5.2 The emergence of life and major evolutionary transitions

Life's journey from simple molecules to complex organisms is a fascinating tale. It begins with basic building blocks like amino acids, which combine to form more intricate structures. These structures eventually develop the ability to replicate, metabolize, and adapt to their environment.

As life evolves, major transitions occur. Cells become more complex, multicellular organisms emerge, and sexual reproduction develops. These changes lead to increased diversity and adaptability, allowing life to conquer new frontiers like land and air.

Characteristics and Origins of Life

Characteristics of living systems

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• Organized structures exhibit complex and ordered arrangements (cells, tissues, organs)
• Metabolize energy through anabolism (photosynthesis) and catabolism (cellular respiration)
• Maintain homeostasis by regulating internal conditions (body temperature, pH levels)
• Grow and develop from simple to more complex forms (embryonic development, metamorphosis)
• Reproduce offspring and pass genetic information to next generation (sexual, asexual reproduction)
• Respond and adapt to environmental stimuli (tropisms, habituation)
• Evolve over generations through changes in genetic composition (natural selection, genetic drift)

Transition to living systems

• Abiotic synthesis of simple organic molecules (amino acids) as demonstrated by Miller-Urey experiment
• Concentration and isolation of organic compounds in hydrothermal vents, clay surfaces, or lipid vesicles prevents degradation and dilution
• Polymerization of complex molecules (proteins, nucleic acids) catalyzed by minerals or other organic compounds
• Emergence of self-replicating systems (RNA) capable of templated replication
• Encapsulation in lipid membranes forms protocells and compartmentalizes chemical reactions
• Development of primitive metabolism couples chemical reactions to generate energy and establish basic metabolic pathways
• Diversification and adaptation of protocells leads to evolution of first living organisms

Major Evolutionary Transitions

Major evolutionary transitions

• Emergence of eukaryotic cells:
  1. Endosymbiotic theory proposes incorporation of prokaryotic cells as organelles (mitochondria, chloroplasts)
  2. Compartmentalization of cellular functions in specialized organelles increases size and complexity
• Development of multicellularity:
  1. Colonial organisms aggregate individual cells that cooperate and divide labor (Volvox)
  2. True multicellularity differentiates cells into tissues and organs with coordinated development and function (plants, animals)
  3. Multicellularity enhances survival, adaptability, and enables specialization (roots, leaves, organs)
• Origin of sexual reproduction increases genetic diversity and accelerates adaptation and evolution (meiosis, fertilization)
• Colonization of land by plants and animals spurs adaptation to terrestrial environments and diversification of life forms (vascular tissues, seeds, limbs)

RNA world hypothesis evidence

• RNA's versatility enables it to store genetic information like DNA and catalyze reactions like enzymes (ribozymes)
• Discovery of naturally occurring catalytic RNA molecules (ribozymes) demonstrates RNA's catalytic properties
• RNA viruses with genomes composed entirely of RNA suggest possible remnants of ancient RNA-based life forms
• Ribosomes contain catalytic RNA components, indicating RNA's central role in protein synthesis
• RNA could have served as an early self-replicating molecule, enabling templated replication without proteins
• Emergence of RNA-based life forms transitioned to DNA and protein-based life:
  1. Development of DNA as more stable genetic material
  2. RNA-catalyzed synthesis of proteins
  3. Gradual shift to modern DNA-RNA-protein system