Question
Describe the Miller-Urey experiment. What were the conditions simulated, the gases used, and the products obtained? How does this experiment support the theory of chemical evolution (abiogenesis)?
(NEET 2022, similar pattern)
Solution — Step by Step
In the 1920s, A.I. Oparin and J.B.S. Haldane independently proposed that life originated from simple inorganic molecules on primitive Earth. The early atmosphere was reducing (no free oxygen) and rich in methane, ammonia, hydrogen, and water vapour. Energy from lightning, UV radiation, and volcanic heat could have driven chemical reactions to form organic molecules.
This was called the theory of chemical evolution or abiogenesis.
Stanley Miller and Harold Urey tested the Oparin-Haldane hypothesis in the lab.
Setup:
- A closed glass apparatus with two chambers connected by tubes
- One chamber contained water (simulating the primitive ocean) that was heated to produce steam
- The other chamber contained a mixture of gases: (methane), (ammonia), (hydrogen), and (water vapour) — simulating the early atmosphere
- Electric sparks were discharged through the gas mixture (simulating lightning)
- A condenser cooled the products back into the water chamber
After running the apparatus for one week, Miller analysed the water and found:
- Amino acids — glycine, alanine, aspartic acid, and others (up to 15 amino acids)
- Simple organic molecules — urea, acetic acid, formaldehyde, hydrogen cyanide
- Sugars and lipid precursors in smaller amounts
The experiment demonstrated that organic molecules essential for life could form spontaneously from inorganic precursors under the right conditions — no living organism required.
Why This Works
The experiment works because energy (electric discharge) breaks the stable bonds in simple molecules (, , ) and creates reactive intermediates (free radicals). These recombine to form more complex organic molecules, including amino acids.
The reducing atmosphere is critical — with free oxygen present, organic molecules would be oxidised and destroyed before they could accumulate. The absence of in the primitive atmosphere allowed organic molecules to persist and eventually form the “primordial soup” from which life emerged.
This supports the idea that the building blocks of life (amino acids, nucleotides) could have formed abiotically and gradually assembled into more complex structures — protocells, self-replicating molecules, and eventually living cells.
Alternative Method — Key Facts for Quick Recall
For NEET, remember these bullet points:
- Who: Stanley Miller and Harold Urey (1953)
- Gases: (no !)
- Energy source: Electric sparks (simulating lightning)
- Products: Amino acids (glycine, alanine), urea, organic acids
- Significance: First experimental evidence for abiogenesis/chemical evolution
- Key condition: Reducing atmosphere (no free oxygen)
Common Mistake
Students often include in the gas mixture — this is wrong. The primitive atmosphere had NO free oxygen. Oxygen accumulated later through photosynthesis by cyanobacteria (Great Oxidation Event, ~2.4 billion years ago). Including oxygen in the Miller-Urey setup would destroy the organic products. Also, the experiment does NOT prove that life originated this way — it only shows that it’s possible for organic molecules to form abiotically. Don’t overstate the conclusion in your NEET answer.