BS

Evolution of Microbial Life and Macroevolution

Evolution of Microbial Life

  • Timeline:

    • Earth formed: 4.5 ext{ BYA}

    • Stable hydrosphere: 4.2 ext{ BYA}

    • Prebiotic chemistry to Origin of life: Before 4.0 ext{ BYA}

    • Worlds progression: Pre-RNA $\rightarrow$ RNA $\rightarrow$ RNA/Protein $\rightarrow$ DNA/RNA/Protein

    • Oldest microbial fossils (stromatolites): ~3.5 ext{ BYA} (resemble modern cyanobacteria)

    • First microbial life evidence (carbon signatures): 3.7-3.8 ext{ BYA} (anoxygenic phototrophic filamentous bacteria)

    • Oxygen-rich atmosphere: 2.8-2.5 ext{ BYA}

Origin of Life Hypotheses

  • Early Theory: Spontaneous generation (disproven by Louis Pasteur in 1862).

  • Modern Hypothesis (4 Stages):

    1. Abiotic synthesis of monomers (nucleotides, amino acids).

    2. Polymerization of monomers (DNA, RNA, proteins).

    3. Packaging of polymers into protobionts (membrane-enclosed).

    4. Evolution of cellular properties (self-replication).

  • Early Earth Conditions:

    • Atmosphere: Reducing environment, rich in H2O, CO2, SO2, H2, H2S (no O2).

    • Energy Sources: Intense volcanic activity, lightning, UV radiation.

  • Stage 1: Monomer Formation:

    • Reducing Atmosphere Hypothesis (Oparin & Haldane, 1920 ext{s}): Complex organic molecules arose spontaneously.

    • Miller & Urey Experiment (1953): Simulated early Earth, generating amino acids. Later experiments showed other organic compounds in neutral (N2, CO2) atmospheres.

    • Deep-Sea Vent Hypothesis: Alkaline deep-sea vents (40-90^ ext{o} ext{C}, pH 9-11) may have been suitable.

  • Stage 2: Polymer Formation:

    • Monomers polymerize, possibly on clay substrates to overcome hydrolysis.

  • Stage 3: Protobiont Packaging:

    • Polymers aggregated into cell-like structures, e.g., liposomes encapsulating polymers.

  • Stage 4: Cellular Properties ("RNA World"):

    • RNA is favored as the first macromolecule due to its 3 key functions:

      1. Information storage.

      2. Capacity for replication.

      3. Enzymatic function (ribozymes).

    • Chemical Selection: Favored RNA molecules with self-replication and ribonucleotide synthesis abilities.

Fossil Record and Geologic Time

  • Fossil Formation: Organisms buried by sediments, hard parts replaced by minerals.

  • Radiometric Dating:

    • Carbon-14: Dates fossils up to 75,000 years (half-life 5,730 years, decays to Nitrogen-14).

    • Potassium-40: Dates volcanic rocks hundreds of millions of years old (half-life 1.3 billion years, decays to Argon-40).

  • Geologic Record: Defined by major transitions; composed of Eons (Archaean, Proterozoic, Phanerozoic) and Eras (Paleozoic, Mesozoic, Cenozoic).

  • Emergence of Life Groups:

    • Prokaryotes: 3.8 ext{ BYA}

    • Atmospheric Oxygen (from prokaryotic photosynthesis): 2.7 ext{ BYA}

    • Single-celled Eukaryotes: 2.1 ext{ BYA}

    • Multicellular Eukaryotes: 1.5 ext{ BYA}

    • Animals: 600 ext{ MYA}

    • Fungi, Plants, Animals colonize land: ~500 ext{ MYA}

    • Hominids: 6-7 ext{ MYA}

    • Humans: ~200,000 years ago

Mechanisms of Macroevolution

  • Macroevolution: Evolutionary processes and patterns at or above the species level.

  • Continental Drift:

    • Slow, continuous movement of Earth's crustal plates.

    • Pangaea formed 250 ext{ MYA}, its breakup altered habitats, triggered mass extinctions, and influenced species distribution (e.g., lungfishes, marsupials).

  • Mass Extinctions:

    • 5 major events: Ordovician-Silurian (86\% species), Late Devonian (75\%), Permian-Triassic (90\%$, greatest, 252 ext{ MYA}), Triassic-Jurassic (80\%), Cretaceous-Tertiary (76\%$, dinosaurs extinct, 65 ext{ MYA}).

    • Current extinction rates are 100-1,000 times background rate, potentially leading to a 6^ ext{th} mass extinction primarily due to human habitat loss.

  • Adaptive Radiations:

    • Rapid formation of new species filling new habitats/roles.

    • Often follow mass extinctions (e.g., mammals after dinosaur extinction) or the evolution of novel adaptations (e.g., land plants with waxy coats, stomates, vascular tissue, seeds, flowers).

  • Role of Developmental Genes:

    • Slight changes in genes controlling development can lead to major morphological differences.

  • Evolutionary Novelties:

    • Complex structures (e.g., eyes) arise through incremental modifications, with each step providing selective advantage.

    • Evolution is not goal-directed; intermediate stages must be adaptive. Example: bird wings may have initially served for insulation or display before flight.

  • Evolutionary Trends:

    • Natural selection can lead to trends (e.g., predator-prey arms races), but trends can cease or reverse if the environment changes.