The Origin of Life
From Chemistry to Biology: Life's Beginning
Conditions on Early Earth
Earth formed ~4.6 billion years ago. Early Earth had:
- No free oxygen (anoxic atmosphere: N₂, CO₂, CH₄, NH₃, H₂O vapour, H₂)
- Intense UV radiation (no ozone layer)
- Frequent volcanic activity, hydrothermal vents
- No ozone layer protecting against UV
- Possible lightning, meteorite bombardment (delivering organic molecules)
The first evidence of prokaryotic life appears ~3.7–3.8 billion years ago (stromatolites). Life's origin required: synthesis of organic monomers → polymers → self-replicating molecules → membrane-bound protocells.
Hypotheses for the Origin of Life
Miller-Urey experiment (1953): simulated early Earth conditions (H₂, CH₄, NH₃, H₂O, electrical sparks) and produced amino acids → proving organic monomers can form abiotically.
RNA World hypothesis: RNA may have preceded both DNA and proteins — RNA can both store information (like DNA) and catalyse reactions (ribozymes). Support: ribozymes exist; ribosome's peptidyl transferase activity is RNA-based.
Hydrothermal vents: deep-sea hydrothermal vents (especially alkaline 'white smokers') may have provided chemical gradients, mineral surfaces for concentrating molecules, and thermal energy.
Panspermia: hypothesis that life (or organic precursors) arrived on Earth via meteorites — shifts the question rather than answering it.
First Cells: Protocells
Experiments show that fatty acids (available abiotically) spontaneously form membrane vesicles (protocells) — primitive cell-like structures. If RNA was encapsulated in a protocell and could self-replicate, natural selection could begin: protocells with better-replicating RNA would outcompete others.
The Miller-Urey experiment (1953) demonstrated that:
Miller and Urey passed electrical sparks through a mixture of inorganic gases (simulating early Earth) and produced amino acids and other organic molecules abiotically — a key demonstration that biochemical building blocks can arise without life.
Correct answer: Amino acids can form abiotically from simple inorganic molecules
The RNA World hypothesis proposes that RNA preceded DNA and proteins because:
RNA's dual capacity — information storage (like DNA) and catalysis (ribozymes, like proteins) — makes it a plausible first informational/catalytic molecule in the origin of life.
Correct answer: RNA can both store genetic information and catalyse chemical reactions
Why is the origin of a self-replicating molecule considered the critical event in the origin of life?
Self-replication introduces inheritance and variation — the prerequisites for natural selection.
Correct answer: Before self-replication, molecular complexity could form but would not be maintained or inherited. Once a molecule can copy itself (even imperfectly), Darwinian evolution can begin: variants arise by copying errors, and those that copy more efficiently increase in frequency. This is the transition from chemistry to biology — heredity with variation under selection.