The History of Life on Earth

Macroevolution

  • Definition: Broad pattern of evolution above the species level.

  • Evidence from fossil record: Emergence of terrestrial vertebrates, impact of mass extinctions, origin of key adaptations (e.g., flight).

Origin of Life on Early Earth

  • Earth's Formation: Approximately 4.6 billion years ago (YA).

  • Early Atmosphere: Little oxygen (O₂), rich in water vapor and volcanic gases (N₂, CO₂, CH₄, NH₃, H₂).

  • Abiotic Synthesis of Organic Molecules:

    • Miller-Urey Experiment (1953): Demonstrated abiotic synthesis of organic molecules in an oxygen-poor atmosphere.

    • Sources: Deep-sea hydrothermal vents, deep-sea alkaline vents, and meteorites (e.g., Murchison meteorite with amino acids, lipids, sugars).

  • Steps to Life (Hypothesized Order):

    1. Abiotic synthesis of small organic monomers.

    2. Joining of organic monomers into polymers (e.g., RNA polymers on hot surfaces).

    3. Packaging of molecules into membranous protocells (fluid-filled vesicles with membrane-like structures, exhibiting growth, reproduction, metabolism).

    4. Origin of self-replicating molecules (RNA likely the first genetic material, with ribozymes acting as enzymes).

The Fossil Record

  • Formation: Accumulation in sedimentary rock layers (strata), mineralization, preservation in ice or bogs.

  • Dating:

    • Relative Age: Order in rock strata.

    • Radiometric Dating: Decay of radioactive isotopes (e.g., Carbon-14 for fossils up to 75,000 years, dating surrounding rocks for older fossils).

Key Evolutionary Milestones

  • First Single-Celled Organisms: Stromatolites (layered prokaryotic rock formations) from 3.5 billion YA.

  • Oxygen Revolution: O₂ accumulated gradually 2.7 to 2.4 billion YA due to photosynthesis, driving some prokaryotes to extinction and leading to aerobic respiration.

  • First Eukaryotes: Oldest fossils from 1.8 billion YA. Originated by endosymbiosis (engulfed cells became mitochondria and chloroplasts).

  • Origin of Multicellularity: Small red algae 1.2 billion YA; larger organisms ~600 million YA.

  • Cambrian Explosion: Rapid appearance of many animal phyla 535–525 million YA.

  • Colonization of Land: Prokaryotes (3.2 billion YA), fungi, plants, animals (~500 million YA), arthropods (~450 million YA), tetrapods (~365 million YA).

Major Processes Shaping Life

  • Plate Tectonics (Continental Drift): Movement of Earth's crust, influencing climate and leading to allopatric speciation.

  • Sea Level Fluctuations: Created bridges and barriers, contributing to allopatric speciation.

  • Mass Extinctions:

    • Causes: Extreme volcanism (e.g., Permian Extinction), meteor crashes (e.g., Cretaceous Extinction 66 million YA), both causing major climate changes.

  • Adaptive Radiation: Rapid evolutionary change leading to many new species, often following mass extinctions or colonization of new regions/habitats.

  • Evolution of Developmental Genes:

    • Function: Control rate, timing, and pattern of development.

    • Heterochrony: Changes in relative timing of reproductive to nonreproductive development (e.g., paedomorphosis).

    • Homeotic Genes (e.g., Hox genes): Master regulatory genes determining where features develop (Ubx gene in insects suppressing leg formation).

  • Evolutionary Novelties: Evolve in stages from simpler structures, often from changes in existing gene expression or co-option for new functions.