Biochemical Origin of Life - Third step: Origin of Organic Compounds

• As the primitive atmosphere cooled to about 1000°C, saturated and unsaturated hydrocarbons were formed from simple gases.
• These hydrocarbons reacted with superheated steam to form oxy- and hydroxy-derivatives such as aldehydes, ketones and organic acids.

By condensation and polymerisation of these derivatives, several organic molecules were formed:

• Sugars, glycerol, fatty acids
• Amino acids
• Nitrogenous organic bases – purines and pyrimidines

In the absence of free oxygen and bacteria, these complex organic molecules were neither oxidised nor decomposed (no putrefaction).

They progressively combined and reacted with each other to form more complex compounds:

• Polysaccharides (carbohydrates)
• Fats (lipids)
• Proteins
• Nucleosides and nucleotides

Nucleotides are linked in various combinations to form nucleic acids, and proteins + nucleic acids aggregate to form giant nucleoprotein molecules.

Some polypeptides acted as primitive enzymes, increasing the rate of formation of specific molecules.

• The synthesis of carbohydrates, fats, amino acids and other complex organic compounds took place in the sea.
• J. B. S. Haldane described this mixture of seawater and dissolved organic compounds as a “hot dilute soup”.

Scientists: Stanley Miller and Harold C. Urey (1953).

Aim: To test whether organic biomolecules could form abiotically under conditions similar to the primitive Earth.

Experimental Setup (Spark-Discharge Apparatus)

A closed apparatus with:

• Boiling flask (water → water vapour)
• Gas chamber containing hydrogen, ammonia, methane and water vapour
• Electrodes providing electric sparks (simulating lightning)
• Condenser and collection chamber for condensed liquid.

Atmosphere in the chamber was reducing and maintained at about 800°C.

Procedure

• Water was heated → water vapour formed and entered gas chamber.
• Gas mixture (H₂, NH₃, CH₄, H₂O vapour) was exposed to electric discharges for several days.
• Products were cooled and collected in another chamber for analysis.

The spark-discharge apparatus set up by Stanley Miller and Harold C. Urey to simulate conditions in the atmosphere of the primitive earth

Observations and Conclusion

• Detected formation of amino acids (e.g., glycine, alanine, aspartic acid) and other complex organic compounds.
• Showed that various organic compounds can form abiotically from a reducing mixture of simple gases subjected to energy sources.

• M. Calvin: Obtained amino acids and sugars by treating a mixture of hydrogen, water vapour, ammonia and methane.
• Bahadur (1954): Obtained all possible amino acids from a mixture of ammonia, ferric chloride and paraformaldehyde exposed to strong sunlight.
• Meteorite analyses: Revealed similar organic compounds, indicating such chemical processes can occur elsewhere in space.

• Abiotic synthesis of organic compounds in these experiments required a reducing atmosphere.
• Free oxygen would destroy many intermediate products, preventing such synthesis.
• Present-day Earth has free oxygen and living organisms, which prevent the abiotic origin of life in the same way now.
• Therefore, life is understood to have originally arisen from inorganic molecules on primitive Earth, but today life comes only from pre-existing life.

• As the Earth cooled to about 1000°C, simple hydrocarbons formed and later reacted with steam to produce organic compounds such as aldehydes, ketones, and organic acids.
• These compounds further formed sugars, amino acids, fatty acids, purines, and pyrimidines.
• Organic molecules accumulated in the primitive ocean as a hot dilute soup.
• Further reactions produced macromolecules such as proteins, fats, and nucleic acids.
• Miller–Urey experimentally supported abiotic synthesis of organic molecules.