Explain why the hypothesis that the first cells arose from pre-existing cells is not accepted, and outline the evidence that supports an abiotic origin of the first cells on early Earth.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The hypothesis that cells only arise from pre‑existing cells cannot apply to the very first cells, as there were no cells on early Earth from which they could have originated.
Simple organic molecules (monomers such as amino acids) can form abiotically from inorganic precursors, as demonstrated by the Miller–Urey experiment using conditions simulating early Earth's atmosphere.
RNA is thought to have been the first self‑replicating molecule (RNA world hypothesis), as RNA can both store genetic information and catalyse reactions (ribozymes), suggesting it could have driven early chemical evolution without proteins.
Lipids can spontaneously form bilayer vesicles (protocells/liposomes) in aqueous environments, providing a plausible mechanism by which a membrane‑bound compartment capable of enclosing replicating molecules could have formed abiotically.
Simple organic molecules (monomers such as amino acids) can form abiotically from inorganic precursors, as demonstrated by the Miller–Urey experiment using conditions simulating early Earth's atmosphere.
RNA is thought to have been the first self‑replicating molecule (RNA world hypothesis), as RNA can both store genetic information and catalyse reactions (ribozymes), suggesting it could have driven early chemical evolution without proteins.
Lipids can spontaneously form bilayer vesicles (protocells/liposomes) in aqueous environments, providing a plausible mechanism by which a membrane‑bound compartment capable of enclosing replicating molecules could have formed abiotically.
Examiner tips
- Use the exact phrase ‘cannot apply to the very first cells’ to gain the first point. Explain each piece of evidence with a brief example (Miller–Urey, RNA world, lipid vesicles).
- Show the logical sequence: no cells → need abiotic origin → evidence for monomers, RNA, membranes.
Common mistakes
- Saying ‘cells can arise from pre‑existing cells’ without noting the absence of any cells at the start. Confusing the Miller–Urey experiment with a demonstration of life rather than just monomer synthesis. Forgetting to mention that RNA can act as both information carrier and catalyst.
Mark scheme (4 marks)
- The hypothesis that cells only arise from pre-existing cells cannot apply to the very first cells, as there were no cells on early Earth from which they could have originated.
- Simple organic molecules (monomers such as amino acids) can form abiotically from inorganic precursors, as demonstrated by the Miller–Urey experiment using conditions simulating early Earth's atmosphere.
- RNA is thought to have been the first self-replicating molecule (RNA world hypothesis), as RNA can both store genetic information and catalyse reactions (ribozymes), suggesting it could have driven early chemical evolution without proteins.
- Lipids can spontaneously form bilayer vesicles (protocells/liposomes) in aqueous environments, providing a plausible mechanism by which a membrane-bound compartment capable of enclosing replicating molecules could have formed abiotically.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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