layer (O₃) for protection
  • Prior to ozone layer, life existed only in water.
• Banded iron formations: Layered sedimentary rocks (deep water deposits) containing iron oxides and silicates.
• Oldest eukaryotic microfossils: Grypania spiralis (alga)
• Eukaryotes: Nuclear membrane, mitochondria, chloroplasts (originating ~2 bya) containing ribosomes (70S, not 80S) with circular DNA, and dividing by binary fission,.
  • Having 16S rRNA instead of 18S rRNA suggests early evolutionary relationships.

Bacterial and Archaeal Cell Structure

• Bacterial cell wall: peptidoglycan (murein), containing N-acetylmuramic acid and N-acetylglucosamine, with cross-linking by D-amino acids. This structure is hydrolyzed by lysozyme.
  • Gram-positive: Thick cell wall, containing teichoic acids.
  • Gram-negative: Thin cell wall, outer membrane (LPS), and periplasmic space.
• Archaeal cell wall: lacks peptidoglycan, has unique lipids (isoprenoids) and ether linkages to glycerol. The cell wall contains pseudopeptidoglycan, B1.3 linkages, and L-amino acids.
• Cell membrane: acts as a barrier between the cytoplasm and external environment (hydrophobic). The phospholipids form a bilayer. Bacteria use fatty acids attached to glycerol via ester bonds, while Archaea use isoprenoids attached via ether bonds. Both maintain fluid structure by using various components that influence fluidity.
• Cell wall: providing structural integrity against osmotic pressure, external environment, and toxins, while allowing nutrients in and waste out.
• Cell membrane uses: Forms osmotic barrier, allowing for energy generation via H+/Na+ gradient. Also used for transport and signaling functions
• S layer: crystalline/ordered structure as outermost layer.

Other Cell Structures

• Capsule/slime layer: Protection from toxins, predators, allows in nutrients, and reproduction.
• Flagella: common form of bacterial movement, often helical shape, rotation driven by PMF, and comprised of flagellin protein.
• Pili/fimbriae: protein filaments used for adhesion and transporting material.
• Spores: allow for survival in harsh conditions, produced when growth is restricted (endospores).
• Cytoplasmic membrane: is the inner membrane that maintains fluidity throughout.

Cell Division and Growth

• Bacterial growth is rapid and often exponential under specific conditions.
• Exponential phase: growth rate is constant, cell number is increasing, and all components increase at the same ratio. Constant doubling time.
• Lag phase: When cells first grow, they adjust their conditions to their new environment
• Stationary phase: nutrients are depleted, and waste products accumulate, so there is no change in cell numbers.
• Death phase: cells die due to depletion of a resource, or waste build-up, etc.

Factors Affecting Growth

• Nutrients: essential elements, water, energy, carbon, nitrogen, and sulfur.
• Temperature: different organisms have optimal growth temperatures.
• Hydrostatic pressure: Organisms that thrive under very high pressure (barophiles)
• Osmotic pressure: the concentration of dissolved solutes in a solution
• pH: Optimal growth for various organisms occurs at a specific pH
• Other factors: light, oxygen, heavy metals, etc

Other factors to consider

• Internal structures

  • Carboxysomes
  • Enterosomes
  • Gas vesicles
  • Magnetosomes
  • Poly-beta-hydroxybutyrate (PHB)
  • Thylakoids

• External structures

  • Exosporium
  • Capsules/slime layer
  • Flagella
  • Pili/fimbriae

• Other important concepts to note:

  • Nucleoid: Contains DNA- the genome, circular, and condensed.. It may also have linear chromosomes, if the structure requires it.
  • Cytoplasm: contains high concentration of ribosomes that participate in protein synthesis.

• Other concepts:

  • Binary fission.
  • Microbes adapt to external conditions by forming biofilms for protection against chemicals, antibiotics, and ROS.
  • Biofilm formation occurs sequentially: initial attachment, irreversible attachment, maturation of the biofilm structure, and dispersion, allowing separation of individual clusters of cells from each other.
  • Types of microbes
    • Gram-positive
    • Gram-negative
    • Psychrophiles
    • Mesophiles
    • Thermophiles
    • Hyperthermophiles