PPT 11

Lecture 11: Abiogenesis and Natural Selection

Introduction to Abiogenesis

• Life originated under conditions vastly different from present-day Earth.

• Early Earth atmosphere possibly consisted of:

  • Water vapor

  • Nitrogen and its oxides

  • Carbon dioxide

  • Methane

  • Ammonia

  • Hydrogen

  • Hydrogen sulfide

The Hadean: The First Geological Eon

• Initial conditions set the stage for the development of life to be explored.

Theory of Chemosynthetic Origin of Life

Concepts of Abiogenesis

• Conditions of early Earth markedly different:

  • High presence of gases like nitrogen, methane, and hydrogen sulfide.

• Theories proposed in the 1920s:

  • A. I. Oparin and J.B.S. Haldane's Hypothesis:

    • Early oceans could have been a "primordial soup" of organic molecules.

    • The atmosphere was reducing (electron-adding), allowing for organic molecule formation.

    • Energy sources for organic synthesis included lightning and intense UV radiation.

Miller-Urey Experiment (1953)

• Conducted experiments based on the early Earth atmosphere hypothesis:

  • Successfully created organic compounds, notably amino acids.

  • Subsequent research by others, under extreme conditions, produced polymers without enzymatic assistance.

Protobionts

Characteristics and Evolution

• Protobionts are collections of abiotically produced molecules enveloped by vesicle-like structures.

• Early replication may have involved RNA:

  • Copy errors could stabilize variants leading to selection.

• Transition from RNA to DNA:

  • DNA provides greater stability and fewer replication errors.

Stages of Creating Simple Cells

1. Abiotic Synthesis: Formation of small organic molecules (e.g., amino acids).

2. Micromolecule Assembly: Combining small molecules into macromolecules (e.g., proteins).

3. Packaging into Protobionts: Forming membranes isolating internal environments.

4. Origin of Self-Replicating Molecules:

   • Initially RNA, followed by DNA.

   • Development of photoautotrophs which significantly contributed to atmospheric oxygen.

Self-Replicating RNA and Ribozymes

• RNA likely the first genetic material:

  • Ribozymes can duplicate RNA given monomer supplies.

  • RNA is versatile, allowing for various forms and enzymatic functions.

Evolution of Eukaryotes

Endosymbiosis Model

• Theory suggests that mitochondria and chloroplasts originated as prokaryotic endosymbionts:

  • Entered as prey or internal parasites, particularly advantageous if anaerobic.

• Evidence supporting this:

  • Inner membranes share traits with prokaryotes.

  • Replication processes resemble prokaryotic fission.

  • Both organelles possess their own DNA and ribosomes.

Historical Context: Before Darwin

Classification and Early Theories

• Carolus Linnaeus (1707-1778):

  • Developed binomial nomenclature for species classification.

  • Established taxonomy.

• Georges Cuvier (1769-1832):

  • Studied fossils, noted changes across strata, and inferred extinctions.

  • Advocated catastrophism as an explanation for observed phenomena.

• James Hutton (1726-1797):

  • Proposed uniformitarianism to explain geological features through gradual processes.

Contributions of Early Evolutionary Thinkers

Lamarck, Malthus, and Lyell

• Jean Baptiste Lamarck (1744-1829):

  • Proposed an early theory of evolution: use and disuse of traits, with hereditary transmission.

  • Recognized environmental influence on evolution.

• Thomas Malthus (1766-1834):

  • Suggested populations grow rapidly, leading to competition and potential societal collapse.

• Charles Lyell (1797-1875):

  • Advocated gradual geological changes and established uniformitarianism based on Hutton's ideas.

Charles Darwin's Contributions

The Beagle Voyage and Evolution

• Charles Darwin (1809-1882):

  • Established foundational theories on evolution through observation and evidence collection during the Beagle voyage.

  • Noted diversity in species across different geographic locations, particularly the Galapagos Islands.

Darwin-Wallace Theory of Evolution

Key Concepts

• Evolution rests on four main principles:

  1. Variation among individuals within a population.

  2. Heritable variations passed down.

  3. Overproduction leading to competition for resources.

  4. Natural selection favoring individuals with traits best suited to the environment.

• Natural Selection Process:

  • Variation leads to differential survival and reproduction tailored to the environment, gradually causing evolution and adaptation in populations.

Adaptive Radiation of Galapagos Finches

• Illustrates evolution based on environmental influences shaping species traits (e.g., beak types).

• Represents a key example of Darwin's concept of "descent with modification.

• Each branch signifies common ancestral lineage.