prebiotic evolution

Pre-Biotic Evolution

  • Alexander Oparin and J.B.S. Haldane proposed a gradual pre-biotic evolution of molecules leading to self-replication and complex molecular aggregates, termed the Oparin-Haldane Hypothesis.

Evolution of the Cosmos

  • Initial state: All forces (electromagnetism, weak and strong nuclear forces, and gravity) were unified.

  • Big Bang: An explosion of pure energy that occurred approximately 15 billion years ago, marking the beginning of the universe.

  • Earth Formation: About 4.5 billion years ago, the Earth formed from the remnants of the Big Bang; initially a molten mass.

  • Oldest rocks are estimated to be 3.5 - 3.8 billion years old, suggesting the crust became solid at least this long ago.

Early Atmosphere

  • Hypothesized composition: 74% carbon dioxide, 15% water vapor, 10% nitrogen.

  • Present atmosphere: 78% nitrogen, 21% oxygen, 1% argon, 0.03% carbon dioxide.

  • Life is believed to have evolved in an oxygen-free atmosphere.

Oldest Fossils

  • Roughly 3.5 billion years old; stromatolites formed from cyanobacteria and photosynthesizing bacteria.

  • Life predates these fossils due to the necessary existence of bacteria before photosynthesis.

Chemical Evolution

Miller and Urey Experiment (1953)

  • Stanley Miller and Harold Urey published a paper on amino acid production under primitive Earth conditions.

  • Experiment: Circulated ammonia, methane, hydrogen, and water vapor in a flask and applied electrical discharge over 7 days.

  • Result: Brown slurry containing a rich variety of organic compounds, including common amino acids, urea, acetic acid, and lactic acid.

  • Absence of oxygen would prevent the breakdown of these compounds, resulting in a concentrated "soup" of organic molecules.

Iron-Sulfur World Theory

  • Proposed by Günter Wächtershäuser (1988-1992).

  • Suggests early life formed on iron sulfide minerals with catalytic transition metals.

  • Pioneer organism originated in volcanic hydrothermal flows at high pressure and high temperature (100 °C).

  • Catalytic centers: Iron, nickel, possibly cobalt, manganese, tungsten, and zinc responsible for autotrophic carbon fixation.

  • Chemical pathways include generating small organic compounds from inorganic gases (e.g., carbon monoxide, carbon dioxide).

Key Steps

  1. Heated water flow with volcanic gases (e.g., carbon monoxide, ammonia).

  2. Catalyzed reactions on transition metal solids.

Experiments

  • Synthetic reactions confirmed the generation of activated acetic acid derivatives essential for further biochemical processes.

  • Specific reactions with nickel hydroxide and hydrogen cyanide produce pairs of amino acids and pyruvic acid.

Early Evolution

  • Defined from the origin of life to the last universal common ancestor (LUCA).

  • Explores coevolution of cellular organization, genetic machinery, and enzymatic metabolism.

Cellularization Stages

  1. Formation of primitive lipids in mineral bases.

  2. Accumulation of lipids leading to semi-cell structures bounded by mineral and lipid membranes.

  3. Development into closed cells (pre-cells) allowing genetic exchange, promoting rapid evolution.

Proto-Ecological Systems

  • Proposed by William Martin and Michael Russell.

  • Cellular life may have evolved in alkaline hydrothermal vents with micro-caverns.

Features of Vents

  • Concentration of newly synthesized molecules.

  • Temperature gradients enabling partial reactions in different regions.

  • Constant energy supply from hydrothermal processes.

  • Facilitated stages of cellular evolution within a single structure.

RNA World Hypothesis

  • Proposes self-replicating RNA molecules predated current life forms dependent on DNA.

  • RNA's roles: Genetic information storage similar to DNA, catalytic functions similar to enzymes.

History and Support

  • First introduced by Walter Gilbert in 1986.

  • Influential earlier concepts by Francis Crick, Leslie Orgel, Carl Woese.

  • Suggests RNA could replicate autonomously and catalyze chemical reactions as ribozymes.

RNA as Enzyme

  • Short RNA molecules acting as ribozymes demonstrate self-replication capability, such as a 189-base ribozyme noted for high fidelity.

  • The ability to catalyze peptide bond formation by RNA suggesting a basis for early protein synthesis.

Implications of RNA World

  • Redefines life from a DNA/protein-centric view to one where RNA played a central role.

  • RNA structures, such as snRNPs, demonstrate important cellular functions beyond mere storage.

Pre-Biotic Cell Membrane Formation

  • Non-living organic compounds create coacervate droplets and proteinoid microspheres, promoting chemical concentration.

  • The significance of these formations in evolving cell membranes remains debated.

Endosymbiotic Theory

  • Proposes that eukaryotic cells developed through symbiotic relationships between prokaryotes.

Steps of Development

  1. Invasion by a purple bacterium leading to mitochondria.

  2. Acquisition of cilia or flagella from spirochetes.

  3. Integration of a photosynthetic cyanobacterium leading to chloroplast formation.

Organelles

  • Mitochondria, chloroplasts, and the nucleus show distinct but shared features among eukaryotic cells, supporting the endosymbiotic theory.

Major Groups of Prokaryotes

Eubacteria

  • Includes most pathogenic species and photosynthetic bacteria; can operate aerobically or anaerobically.

Archaebacteria

  1. Methanogens - Generate methane from CO2.

  2. Halophiles - Utilize a simple photosynthetic system.

  3. Thermophiles - Thrive in high-temperature environments.

Urkaryotes

  • Ancestral lineage leading to eukaryotes, linking to all higher organisms.

Conclusion

  • The formation of life is hypothesized to proceed from organic molecule synthesis through aggregation to stable entities capable of replication. Potential for energy utilization has evolved from anaerobic processes to complex metabolic pathways encountered in modern organisms.