# Explain how the first cells could have arisen from non-living matter through the progressive increase in molecular complexity on early Earth.

> IB DP Biology Higher Level (2023 syllabus) — A2.1 Origins of cells · Explain · 4 marks

> The origin of life is thought to have involved a series of stages in which simple inorganic molecules gave rise to increasingly complex organic systems capable of self-replication and compartmentalisation.

## Mark scheme (4 marks)

1. Simple inorganic molecules (e.g. H₂O, CO₂, NH₃, H₂S) reacted abiotically to form organic monomers such as amino acids or nucleotides under early Earth conditions (e.g. lightning, UV radiation, heat).
2. Monomers polymerised abiotically to form macromolecules such as polypeptides or polynucleotides, possibly on mineral or clay surfaces that concentrated the molecules.
3. Amphipathic / phospholipid-like molecules spontaneously formed bilayers that enclosed these macromolecules, creating membrane-bound compartments (protocells / liposomes).
4. Protocells that could replicate their nucleic acids and pass heritable information to offspring were subject to natural selection, driving the emergence of the first true cells.

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