Explain how the chemical conditions on early Earth could have supported the abiotic synthesis of organic monomers necessary for the origin of life.

IB DP Biology Standard Level (2023 syllabus) — A2.1 Origins of cells · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Scientists hypothesise that life on Earth originated approximately 3.5–4 billion years ago, when the planet's surface and atmosphere differed dramatically from today.

Model answer (4 marks)

The early atmosphere was anoxic and reducing, containing gases such as CH₄, NH₃, H₂ and H₂O vapour. These simple inorganic molecules could not be oxidised, so they remained available for synthesis of organics. Energy from lightning, UV radiation or hydrothermal vents supplied the activation energy for reactions between the gases. Such conditions allowed the formation of organic monomers – amino acids, simple sugars, nucleobases and nucleotides – as shown by the Miller–Urey experiment.

Examiner tips

Mention the reducing atmosphere first to show understanding of conditions; list the key gases; name the energy sources; cite the Miller–Urey experiment as evidence of abiotic synthesis.
Use precise terminology: anoxic, reducing, monomers, amino acids, nucleobases.
Keep answer concise – 4 marks allow one sentence per point.

Common mistakes

Confusing oxidising atmosphere with reducing; writing that oxygen was present.
Failing to name the key gases or energy sources; giving irrelevant details such as photosynthesis.
Using vague terms like ‘energy’ without specifying lightning, UV or heat from vents.

Mark scheme (4 marks)

The early atmosphere lacked free oxygen / was a reducing atmosphere, preventing oxidation of organic molecules.
Simple inorganic molecules / gases (such as methane, ammonia, hydrogen, water vapour) were present and could serve as raw materials for organic synthesis.
Energy sources such as lightning, UV radiation, or heat from hydrothermal vents could drive chemical reactions between these simple molecules.
These reactions could produce organic monomers such as amino acids / simple sugars / nucleotides / nucleobases abiotically, as demonstrated experimentally (e.g. Miller–Urey experiment).

Key terms in this question

abiotic synthesis · organic monomers