Life on Earth has existed for around 3.5×109 years, during which time it has diversified.
Existing cells are products of evolution.
Early cells may have used RNA as genetic information; ribozymes likely played a role.
Prokaryotic cells existed before eukaryotic cells (fossil evidence: stromatolites as ancient microbial mats).
Most existing eukaryotic cells likely arose via endosymbiotic events.
RNA World and ribozymes
RNA world hypothesis: RNA was the primary carrier of genetic information; ribozymes assisted copying and protection of RNA.
DNA later replaced RNA as the genetic material because it is more stable.
Proteins replaced many ribozymes due to superior catalytic properties.
Ribozymes are RNA molecules that catalyse reactions; ribosomes catalyse peptide bond formation via RNA; ribosomal RNA is catalytic, protein part is largely structural.
LUCA (Last Universal Common Ancestor)
LUCA refers to the most recent common ancestor of all life today, not the first life.
All existing life shares a common ancestral population.
Origins and early environments
Deep-sea hydrothermal vents are candidate origins: warm, hydrogen-rich fluids provide energy and minerals for early metabolism.
The Miller–Urey experiment (1953) simulated early Earth conditions (H2O, CH4, NH3, H2) and produced 13/20 amino acids, demonstrating abiotic synthesis is plausible.
Stromatolites: oldest fossil evidence of life (~3.7×109$) years ago; stromatolites still form today in places like Shark Bay and the Bahamas.
Oxygenation event: after ~2×109 years, cyanobacteria-like organisms produced oxygen via oxygenic photosynthesis; atmospheric O2 rose to 0.45%, enabling ozone formation and the eventual rise of eukaryotes, while causing extinctions of many anaerobes.
Protocells and membranes
Lipids can spontaneously form micelles and vesicles (amphipathic nature: hydrophilic heads outward, hydrophobic tails inward).
Compartments within protocells allow accumulation of reactants and selective chemistry, a step toward cellular life.
Endosymbiotic theory and eukaryotic evolution
Protists are among the oldest primitive eukaryotes; main groups include Protozoa and Algae.
Lynn Margulis proposed endosymbiotic theory: chloroplasts and mitochondria originated as free-living prokaryotes engulfed by a host cell and retained in a symbiotic relationship.
Evidence supporting endosymbiosis:
Inner membranes resemble prokaryotic membranes more than those of the host.
Mitochondria and chloroplasts divide by binary fission like prokaryotes.
They contain circular DNA and their own ribosomes, which are more prokaryote-like than eukaryotic.
They rely on nuclear-encoded proteins for many functions, indicating integration with the host.
Key takeaway points for quick recall
Life began at least 3.5×109 years ago; stromatolites provide early fossil evidence.
RNA may have been the first genetic material; ribozymes could catalyse essential reactions.
LUCA is the common ancestral node of all current life, not the first life.
Endosymbiosis explains why mitochondria and chloroplasts have prokaryote-like features.
Energy-rich environments like hydrothermal vents and abiotic synthesis experiments (Miller–Urey) support possible pathways for the origin of life.