Life's origins remain a mystery, but scientists have developed intriguing theories. The RNA world, iron-sulfur world, and panspermia hypotheses offer different perspectives on how life began. Each theory has strengths and weaknesses, supported by various lines of evidence.

Understanding these origin scenarios is crucial for astrobiology. They guide our search for extraterrestrial life, helping us identify potential habitable environments and biosignatures on other planets and moons. The quest to unravel life's beginnings continues to shape our exploration of the cosmos.

Origins of Life Theories and Hypotheses

Theories of life's origins

• RNA world hypothesis
  • Self-replicating RNA molecules were the first forms of life on Earth
  • RNA can store genetic information and catalyze chemical reactions like enzymes do
  • RNA may have later evolved into the more stable DNA for information storage and proteins for catalytic functions
• Iron-sulfur world theory
  • Life originated in hydrothermal vents on the ocean floor where iron-sulfur compounds are abundant
  • Iron-sulfur compounds could have catalyzed the formation of simple organic molecules like amino acids and sugars
  • Gradients in temperature, pH, and redox potential in the vents could have driven the emergence of primitive metabolic pathways
• Panspermia hypothesis
  • Life originated elsewhere in the universe (on another planet or moon) and was transported to Earth
  • Microorganisms could have survived the journey through space on asteroids, comets, or interplanetary dust particles
  • Panspermia does not explain how life initially arose, only how it could have spread throughout the universe (Earth, Mars, Europa)

Evidence for origin hypotheses

• RNA world hypothesis
  • Strengths
    • RNA can perform both genetic (information storage) and catalytic functions (enzymatic activity)
    • Ribozymes, which are catalytic RNA molecules, have been discovered in various living organisms (bacteria, viruses)
    • RNA can self-replicate under specific laboratory conditions, demonstrating its potential as a precursor to life
  • Weaknesses
    • The spontaneous formation of RNA molecules in prebiotic conditions is challenging due to the instability of nucleotides
    • The transition from an RNA-based system to the modern DNA-RNA-protein system is not fully understood
• Iron-sulfur world theory
  • Strengths
    • Hydrothermal vents provide a stable, energy-rich environment with a continuous supply of chemicals (hydrogen, carbon dioxide)
    • Iron-sulfur compounds (pyrite, greigite) are abundant in hydrothermal vents and can catalyze redox reactions
    • The theory explains the emergence of primitive metabolic pathways and the origin of the genetic code based on amino acid binding to metal surfaces
  • Weaknesses
    • The formation of complex organic molecules like proteins in hydrothermal vents is not well-demonstrated experimentally
    • The transition from inorganic catalysts (iron-sulfur) to organic catalysts (enzymes) is not fully explained
• Panspermia hypothesis
  • Strengths
    • Microorganisms can survive in extreme conditions, such as the vacuum and radiation of space (tardigrades, bacteria)
    • The discovery of organic molecules (amino acids, nucleobases) in meteorites supports the possibility of interplanetary transport
  • Weaknesses
    • The hypothesis does not address the ultimate origin of life, only its potential spread through the universe
    • The survival of microorganisms during atmospheric entry and impact on a planet's surface is questionable
    • The hypothesis is difficult to test experimentally since it relies on rare events (asteroid impacts) and long timescales

Comparison of origin scenarios

• RNA world and iron-sulfur world theories
  1. Both propose that life originated on Earth in specific environments (prebiotic soup or hydrothermal vents)
  2. Imply that life could emerge on other planets or moons with similar conditions (liquid water, organic compounds, energy sources)
  3. Suggest that the emergence of life requires specific chemical and physical prerequisites (monomers, catalysts, concentration mechanisms)
• Panspermia hypothesis
  • Proposes that life originated elsewhere and was transported to Earth, implying that life could be more widespread in the universe
  • Suggests that the emergence of life on a planet or moon could be influenced by external factors (delivery of organic materials by comets)
• Implications for the search for extraterrestrial life
  • The RNA world and iron-sulfur world theories suggest looking for planets or moons with liquid water, organic compounds, and energy sources (Mars, Europa, Enceladus)
  • The panspermia hypothesis suggests that life could be found in a wider range of environments, including those that may not have the necessary conditions for the de novo emergence of life (Titan, comets)
  • Understanding the origin of life on Earth can inform the search for habitable environments and biosignatures on other planets and moons (methane, oxygen, phosphine)