Q.(a) Explain the significance of the experiment carried out by S.L. Miller. Name the scientists whose hypothesis prompted him to carry out this experiment.

Origin of Life: The Miller-Urey Experiment

(a) Significance and Historical Context

Concept: Abiogenesis – the chemical origin of life from inorganic molecules.

S.L. Miller's 1953 experiment provided the first experimental evidence that organic molecules (the building blocks of life) could form spontaneously from simple inorganic precursors under early Earth conditions. He was testing the hypothesis proposed by A.I. Oparin (1924) and J.B.S. Haldane (1929), who independently suggested that life arose through chemical evolution in a reducing atmosphere rich in methane, ammonia, hydrogen, and water vapor. …

The conceptual foundation

Before Miller's work, the origin of life was largely speculative. Two scientists—A.I. Oparin (Soviet biochemist, 1924) and J.B.S. Haldane (British biologist, 1929)—independently proposed that life arose through gradual chemical evolution in a reducing atmosphere, not through spontaneous generation or divine creation. Their hypothesis rested on a simple but profound idea: if early Earth had the right mix of simple gases and energy sources, complex organic molecules would form naturally, accumulating in a "primordial soup" that eventually gave rise to the first living systems.

Miller set out to test whether this was chemically plausible.

(a) The Miller-Urey experiment and its significance

What Miller did

1. Simulated early Earth conditions: Miller constructed a closed apparatus containing gases thought to be present in the primitive atmosphere—methane (CH₄), ammonia (NH₃), hydrogen (H₂), and water vapour (H_2O). Crucially, this was a reducing atmosphere (no free oxygen).

2. Provided energy: He passed electric sparks through the gas mixture to simulate lightning, a major energy source on early Earth. The apparatus also included a heating element to evaporate water and a condenser to cool vapours back into liquid, mimicking the water cycle.

3. Observed organic synthesis: After just one week, the water in the flask turned reddish-brown. Chemical analysis revealed the presence of several amino acids—including glycine, alanine, and aspartic acid—along with other organic compounds like urea and simple carboxylic acids.

Why this mattered

The experiment was revolutionary for three reasons:

• Proof of principle: It demonstrated that the building blocks of life (amino acids are the monomers of proteins) could arise from non-living matter through purely chemical processes, without any biological intervention. This was the first experimental evidence for abiogenesis—life from non-life.

• Support for Oparin-Haldane: The results directly validated their hypothesis that organic molecules could accumulate in early oceans, setting the stage for more complex chemistry and eventually, self-replicating systems.

• Shifted the paradigm: Before Miller, many believed a "vital force" was needed to create organic molecules. Miller showed that given the right conditions, chemistry alone suffices.

(b) How meteorite analysis supports the hypothesis

The extraterrestrial evidence

When meteorites—particularly carbonaceous chondrites like the famous Murchison meteorite (fell in Australia, 1969)—are analysed, they contain a surprising inventory of organic compounds:

1. Amino acids: Over 70 different amino acids have been identified in meteorites, including many of the 20 found in living organisms. Importantly, these include both L- and D-forms (roughly equal amounts), confirming they formed through non-biological processes (life on Earth uses only L-amino acids).

2. Nucleobases: Adenine, guanine, and other components of DNA and RNA have been detected.

3. Sugars and lipid precursors: Simple carbohydrates and fatty acids are also present.

Why this strengthens the Oparin-Haldane-Miller picture

Meteorite data provides independent corroboration in two key ways: …