Urey and Miller’s Experiment

Stanley L. Miller and Harold C. Urey provided the first experimental evidence for chemical evolution. The experiment showed that simple molecules present in the early atmosphere could combine to form organic building blocks of life.

Background Idea:

• The first form of life was proposed to have arisen from pre-existing non-living organic molecules such as RNA and proteins.
• The formation of life was considered to have been preceded by chemical evolution, that is, the formation of diverse organic molecules from inorganic constituents.
• The conditions described for the early Earth included high temperature, volcanic storms, and a reducing atmosphere containing methane and ammonia.

• A glass apparatus called a spark-discharge apparatus was designed.
• The apparatus was sterilised and evacuated before use.
• Methane, ammonia, and hydrogen gases were introduced into the chamber in a 1:2:2 ratio.
• A tube carrying water vapour was also connected to the chamber.
• The heating mantle and condenser were used to simulate evaporation and precipitation.

• An electric discharge in the form of a carbon arc spark was passed through the chamber to mimic lightning.
• In another description of the same experiment, electric discharge was created in a closed flask containing methane, hydrogen, ammonia, and water vapour at 800°C.
• These conditions were intended to reproduce conditions similar to those of the primitive Earth on a laboratory scale.

• The mixture of methane, ammonia, and hydrogen was exposed continuously to electric discharge for several days.
• During this period, the gases interacted and were then condensed.
• The condensed liquid collected in the U-tube turned brown.

Diagrammatic representation of Miller’s experiment

Observation:

Chemical analysis of the brown liquid showed the presence of simple organic compounds such as urea, amino acids, and lactic acid.

In similar experiments, the formation of sugars, nitrogen bases, pigments, and fats was also observed.

Significance:

• The experiment strongly supported the view that simple molecules in the early atmosphere could combine to form organic compounds.
• It supported the idea of chemical evolution.
• On the basis of such limited evidence, the first part of the proposed account of chemical evolution came to be more or less accepted.

• The first non-cellular forms of life could have originated about 3 billion years ago.
• These may have been giant molecules such as RNA, proteins, and polysaccharides.
• The first cellular forms of life may not have originated until about 2000 million years ago.
• These were probably single-celled and lived only in water.

• Purpose - Stanley Miller and Harold Urey provided the first experimental evidence in support of Oparin's chemical evolution theory.
• Apparatus - A sterilised and evacuated spark-discharge glass apparatus was used; CH₄, NH₃, and H₂ gases were pumped in the ratio 1:2:2 along with water vapour.
• Lightning Effect - Electric discharge carbon arc spark was used to mimic lightning; heating mantle and condenser simulated evaporation and precipitation.
• Result - After several days of continuous electric discharge, the gases interacted and condensed; the collected liquid turned brown and contained simple organic compounds like urea, amino acids, and lactic acid.
• Significance - The experiment strongly proved that simple molecules present in Earth's early atmosphere can combine to form organic building blocks of life.